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Intersection sight distance analysis and guidelines
ARTICLE IN PRESS Transport Policy 16 (2009) 143–150
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Intersection sight distance analysis and guidelines Faisal Awadallah Faculty of Engineering, Birzeit University, Birzeit, West Bank, Palestine
a r t i c l e in fo
abstract
Available online 15 May 2009
Guidelines for intersection sight distance are mostly outlined in general terms without showing effects of all the variables and assumptions involved. This paper provides theoretical analysis and sets guidelines based on the laws of motion for three types for intersection sight distances, namely (a) approach sight distance, (b) sign visibility sight distance, and (c) stop-line sight distance. The paper introduces guidelines for satisfying intersection sight distances for Stop sign, Yield sign and ‘no traffic control’ approaches in a systematic and in equation format that includes all the pertinent variables. The paper is especially useful for practitioners and policy makers. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Intersection sight distance Intersection traffic controls Intersection hierarchy
1. Introduction
2.1. ‘No traffic control’ approaches
Intersections are a main challenge to traffic engineers. They are the main source of traffic delays, capacity constraints, safety hazards, and drivers’ confusion. However, they are an unavoidable necessity for a traffic network. Intersection sight distance (ISD) requirements have long been recognized as a safety consideration for accidents analysis, intersection design, and traffic control. Nonetheless, the regulations for sight distance requirements at intersections for various traffic controls have been general and sometimes vague. Most standards and guidelines use tables and figures to simplify the procedure, thus, the analysis becomes like a ‘black box’; the user does not know the underlying variables and assumption that provide the intersection sight distances. Despite, the emphasis on intersection sight distance as a concept and criteria in the literature; in practice, many jurisdictions throughout the world do not have set guidelines or implementing procedures for ISD requirements for traffic control and safety considerations. ISD in various parts of the world shows substantial differences in standards and guidelines.
In a study of ISD for nine countries namely Australia, Canada, France, Germany, The Netherlands, South Africa, Sweden, Switzerland, and the USA; only the USA apparently has a policy for sight distance on approaches of intersections with no traffic control other than the basic right-of-way rule (Harwood et al., 1995). For no control intersections, AASHTO (2004) provides a table for recommended sight distances for design approach speeds. The table is based on a perception–reaction time of 2.5 s and assuming approaching vehicles to a ‘no traffic control’ intersection reduce speed to 50% of their midblock running speed. Adjustments of ISD for grades more than 3% is provided in a separate table; but no provisions are available for various design vehicles or skewed intersections approaches. Earlier AASHTO versions did not considered reduction of approaching vehicles speeds compared with the midblock speeds for ISD for ‘no traffic control’. Various international standards for ISD consider a speed reduction for approaching vehicles. Also, AASHTO (1994) ISD for ‘no stop control’ is based on a traveled distance by vehicles at the design speed in 3 s.
2. Background ISD requires unobstructed sight distance along certain approaches on both intersecting roads, depending on traffic flow regulation (mainly one-way and two-way traffic). Three cases of ISD design policies (based on traffic control) have been reviewed namely ‘no traffic control’, Yield sign, and Stop-sign approaches. Tel.: +972 2 298 2002; fax: +972 2 298 2984.
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2.2. Yield-sign approaches In a study of ISD for Yield-sign approaches, the sight distances along the minor and the main roads were compared for four countries namely Germany, South Africa, Sweden, and the USA (Harwood et al., 1995). The values of sight distances along the main road are a function of truck usage for the German and South African guidelines. The sight distance values for the main road vary for each of the four guidelines with the design or operating speed. Also, the values vary for the same design speed among the guidelines of the four countries in the range of 0–75% (including
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values specified in some countries for high volume of trucks, passenger cars only or general traffic). However, the most striking comparison is the sight distances along the minor road approaches in the same four countries, where sight distance is not a function of design speed or operation speed in Germany, South Africa, and Sweden. Thus, the difference between the guidelines of sight distance for the same design speed in the four countries is more that 10-fold in some cases. The sight distance along a minor approach for German guidelines is only 10 m for all design speeds, while it is 20 m for Swedish guidelines (Harwood et al., 1995). Thus for example, for a Yield-sign control on a minor road with a 70 km/h design speed; it would be impossible for a vehicle to stop safely at or before the intersection if the driver maintains the design speed up to 10 m from the Yield-sign intersection (when the sight distance for the minor road is only 10 m) and the driver is confronted with crossing vehicles. Therefore, the regulations and drivers’ instructions in drivers’ license tests must be different between the USA Yield-sign regulations and that of fixed sight distance for minor roads, such as Germany. Thus in Germany for example, drivers facing a Yield sign must reduce their speed to about 10 km/h regardless of the design or operating speed on such road when they arrive within 10 m from the intersection in order to be able to stop safely if needed. In addition, a driver on a 70 km/h design road and located 100 m from a Yield sign, where sufficient unobstructed view triangles allow the driver to cross the intersection safely; then it can be argued that most drivers would not slow down to10 km/h within 10 m from such an intersection as required for such Yieldsign regulation. It may be further argued that it is unreasonable to assume that drivers are able to reduce speed as needed depending on the obstruction distance from a Yield sign. AASHTO (2004) Yield-sign standards provide sufficient sight distance for vehicles to stop or cross safely based on a speed reduction to 60% of the midblock running speed. A clear approach triangle is provided with minor road approach distance and a crossing road clear distance. Tables for minor and major roads ISD are provided for passenger cars crossing a two-lane highway with no median and grades 3% or less. Adjustment factors for grades are provided, but adjustments for various design vehicles and intersection widths are only possible via a designated equation. The AASHTO (2004) also provides ISD for left- and right-turning movement for Yield signs; however, the approach speed for these maneuvers is assumed to be 16 km/h. Thus, because the ISD crossing requirements for Yield sign is equal to or higher than for turning movements, the only need for ISD turning requirement would be for T-intersections. Also in such a case, if a vehicle needs to stop; the stop-line departure triangle needs to be applied. The criteria for ISD for Yield signs vary considerably among various standards and guidelines and it may be general, such as in the Manual of Uniform Traffic Control Devices (MUTCD) guidelines for using Yield sign on minor approaches, which states: When the ability to see all potentially conflicting traffic is sufficient to allow a road user traveling at the posted speed, the 85th-percentile speed, or the statutory speed to pass through the intersection or to stop in a reasonably safe manner (MUTCD, 2003) 2.3. Stop-sign approaches AASHTO (2004) addresses ISD for Stop-sign control on minor roads for the crossing and for turning maneuvers. It provides detail analysis for sight distance requirement for each maneuver. The ISD values are based on the time gap needed for minor road vehicles to cross the intersection and the design speed of the main road. The time gaps are provided for various design vehicles for
vehicles turning either left or right onto a two-lane highway with no median and grades 3% or less. The ISD requirements for the crossing maneuver are satisfied by satisfying that of the right-turn maneuver. Adjustments are provided for multi-lane highways, and minor approach grades exceeding 3%. Acceleration rates and start-up lost time are accounted for within the time gap. Harwood et al. (1999) recommended that sight distance along the major road for a passenger car at a stopped controlled intersection to be based on a distance equal to 7.5 s of travel time at the design speed of the major road. Many studies have focused on the ‘critical gap’ or the minimal gap size acceptable to a population of drivers (Gattis and Low, 1999; Mahmassani and Sheffi, 1981). Sight distance for stop control should be differentiated from gap acceptance. Time gap is the time in seconds between the passages of two successive vehicles. The sight distance requirements should accommodate at least one critical time gap. Acceptable time gap varies among drivers; Neudorff (1985) states the acceptable gap size to accommodate the passage of one vehicle from a side street is 5–9 s.
3. Intersection traffic control The main need of intersection traffic control is to regulate traffic and to ensure safety. Additional benefits may be the reductions of delays, and increasing of intersection vehicular and pedestrian capacities. The intersection traffic control or hierarchy are as follows: Yield or Stop signs on minor approaches, Stop signs at all approaches (not common), temporal separation via traffic signals, and finally spatial separation via interchanges. In addition, no traffic control on all intersection approaches is common on low volume and low-speed roads (especially unpaved roads) with sufficient sight distances. Rotary (roundabout) intersections have particular traffic controls and regulations, but they are not within the scope of this research. The main criteria for selection of appropriate intersection traffic controls are volume (daily distribution and peaking), delay, and safety. Heavy volumes and high delay intersections require traffic signals or interchanges. Most warrants for intersection signalization involve vehicular or pedestrians volume guidelines (MUTCD, 2003). Signalized intersections with high delays; intersections that have demand volume (approach arrival) to capacity (v/c) ratios greater than one, or intersections with expressways require interchanges. Average intersection delays greater that 60 s/vehicle are considered level of service F by the Highway Capacity Manual (HCM, 2000). Selection of type of signalization or interchange type is based on volume and turning movement analysis. When signalization is not warranted according to the MUTCD (2003) or any other standard; then Stop, Yield, and ‘no traffic control’ are the remaining options. ‘No traffic control’ intersections are justified when volumes are very low, including peak periods and there are sufficient sight distances on all approaches for vehicles to stop safely at the intersection or to be able to cross and clear the intersection safely. Intersections without any traffic control are usually minor, unpaved roads with low speeds. MUTCD (2003) warrants Yield signs on minor road approaches with low intersection volumes and with sufficient sight distance for vehicles to stop safely at the intersection or to be able to cross and clear the intersections safely. Yield-sign approaches should not cross high-speed roads or main roads, especially four or more lane roads. The essence of Yield-sign recommendation is to accommodate for a simple decision by drivers to stop, or proceed (with or without the need to adjust speed) to cross the intersection safely. Thus multi-lanes, high speed or high volume for the main road would complicate the decision process.
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Sufficient time to decide, react, and implement the decision to stop or proceed is essential. Yield signs are also warranted and commonly used for merging traffic movements, especially at right-turn channelized intersection approaches or ramp exits without sufficient-length acceleration lanes. Stop signs are warranted when any of the Yield-sign guideline warrants criteria are not met, i.e., minor road crossing main road, especially four or more lane roads, moderate volume, high-speed main roads, or approach sight distance is not sufficient. Thus if any of these criteria are not met; it would require at least a Stop sign. If signalization is warranted (e.g., according to MUTCD 2003 standards) then a Stop sign would not be sufficient. All-ways Stop signs have been used mostly in residential areas with low volume and low-speed streets. The all-ways Stop is sometimes intended to control speed at residential streets and to improve safety, especially when pedestrian and cyclist safety is involved. But this practice is not generally allowed in most countries throughout the world, and it has limited usage in the USA.
4. Intersection sight distance analysis There are several sight distance requirements that ought to be satisfied for various intersection traffic control settings. Three types of intersection sight distance requirements are outlined in this paper, namely Approach sight distance, stop-line sight distance, and sign visibility sight distance. ‘No traffic control’ intersections require satisfaction of approach sight distance only; Yield-sign control approaches need to satisfy approach sight distance and sign visibility sight distance; and Stop-sign control approaches need to satisfy sign visibility sight distance and stopline sight distance. A summary of the various traffic controls and the types of ISD required is provided in Table 1. 4.1. Approach sight distance Approach sight distance is required on all approaches for ‘no control’ intersections and on Yield-controlled approaches. When there is no traffic control on an intersection, drivers on all approaches are expected to give the right-of-way to vehicles traveling at the crossing road at ‘set’ speed. The ‘set’ speed for calculating the required sight distance is recommended to be the posted speed limit, statutory speed limit, or the 85th-percentile speed. The ‘set’ speed or approaching speed may be also a percentage of the above midblock speeds. The design speed is based on the designed feature of the roadway and could be misleading in cases where the posted speed is substantially less than the design speed due to land use and other surrounding environment conditions. The ‘set’ speed may be termed design speed for ISD calculations. The approach sight distance requirement for each approach is based on providing clear sight distance triangles from the decision point to both sides of the intersecting road. Both sides are needed for two-way traffic; a triangle on one side only would be needed for one-way traffic in the direction of oncoming traffic to the intersection. The decision point is a minimum required stopping Table 1 Intersection traffic controls vs. required intersection sight distances. Type of traffic control
Required intersection sight distance Approach
No control Yield Stop
x x
Sign visibility
Stop-line
x x
x
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Fig. 1. Illustration of intersection approach sight distance analysis.
sight distance (MRSSD) from the conflict point or it may be assumed from the stop-line since inside the intersection would be considered a conflict area (see Fig. 1). The MRSSD is calculated by Eq. (1), which is based on Eqs. (3-2) and (3-3) from AASHTO (2004) but with compatible units’ format: MRSSD ¼ V s t r þ
V 2s 2a 2gG
(1)
where MRSSD is the minimum required stopping sight distance (m), Vs the velocity or ‘set’ speed for side or traffic controlled road (m/s), tr the driver’s perception–reaction time (s), default 2.5 s, g the gravitational constant (9.81 m/s2), a is the deceleration rate (m/s2), default 3.4 m/s2, and G the grade in decimal form (e.g., 1% ¼ 0.01). From the decision point, a triangle of non-obstructed view is formed by establishing the line of sight from the decision point and tangent to the nearest obstruction until it intersects the travel lane towards the intersection of the crossing road. This point is termed the sight point (see Fig. 1). The side of this triangle from the conflict point to the sight point is termed the available nonobstructed view distance (DANOV); this distance must be compared with the required non-obstructed view distance (DRNOV). DANOV does not need to be a straight or level line, but it is a distance along the main road from the collision point to the sight point (Fig. 1). There are two methods of determining DANOV: (a) A driver at a stopped vehicle (critical eye height is for lowheight design vehicles) located at the decision point watches a very slow moving low-height vehicle (not van or higher vehicles) driving on the crossing road towards the intersection. When the driver at the decision point first sees the very slow moving vehicles from the crossing road (just clears the obstruction as viewed from the decision point); the sight point and the DANOV is established (see Fig. 1). The very slow moving vehicle should continue traveling to reach the intersection and it must be continuously seen by the stationary driver at the decision point. Indeed police supervision for such field test is needed and it should be conducted at very low traffic periods. In addition, it is possible to perform this field exercise via a sighting rod of design driver’s eye height (1.08 m) and target rod for a height of the design vehicle (1.08 m) also. The proposed values for drivers’ eye height and design vehicle height for ISD are those of the USA (AASHTO, 2004), but other values may be used. The outlined procedure for determining the available ISD is similar to others used by numerous jurisdictions, especially in the USA. This field test procedure may be further simplified by calculating the required non-obstructed view distance (DRNOV)
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first and establishing the required sight point. The decision point is established based on the MRSSD calculation. Thus, if a driver (or person viewing via sighting rod) at the decision point sees a low vehicle (or target rod) at the required sight point; the approach sight distance requirement would be satisfied. (b) A map of the intended intersection with sufficient approach distances and with all obstructions greater than one meter above the elevation of the decision point must be shown on the map. From the decision point (MRSSD from the approach stop-line), a line of sight is drawn tangent to the nearest obstruction; the intersection of the line of sight with the line drawn from the conflict point in the lane approaching the intersection of the crossing road would be the sight point (see Fig. 1). This process may be also performed with 3-D digital maps (Nehate and Rys, 2006). Nonetheless, the field test method is preferable since it represent the actual conditions. Required non-obstructed view distance (DRNOV) is based on providing sufficient non-obstructed distance on the crossing road as seen from the decision point. This non-obstructed view distance should be far enough to enable a driver at the decision point to continue at the ‘set’ speed and to cross and clear the intersection before the crossing vehicle arrives at the conflict point and it would be at least a set clearance distance/time from the conflict point. The set speed could be lower than the posted or midblock running speed depends on the traffic regulations and ISD standards. AASHTO (2004) assumes 50% and 60% of midblock running speed for ISD for approaches of ‘no traffic control’ and Yield traffic control, respectively. If a vehicle is seen at the available sight point from the decision point, yet a driver of a vehicle at the decision point can not proceed to cross the intersection safely (only he/she can stop safely); then also if there are no vehicles between the sight point and the conflict point; it is not safe to proceed since a vehicle hidden by the obstruction may reach the conflict point before the vehicle passing the decision point clears the intersection. The required non-obstructed view distance (DRNOV) may be calculated by first finding the time required for a vehicle traveling at ‘set’ speed at the decision point to cross and clear the intersection (tcc), Eq. (2). Gap headway time (tgh) is set as a safety factor. Thus when the vehicle just clears the intersection, the vehicle from the crossing road would be at least a set gap headway time away from the conflict point. Hence, given the time to cross and clear the intersection (tcc), the gap headway time (tgh), and the speed at the crossing road; the required nonobstructed view distance (DRNOV) is found by Eq. (3): t cc ¼
ðMRSSD þ w þ LÞ Vs
(2)
where tcc is the crossing and clearing time (s), w the intersection width (m), and L the design vehicle length (m), default 6 m (for passenger cars only): DRNOV ¼ ðt cc þ t gh ÞV c
(3)
where DRNOV is the required non-obstructed view distance (m), tgh the gap headway time (s), default 2.0 s, and Vc the velocity or ‘set’ speed for crossing or main road (m/s). The effects of trucks or grades are accounted for in the calculation of the MRSSD. In order for a ‘no traffic control’ intersection to be satisfied by sight distance requirement; the available non-obstructed view distance (DANOV) must be greater than the required nonobstructed view distance (DRNOV), Eq. (4), for all approaches. This will allow sufficient distance to see a vehicle on the crossing road far enough to be able to continue at the ‘set’ speed and to cross and clear the intersection safely; to be able to stop safely at the
intersection if there is conflicting traffic on the crossing road; or to slow down to give way for a vehicle or more on the crossing road to pass then to cross the intersection without a need to stop: DANOV XDRNOV
(4)
where DANOV is the available non-obstructed view distance (m), and DRNOV the required non-obstructed view distance (m). When the approach sight distance requirement, Eq. (4), is satisfied for ‘no control’ or Yield-sign approach intersections, then this condition should be maintained by no parking zones between each of the decision point and the intersection and the required sight point and the intersection. When a ‘no control’ or Yield-sign approach sight distance requirements are not satisfied, specifically Eq. (4) is not satisfied; then a Stop sign is required. 4.2. Sign visibility sight distance Sign visibility sight distance is required for Yield- and Stopcontrolled intersections. Signs should be visible from a sufficient distance to stop safely before entering the intersection. Thus, the available sight distance (ASD) for a Yield or a Stop sign should be greater than the minimum required stopping sight distance, Eq. (5). The ASD for Yield, Stop, or any other sign is the distance before reaching the sign, where the sign is first recognized at the critical visibility conditions. ASD is mainly a function of ambient light conditions (e.g., day time/night time). The critical ASD for traffic signs is usually taken for night time. Adverse weather condition could be more critical than just night time condition for ASD, but it is assumed when the visibility is poor due to weather condition; drivers must reduce their speed. Night time ASD for traffic signs is a function of sign retroreflective material, size, type, and position (Schnell et al., 2004); in addition to vehicle’s headlights positions, angle, and intensity (low beam should be used, since high beam is not allowed at all times). The ASD for a sign may be determined via field testing using ‘design’ vehicle and drivers with minimum acceptable visual acuity. The field test simply consists of a driver driving toward a sign and once it is recognized a button is pushed that start reading of a measuring wheel. At the position of the sign the measured distance should be recorded and it equals to the ASD for such a sign. Several runs, especially with different drivers are recommended. Also, estimating ASD for a particular sign may be performed via a reflectormeter that measures sign retroreflectivity and correlation techniques may be used to estimate the ASD (Awadallah, 1988). Eq. (5) is a basic highway and traffic design requirement, which is based on an AASHTO (2004) requirement in written format in the chapter on elements of design: ASDXMRSSD
(5)
where ASD is the available sight distance (m), and MRSSD the minimum required stopping sight distance (m). 4.3. Stop-line sight distance The stop-line sight distance requirement mainly applies for stop-sign control. Yield sign and ‘no traffic control’ intersections would have this requirement satisfied for passenger cars by satisfying Eq. (4). AASHTO (2004) explicitly states ‘Yield-controlled approaches generally need greater sight distance than stop-controlled approaches.’ However, Eq. (4) may not satisfy stop-line sight distance requirements for some heavy trucks since their acceleration rate is very low. Thus, Eq. (4) would be satisfied for a heavy truck to pass and clear the intersection or to stop safely at the intersection if needed, but once heavy truck stops at a Yield sign, the sight distance provided by Eq. (4) may not be
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Fig. 2. Illustration of stop-line sight distance analysis.
sufficient for a heavy truck to cross and clear the intersection. Signalized intersections stop-line sight distance requirement is not necessary when the signal is functioning. But if the signal is not functioning (e.g., due to power outages or during a night time flashing phase), the minor street approaches would be essentially Stop-sign controlled; then stop-line sight distance requirements would be needed. Stop-line sight distance requirement for minor approaches needs to provide sufficient sight distance on both sides of the crossing road in order for a driver of a stopped vehicle on the stopline to cross and clear the intersection or turn into the main road safely. A problem of insufficient sight distance for stop-lines of minor street approaches is only possible when the crossing road is curved and an obstruction is near the side of the road (see Fig. 2). If intersecting roads are straight, level, and perpendicular to each other; the reason for insufficient stop-line sight distance would be most likely due to the stop-line being far from the intersecting road to allow for a pedestrian crosswalk, or parking on the main street are obstructing the view. Thus in such cases the remedy is clear. A differentiation should be made between sufficient stop-line sight distance and traffic gap acceptance for crossing and turning vehicles. Traffic gap patterns and drivers’ gap acceptance influence minor street delays, which is a criterion for intersection signalization warrants. Basically, the available stop-line sight distance should accommodate at least the minimum gap acceptance for such approach. The procedure for determining the sufficiency of the stop-line sight distance is performed by first determining the location of the sight point, and then the stop-line available non-obstructed view distance (DSANOV) (see Fig. 2). Similarly to determining the available non-obstructed view distance (DANOV) in Section 4.1; the stop-line available non-obstructed view distance (DSANOV) is obtained via a field test or via a map of the intersection with all obstructions greater than 1 m from the elevation of the stop-line. The sight point is determined from the line of sight that is tangent to the obstruction as viewed from the stop-line. Stop-line available non-obstructed view distance (DSANOV) is the distance from the point of sight along the path of the approaching lane to the conflict point within the intersection (see Fig. 2). The stop-line available non-obstructed view distance (DSANOV) should be compared with the stop-line required non-obstructed view distance (DSRNOV). However, there are two types of stop-line required non-obstructed view distances, namely for crossing and for turning movements.
4.3.1. Crossing movement The stop-line required non-obstructed view distance-crossing (DSRNOV-C) is based on providing sufficient non-obstructed
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distance on the crossing road as seen by a driver stopped at the stop-line. This stop-line non-obstructed view distance should be far enough to enable a driver at the stop-line to react, accelerate, cross and clear the intersection before the crossing vehicle arrives at the conflict point and it would be at least a set gap headway time from the conflict point. The time for the stopped vehicle to cross and clear the intersection (ts-cc) is calculated by Eq. (6). This equation consists of two components, start-up lost time and the acceleration time needed to cross and clear the intersection. The start-up lost time does not include the decision time; it is only the reaction time after the decision to ‘go’; similar to the start-up lost time for the first vehicle on a traffic signal’s approach when the signal turns green. The start-up lost time for the first five vehicles for a signalized intersection is 1–2 s. Kunzman (1978) estimated the average cumulative start-up lost time to be 1.1 s. Therefore, the default start-up lost time for only one vehicle (the 1st vehicle) for Eq. (6) should be 0.5–1.0 s. The stop-line required non-obstructed view distance-crossing (DSRNOV-C) is calculated by Eq. (7). If the stop-line available non-obstructed view distance (DSANOV) is greater than the stop-line required non-obstructed view distance-crossing (DSRNOV-C), Eq. (8); then the stop-line sight distance is satisfied for the crossing movement: 2ðw þ LÞ 0:5 þ t sl (6) t s-cc ¼ a where ts-cc is the stop-line crossing and clearing time (s), tsl the start-up lost time (s), default 0.5–1.0 s for passengers cars (for trucks values are higher), a the acceleration rate (m/s2), default 3.0 m/s2 for passenger cars (for trucks values may be as low as 1.0 m/s2 or less), w the intersection width from the stop-line (m), default value for the distance from stop-line to the edge of minor road is 4.4 m and drivers’ eyes are located 2.4 m from the front of a passenger car (AASHTO, 2004): DSRNOV-C ¼ ðt s-cc þ t gh ÞV c
(7)
where DSRNOV-C is the stop-line required non-obstructed view distance-crossing (m): DSANOV XDSRNOV-C
(8)
where DSANOV is the stop-line available non-obstructed view distance (m), and DSRNOV-C the stop-line required non-obstructed view distance-crossing (m). 4.3.2. Turning movement The stop-line required non-obstructed view Distance-turning (DSRNOV-T) is based on providing a distance along the main road, where a vehicle approaching from the main (crossing) road is far enough to allow the stopped vehicle at the stop-line to turn left or right and accelerate to a speed ‘close’ to the speed limit of the ‘turned to’ main road before vehicles traveling on the main road reach within a set time/distance headway gap from the turned vehicle. The time for the stopped vehicle to turn and reach a ‘set’ speed (for AASHTO, 2004, it is set at 70% of the main road speed limit) consist of start-up lost time and acceleration time (ts-t), which is calculated by Eq. (9). The start-up lost time for left-turns should be more than for right-turns; since the former needs the operator to check both sides of the road even after the ‘go’ decision. The stop-line required non-obstructed view distanceturning (DSRNOV-T) is calculated by Eq. (10). If the stop-line available non-obstructed view distance (DSANOV) is greater than the stop-line required non-obstructed view distance-turning (DSRNOV-T), Eq. (11); then the stop-line sight distance is satisfied for the turning movement: pV c þ t sl (9) t s-t ¼ a
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where ts-t is the stop-line turning and acceleration time (s), p the percentage of main road speed in decimal form (that main road vehicles are expected to reduce the speed to accommodate turning vehicles), suggested default 70% (0.7), tsl the start-up lost time (s), default 0.5 s for right-turn and 1.0 s for left-turn, for passenger cars: DSRNOV-T ¼ ðt s-t þ t gh ÞV c
(10)
where DSRNOV-T is the stop-line required non-obstructed view distance-turning (m): (11)
DSANOV XDSRNOV-T
where DSRNOV-T is the stop-line required non-obstructed view distance-turning (m), and DSANOV the stop-line available nonobstructed view distance (m). The stop-line sight distance would be satisfied for a four leg intersection by satisfying both of Eqs. (8) and (11) for both sides’ approaches, or by comparing the higher of DSRNOV-C or DSRNOV-T to DSANOV for both sides of the crossing road. For a stop-line at a T-intersection; only turning movement requirement would be necessary. In order to satisfy Eqs. (8) or (11), simply, obstructions should be removed to increase the stop-line available non-obstructed view distance (DSANOV) to become greater than the required nonobstructed view distance-crossing or/and turning. If this is not possible, reduction of main road speed limit, as a speed zone approaching the intersection, is an option in order to reduce the DSRNOV-C or/and DSRNOV-T to become lower than the DSANOV. Finally, if Eqs. (8) or (11) could not be satisfied by any mean possible (or it particularly requires a reduction of the speed of the main road drastically, which would not be reasonable); then this would be a typical hidden entrance/exist and it should not be used specifically for vehicles existing to cross or turn into the main
road. Vehicles turning right or left into such road or entrance would involve vehicles slowing or stopping (especially left-turns without a left-turn bay) on the main road. However, this case should have satisfactory sight distance based on the appropriate geometric design of roads, where the available sight distance should be greater than the minimum required sight distance throughout designed roads.
4.4. Summary and comparisons with AASHTO (2004) The ISD guidelines and equations in this research were developed from the basic laws of motion. Table 2 provides a summary of the equations used for the various ISD and intersection traffic controls. Fig. (3) shows that values for ISD for ‘no control’ intersection approaches from this research are closely correlated to AASHTO (2004) when using the same assumptions, especially vehicles approaching ‘no traffic control’ intersections slow to 50% of midblock running speeds. Also, using the AASHTO (2004) assumption of vehicles slowing down to 60% of their midblock speeds when approaching a Yield-sign approach resulted in similar high correlations as can be seen in Fig. 4. Finally, Fig. 5 provides comparisons of ISD for Stop-sign approaches for right-turn, left-turn, and crossing maneuvers. While the crossing maneuvers results of this research are very close to that of AASHTO (2004); both turning maneuvers for this research provide somewhat lower ISD for low design speeds and somewhat higher ISD for higher design speeds. This is mainly due to use of time gap instead of distance gap for safety clearance. For speeds less than 50 km/h (30 mph) for the main road, the equations developed from the laws of motion provide safe results for the assumptions of 2 s gap and reduction of main road vehicles’ speeds to 70%. But the distance gap may be short
Table 2 Intersections sight distances requirements. Type of sight distance
Type of applicable traffic control
Requirements in equation form
Sign visibility
Yield-sign and Stop-sign
ASD4MRSSD MRSSD ¼ Vstr ¼ V2s /2a72gG where ASD is the available sight distance (m), MRSSD the minimum required stopping sight distance (m), Vs the velocity or ‘set’ speed for side or traffic controlled road (m/s), tr the driver’s perception–reaction time (s), default 2.5 s, g the gravitational constant (9.81 m/s2), a the deceleration rate (m/s2), default 3.4 m/s2, and G the grade in decimal form (e.g., 1% ¼ 0.01)
Approach sight distance
Yield-sign and ‘no traffic control’
DANOVXDRNOV DRNOV ¼ (tcc+tgh)Vc tcc ¼ (MRSSD+w+L)/Vs where DANOV is the available non-obstructed view distance (m), DRNOV the required non-obstructed view distance (m), tcc the crossing and clearing time (s), tgh the gap headway time (s), Vc the velocity or ‘set’ speed for crossing or main road (m/ s), w the intersection width (m) and L the design vehicle length (m), default 6 m (for passenger cars only)
Stop-line sight distance
Stop-sign and some traffic signals’ minor approaches (e.g., flashing phase)
(a) Crossing movement DSANOVXDSRNOV-C DSRNOV-C ¼ (ts-cc+tgh)Vc ts-cc ¼ [(2(w+L)/a]0.5+tsl where DSANOV is the stop-line available non-obstructed view distance (m), DSRNOV-C the stop-line required non-obstructed view distance-crossing (m), ts-cc the stop-line crossing and clearing time (s), tsl the start-up lost time (s), default 0.5–1.0 s for passengers cars (for trucks values are higher), a the acceleration rate (m/s2), default 3.0 m/s2 for passenger cars (for trucks values may be as low as 1.0 m/s2 or less), w the intersection width from the stop-line (m), default values for distance from stop-line to edge of minor road is 4.4 m and drivers’ eye are 2.4 m from front of passenger cars (AASHTO 2004), tgh the gap headway time (s), tsl the start-up lost time (s), default 1.0 s (b) Turning movement DSANOVXDSRNOV-T DSRNOVT ¼ (tst+tgh)Vc ts-t ¼ (pVc+a)+tsl where DSRNOV-T is the stop-line required non-obstructed view distance-turning (m), DSANOV the stop-line available nonobstructed view distance (m), ts-t the stop-line turning and acceleration time (s), p the percentage of main road speed in decimal form (that main road vehicles are expected to reduce the speed to accommodate turning vehicles), suggested default 70% (0.7), tsl the start-up lost time (s), default 0.5 s for right-turn and 1.0 s for left-turn, for passenger cars
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60 AASHTO 2004 Left Dsrnov-left
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AASHTO 2004 right & crossing
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0 0
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100
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Crossing road ISD (m)
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Fig. 5. Comparisons of ISD for Stop-sign control, right-turn, left-turn, and crossing maneuvers.
Fig. 3. Comparisons of ISD for ‘no traffic control’ intersections.
for main road vehicles to accommodate turning vehicles from the minor street, and the reduction of approach speed to a Yield-sign or ‘no traffic control’ approaches.
120
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100 5. Conclusion
80
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40 AASHTO 2004 20
Drnov (60% approach speed)
0 0
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150 100 Crossing road ISD (m)
200
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Fig. 4. Comparisons of ISD for Yield-sign approaches of a minor road speed of 50 km/h.
especially due to psychological and human expectation reasons; thus a minimum ISD for stop-sign approaches may be augmented (e.g., 45 m). The difference between the research results for higher design values for turning movements is not substantial up to 100 km/h; but at higher speeds, it is not practical to have a stopcontrolled approach on a highway with a design speed of expressways. Variations due to changes in grades or design vehicles are impeded within various equations for ISD in this research. The percent grade is incorporated in the MRSSD equation; while the design vehicle may be addressed via vehicle length, and acceleration and deceleration rates in the various ISD equations. Other variations that depend on the drivers’ characteristics and attitudes may be addressed, such as start-up lost time, the reduction speed
The importance of intersection sight distance has been a common knowledge among traffic engineers and educators, but the requirements and procedures for satisfying intersection sight distances are neither uniform nor well known in practice. The main benefit of this research that it provides ISD based on the laws of motions and the equations are flexible to changes, especially assumptions; thus traffic engineers obtain results from clear scientific equations, which can be easily defended. The proposed ISD guidelines explicitly cover the various variables of significance and provide the procedures and the underlying assumptions. Intersecting roads need not be straight, level or perpendicular to each other to easily follow the proposed intersection sight distance requirements. Thus comparisons, changes for various conditions, and sensitivity analysis are possible. The default values are obtained from various research, studies, and jurisdictions’ guidelines and practices. However, they can be easily changed and their sensitivity can be easily measured. Further studies to obtain more relevant default values for specific conditions and regions are recommended. References American Association of State Highway and Transportation Officials (AASHTO), 1994. American Association of State Highway and Transportation Officials (AASHTO), 2004. Awadallah, F., 1988. Prediction of the service life of warning signs. Public Roads 51 (4). Gattis, J., Low, S., 1999. Gap acceptance at atypical stop-controlled intersections. Journal of Transportation Engineering 125 (3), 201–207. Harwood, D., Mason, J., Brydia, R., Joubert, H., Lamm, R., Psarianos, B., 1995. International sight distance design practices. In: Proceedings of the International Symposium on Highway Geometric Design Practices, Boston, MA, USA.
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Harwood, W., Mason, J., Brydia, R., 1999. Design policies for sight distance at stopcontrolled intersections based on gap acceptance. Transportation Research Part A: Policy and Practice 33 (3/4), 199–216. Highway Capacity Manual (HCM), Special Report 209, 2000. Transportation Research Board, National Research Council, Washington, DC, USA. Kunzman, W., 1978. Another look at signalized intersection capacity. Institute of Transportation Engineers (ITE) Journal 48 (8). Mahmassani, H., Sheffi, Y., 1981. Using gap sequences to estimate gap acceptance functions. Transportation Research 15B, 143–148.
Manual of Uniform Traffic Control Devices (MUTCD) for Streets and Highways, 2003. Federal Highway Administration, US Department of Transportation, Washington, DC, USA Nehate, G., Rys, M., 2006. 3D calculations of stopping-sight distance from GPS data. Journal of Transportation Engineering 132 (9), 691–698. Neudorff, L., 1985. Gap acceptance criteria for signal warrants. Institute of Transportation Engineers (ITE) Journal 55 (2). Schnell, T., Aktan, F., Li, C., 2004. Traffic sign luminance requirements of nighttime drivers of symbolic signs, Transportation Research Record 1862.
Contents lists available at ScienceDirect
Transport Policy journal homepage: www.elsevier.com/locate/tranpol
Intersection sight distance analysis and guidelines Faisal Awadallah Faculty of Engineering, Birzeit University, Birzeit, West Bank, Palestine
a r t i c l e in fo
abstract
Available online 15 May 2009
Guidelines for intersection sight distance are mostly outlined in general terms without showing effects of all the variables and assumptions involved. This paper provides theoretical analysis and sets guidelines based on the laws of motion for three types for intersection sight distances, namely (a) approach sight distance, (b) sign visibility sight distance, and (c) stop-line sight distance. The paper introduces guidelines for satisfying intersection sight distances for Stop sign, Yield sign and ‘no traffic control’ approaches in a systematic and in equation format that includes all the pertinent variables. The paper is especially useful for practitioners and policy makers. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Intersection sight distance Intersection traffic controls Intersection hierarchy
1. Introduction
2.1. ‘No traffic control’ approaches
Intersections are a main challenge to traffic engineers. They are the main source of traffic delays, capacity constraints, safety hazards, and drivers’ confusion. However, they are an unavoidable necessity for a traffic network. Intersection sight distance (ISD) requirements have long been recognized as a safety consideration for accidents analysis, intersection design, and traffic control. Nonetheless, the regulations for sight distance requirements at intersections for various traffic controls have been general and sometimes vague. Most standards and guidelines use tables and figures to simplify the procedure, thus, the analysis becomes like a ‘black box’; the user does not know the underlying variables and assumption that provide the intersection sight distances. Despite, the emphasis on intersection sight distance as a concept and criteria in the literature; in practice, many jurisdictions throughout the world do not have set guidelines or implementing procedures for ISD requirements for traffic control and safety considerations. ISD in various parts of the world shows substantial differences in standards and guidelines.
In a study of ISD for nine countries namely Australia, Canada, France, Germany, The Netherlands, South Africa, Sweden, Switzerland, and the USA; only the USA apparently has a policy for sight distance on approaches of intersections with no traffic control other than the basic right-of-way rule (Harwood et al., 1995). For no control intersections, AASHTO (2004) provides a table for recommended sight distances for design approach speeds. The table is based on a perception–reaction time of 2.5 s and assuming approaching vehicles to a ‘no traffic control’ intersection reduce speed to 50% of their midblock running speed. Adjustments of ISD for grades more than 3% is provided in a separate table; but no provisions are available for various design vehicles or skewed intersections approaches. Earlier AASHTO versions did not considered reduction of approaching vehicles speeds compared with the midblock speeds for ISD for ‘no traffic control’. Various international standards for ISD consider a speed reduction for approaching vehicles. Also, AASHTO (1994) ISD for ‘no stop control’ is based on a traveled distance by vehicles at the design speed in 3 s.
2. Background ISD requires unobstructed sight distance along certain approaches on both intersecting roads, depending on traffic flow regulation (mainly one-way and two-way traffic). Three cases of ISD design policies (based on traffic control) have been reviewed namely ‘no traffic control’, Yield sign, and Stop-sign approaches. Tel.: +972 2 298 2002; fax: +972 2 298 2984.
E-mail address: fi[email protected] 0967-070X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tranpol.2009.04.001
2.2. Yield-sign approaches In a study of ISD for Yield-sign approaches, the sight distances along the minor and the main roads were compared for four countries namely Germany, South Africa, Sweden, and the USA (Harwood et al., 1995). The values of sight distances along the main road are a function of truck usage for the German and South African guidelines. The sight distance values for the main road vary for each of the four guidelines with the design or operating speed. Also, the values vary for the same design speed among the guidelines of the four countries in the range of 0–75% (including
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values specified in some countries for high volume of trucks, passenger cars only or general traffic). However, the most striking comparison is the sight distances along the minor road approaches in the same four countries, where sight distance is not a function of design speed or operation speed in Germany, South Africa, and Sweden. Thus, the difference between the guidelines of sight distance for the same design speed in the four countries is more that 10-fold in some cases. The sight distance along a minor approach for German guidelines is only 10 m for all design speeds, while it is 20 m for Swedish guidelines (Harwood et al., 1995). Thus for example, for a Yield-sign control on a minor road with a 70 km/h design speed; it would be impossible for a vehicle to stop safely at or before the intersection if the driver maintains the design speed up to 10 m from the Yield-sign intersection (when the sight distance for the minor road is only 10 m) and the driver is confronted with crossing vehicles. Therefore, the regulations and drivers’ instructions in drivers’ license tests must be different between the USA Yield-sign regulations and that of fixed sight distance for minor roads, such as Germany. Thus in Germany for example, drivers facing a Yield sign must reduce their speed to about 10 km/h regardless of the design or operating speed on such road when they arrive within 10 m from the intersection in order to be able to stop safely if needed. In addition, a driver on a 70 km/h design road and located 100 m from a Yield sign, where sufficient unobstructed view triangles allow the driver to cross the intersection safely; then it can be argued that most drivers would not slow down to10 km/h within 10 m from such an intersection as required for such Yieldsign regulation. It may be further argued that it is unreasonable to assume that drivers are able to reduce speed as needed depending on the obstruction distance from a Yield sign. AASHTO (2004) Yield-sign standards provide sufficient sight distance for vehicles to stop or cross safely based on a speed reduction to 60% of the midblock running speed. A clear approach triangle is provided with minor road approach distance and a crossing road clear distance. Tables for minor and major roads ISD are provided for passenger cars crossing a two-lane highway with no median and grades 3% or less. Adjustment factors for grades are provided, but adjustments for various design vehicles and intersection widths are only possible via a designated equation. The AASHTO (2004) also provides ISD for left- and right-turning movement for Yield signs; however, the approach speed for these maneuvers is assumed to be 16 km/h. Thus, because the ISD crossing requirements for Yield sign is equal to or higher than for turning movements, the only need for ISD turning requirement would be for T-intersections. Also in such a case, if a vehicle needs to stop; the stop-line departure triangle needs to be applied. The criteria for ISD for Yield signs vary considerably among various standards and guidelines and it may be general, such as in the Manual of Uniform Traffic Control Devices (MUTCD) guidelines for using Yield sign on minor approaches, which states: When the ability to see all potentially conflicting traffic is sufficient to allow a road user traveling at the posted speed, the 85th-percentile speed, or the statutory speed to pass through the intersection or to stop in a reasonably safe manner (MUTCD, 2003) 2.3. Stop-sign approaches AASHTO (2004) addresses ISD for Stop-sign control on minor roads for the crossing and for turning maneuvers. It provides detail analysis for sight distance requirement for each maneuver. The ISD values are based on the time gap needed for minor road vehicles to cross the intersection and the design speed of the main road. The time gaps are provided for various design vehicles for
vehicles turning either left or right onto a two-lane highway with no median and grades 3% or less. The ISD requirements for the crossing maneuver are satisfied by satisfying that of the right-turn maneuver. Adjustments are provided for multi-lane highways, and minor approach grades exceeding 3%. Acceleration rates and start-up lost time are accounted for within the time gap. Harwood et al. (1999) recommended that sight distance along the major road for a passenger car at a stopped controlled intersection to be based on a distance equal to 7.5 s of travel time at the design speed of the major road. Many studies have focused on the ‘critical gap’ or the minimal gap size acceptable to a population of drivers (Gattis and Low, 1999; Mahmassani and Sheffi, 1981). Sight distance for stop control should be differentiated from gap acceptance. Time gap is the time in seconds between the passages of two successive vehicles. The sight distance requirements should accommodate at least one critical time gap. Acceptable time gap varies among drivers; Neudorff (1985) states the acceptable gap size to accommodate the passage of one vehicle from a side street is 5–9 s.
3. Intersection traffic control The main need of intersection traffic control is to regulate traffic and to ensure safety. Additional benefits may be the reductions of delays, and increasing of intersection vehicular and pedestrian capacities. The intersection traffic control or hierarchy are as follows: Yield or Stop signs on minor approaches, Stop signs at all approaches (not common), temporal separation via traffic signals, and finally spatial separation via interchanges. In addition, no traffic control on all intersection approaches is common on low volume and low-speed roads (especially unpaved roads) with sufficient sight distances. Rotary (roundabout) intersections have particular traffic controls and regulations, but they are not within the scope of this research. The main criteria for selection of appropriate intersection traffic controls are volume (daily distribution and peaking), delay, and safety. Heavy volumes and high delay intersections require traffic signals or interchanges. Most warrants for intersection signalization involve vehicular or pedestrians volume guidelines (MUTCD, 2003). Signalized intersections with high delays; intersections that have demand volume (approach arrival) to capacity (v/c) ratios greater than one, or intersections with expressways require interchanges. Average intersection delays greater that 60 s/vehicle are considered level of service F by the Highway Capacity Manual (HCM, 2000). Selection of type of signalization or interchange type is based on volume and turning movement analysis. When signalization is not warranted according to the MUTCD (2003) or any other standard; then Stop, Yield, and ‘no traffic control’ are the remaining options. ‘No traffic control’ intersections are justified when volumes are very low, including peak periods and there are sufficient sight distances on all approaches for vehicles to stop safely at the intersection or to be able to cross and clear the intersection safely. Intersections without any traffic control are usually minor, unpaved roads with low speeds. MUTCD (2003) warrants Yield signs on minor road approaches with low intersection volumes and with sufficient sight distance for vehicles to stop safely at the intersection or to be able to cross and clear the intersections safely. Yield-sign approaches should not cross high-speed roads or main roads, especially four or more lane roads. The essence of Yield-sign recommendation is to accommodate for a simple decision by drivers to stop, or proceed (with or without the need to adjust speed) to cross the intersection safely. Thus multi-lanes, high speed or high volume for the main road would complicate the decision process.
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Sufficient time to decide, react, and implement the decision to stop or proceed is essential. Yield signs are also warranted and commonly used for merging traffic movements, especially at right-turn channelized intersection approaches or ramp exits without sufficient-length acceleration lanes. Stop signs are warranted when any of the Yield-sign guideline warrants criteria are not met, i.e., minor road crossing main road, especially four or more lane roads, moderate volume, high-speed main roads, or approach sight distance is not sufficient. Thus if any of these criteria are not met; it would require at least a Stop sign. If signalization is warranted (e.g., according to MUTCD 2003 standards) then a Stop sign would not be sufficient. All-ways Stop signs have been used mostly in residential areas with low volume and low-speed streets. The all-ways Stop is sometimes intended to control speed at residential streets and to improve safety, especially when pedestrian and cyclist safety is involved. But this practice is not generally allowed in most countries throughout the world, and it has limited usage in the USA.
4. Intersection sight distance analysis There are several sight distance requirements that ought to be satisfied for various intersection traffic control settings. Three types of intersection sight distance requirements are outlined in this paper, namely Approach sight distance, stop-line sight distance, and sign visibility sight distance. ‘No traffic control’ intersections require satisfaction of approach sight distance only; Yield-sign control approaches need to satisfy approach sight distance and sign visibility sight distance; and Stop-sign control approaches need to satisfy sign visibility sight distance and stopline sight distance. A summary of the various traffic controls and the types of ISD required is provided in Table 1. 4.1. Approach sight distance Approach sight distance is required on all approaches for ‘no control’ intersections and on Yield-controlled approaches. When there is no traffic control on an intersection, drivers on all approaches are expected to give the right-of-way to vehicles traveling at the crossing road at ‘set’ speed. The ‘set’ speed for calculating the required sight distance is recommended to be the posted speed limit, statutory speed limit, or the 85th-percentile speed. The ‘set’ speed or approaching speed may be also a percentage of the above midblock speeds. The design speed is based on the designed feature of the roadway and could be misleading in cases where the posted speed is substantially less than the design speed due to land use and other surrounding environment conditions. The ‘set’ speed may be termed design speed for ISD calculations. The approach sight distance requirement for each approach is based on providing clear sight distance triangles from the decision point to both sides of the intersecting road. Both sides are needed for two-way traffic; a triangle on one side only would be needed for one-way traffic in the direction of oncoming traffic to the intersection. The decision point is a minimum required stopping Table 1 Intersection traffic controls vs. required intersection sight distances. Type of traffic control
Required intersection sight distance Approach
No control Yield Stop
x x
Sign visibility
Stop-line
x x
x
145
Fig. 1. Illustration of intersection approach sight distance analysis.
sight distance (MRSSD) from the conflict point or it may be assumed from the stop-line since inside the intersection would be considered a conflict area (see Fig. 1). The MRSSD is calculated by Eq. (1), which is based on Eqs. (3-2) and (3-3) from AASHTO (2004) but with compatible units’ format: MRSSD ¼ V s t r þ
V 2s 2a 2gG
(1)
where MRSSD is the minimum required stopping sight distance (m), Vs the velocity or ‘set’ speed for side or traffic controlled road (m/s), tr the driver’s perception–reaction time (s), default 2.5 s, g the gravitational constant (9.81 m/s2), a is the deceleration rate (m/s2), default 3.4 m/s2, and G the grade in decimal form (e.g., 1% ¼ 0.01). From the decision point, a triangle of non-obstructed view is formed by establishing the line of sight from the decision point and tangent to the nearest obstruction until it intersects the travel lane towards the intersection of the crossing road. This point is termed the sight point (see Fig. 1). The side of this triangle from the conflict point to the sight point is termed the available nonobstructed view distance (DANOV); this distance must be compared with the required non-obstructed view distance (DRNOV). DANOV does not need to be a straight or level line, but it is a distance along the main road from the collision point to the sight point (Fig. 1). There are two methods of determining DANOV: (a) A driver at a stopped vehicle (critical eye height is for lowheight design vehicles) located at the decision point watches a very slow moving low-height vehicle (not van or higher vehicles) driving on the crossing road towards the intersection. When the driver at the decision point first sees the very slow moving vehicles from the crossing road (just clears the obstruction as viewed from the decision point); the sight point and the DANOV is established (see Fig. 1). The very slow moving vehicle should continue traveling to reach the intersection and it must be continuously seen by the stationary driver at the decision point. Indeed police supervision for such field test is needed and it should be conducted at very low traffic periods. In addition, it is possible to perform this field exercise via a sighting rod of design driver’s eye height (1.08 m) and target rod for a height of the design vehicle (1.08 m) also. The proposed values for drivers’ eye height and design vehicle height for ISD are those of the USA (AASHTO, 2004), but other values may be used. The outlined procedure for determining the available ISD is similar to others used by numerous jurisdictions, especially in the USA. This field test procedure may be further simplified by calculating the required non-obstructed view distance (DRNOV)
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first and establishing the required sight point. The decision point is established based on the MRSSD calculation. Thus, if a driver (or person viewing via sighting rod) at the decision point sees a low vehicle (or target rod) at the required sight point; the approach sight distance requirement would be satisfied. (b) A map of the intended intersection with sufficient approach distances and with all obstructions greater than one meter above the elevation of the decision point must be shown on the map. From the decision point (MRSSD from the approach stop-line), a line of sight is drawn tangent to the nearest obstruction; the intersection of the line of sight with the line drawn from the conflict point in the lane approaching the intersection of the crossing road would be the sight point (see Fig. 1). This process may be also performed with 3-D digital maps (Nehate and Rys, 2006). Nonetheless, the field test method is preferable since it represent the actual conditions. Required non-obstructed view distance (DRNOV) is based on providing sufficient non-obstructed distance on the crossing road as seen from the decision point. This non-obstructed view distance should be far enough to enable a driver at the decision point to continue at the ‘set’ speed and to cross and clear the intersection before the crossing vehicle arrives at the conflict point and it would be at least a set clearance distance/time from the conflict point. The set speed could be lower than the posted or midblock running speed depends on the traffic regulations and ISD standards. AASHTO (2004) assumes 50% and 60% of midblock running speed for ISD for approaches of ‘no traffic control’ and Yield traffic control, respectively. If a vehicle is seen at the available sight point from the decision point, yet a driver of a vehicle at the decision point can not proceed to cross the intersection safely (only he/she can stop safely); then also if there are no vehicles between the sight point and the conflict point; it is not safe to proceed since a vehicle hidden by the obstruction may reach the conflict point before the vehicle passing the decision point clears the intersection. The required non-obstructed view distance (DRNOV) may be calculated by first finding the time required for a vehicle traveling at ‘set’ speed at the decision point to cross and clear the intersection (tcc), Eq. (2). Gap headway time (tgh) is set as a safety factor. Thus when the vehicle just clears the intersection, the vehicle from the crossing road would be at least a set gap headway time away from the conflict point. Hence, given the time to cross and clear the intersection (tcc), the gap headway time (tgh), and the speed at the crossing road; the required nonobstructed view distance (DRNOV) is found by Eq. (3): t cc ¼
ðMRSSD þ w þ LÞ Vs
(2)
where tcc is the crossing and clearing time (s), w the intersection width (m), and L the design vehicle length (m), default 6 m (for passenger cars only): DRNOV ¼ ðt cc þ t gh ÞV c
(3)
where DRNOV is the required non-obstructed view distance (m), tgh the gap headway time (s), default 2.0 s, and Vc the velocity or ‘set’ speed for crossing or main road (m/s). The effects of trucks or grades are accounted for in the calculation of the MRSSD. In order for a ‘no traffic control’ intersection to be satisfied by sight distance requirement; the available non-obstructed view distance (DANOV) must be greater than the required nonobstructed view distance (DRNOV), Eq. (4), for all approaches. This will allow sufficient distance to see a vehicle on the crossing road far enough to be able to continue at the ‘set’ speed and to cross and clear the intersection safely; to be able to stop safely at the
intersection if there is conflicting traffic on the crossing road; or to slow down to give way for a vehicle or more on the crossing road to pass then to cross the intersection without a need to stop: DANOV XDRNOV
(4)
where DANOV is the available non-obstructed view distance (m), and DRNOV the required non-obstructed view distance (m). When the approach sight distance requirement, Eq. (4), is satisfied for ‘no control’ or Yield-sign approach intersections, then this condition should be maintained by no parking zones between each of the decision point and the intersection and the required sight point and the intersection. When a ‘no control’ or Yield-sign approach sight distance requirements are not satisfied, specifically Eq. (4) is not satisfied; then a Stop sign is required. 4.2. Sign visibility sight distance Sign visibility sight distance is required for Yield- and Stopcontrolled intersections. Signs should be visible from a sufficient distance to stop safely before entering the intersection. Thus, the available sight distance (ASD) for a Yield or a Stop sign should be greater than the minimum required stopping sight distance, Eq. (5). The ASD for Yield, Stop, or any other sign is the distance before reaching the sign, where the sign is first recognized at the critical visibility conditions. ASD is mainly a function of ambient light conditions (e.g., day time/night time). The critical ASD for traffic signs is usually taken for night time. Adverse weather condition could be more critical than just night time condition for ASD, but it is assumed when the visibility is poor due to weather condition; drivers must reduce their speed. Night time ASD for traffic signs is a function of sign retroreflective material, size, type, and position (Schnell et al., 2004); in addition to vehicle’s headlights positions, angle, and intensity (low beam should be used, since high beam is not allowed at all times). The ASD for a sign may be determined via field testing using ‘design’ vehicle and drivers with minimum acceptable visual acuity. The field test simply consists of a driver driving toward a sign and once it is recognized a button is pushed that start reading of a measuring wheel. At the position of the sign the measured distance should be recorded and it equals to the ASD for such a sign. Several runs, especially with different drivers are recommended. Also, estimating ASD for a particular sign may be performed via a reflectormeter that measures sign retroreflectivity and correlation techniques may be used to estimate the ASD (Awadallah, 1988). Eq. (5) is a basic highway and traffic design requirement, which is based on an AASHTO (2004) requirement in written format in the chapter on elements of design: ASDXMRSSD
(5)
where ASD is the available sight distance (m), and MRSSD the minimum required stopping sight distance (m). 4.3. Stop-line sight distance The stop-line sight distance requirement mainly applies for stop-sign control. Yield sign and ‘no traffic control’ intersections would have this requirement satisfied for passenger cars by satisfying Eq. (4). AASHTO (2004) explicitly states ‘Yield-controlled approaches generally need greater sight distance than stop-controlled approaches.’ However, Eq. (4) may not satisfy stop-line sight distance requirements for some heavy trucks since their acceleration rate is very low. Thus, Eq. (4) would be satisfied for a heavy truck to pass and clear the intersection or to stop safely at the intersection if needed, but once heavy truck stops at a Yield sign, the sight distance provided by Eq. (4) may not be
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Fig. 2. Illustration of stop-line sight distance analysis.
sufficient for a heavy truck to cross and clear the intersection. Signalized intersections stop-line sight distance requirement is not necessary when the signal is functioning. But if the signal is not functioning (e.g., due to power outages or during a night time flashing phase), the minor street approaches would be essentially Stop-sign controlled; then stop-line sight distance requirements would be needed. Stop-line sight distance requirement for minor approaches needs to provide sufficient sight distance on both sides of the crossing road in order for a driver of a stopped vehicle on the stopline to cross and clear the intersection or turn into the main road safely. A problem of insufficient sight distance for stop-lines of minor street approaches is only possible when the crossing road is curved and an obstruction is near the side of the road (see Fig. 2). If intersecting roads are straight, level, and perpendicular to each other; the reason for insufficient stop-line sight distance would be most likely due to the stop-line being far from the intersecting road to allow for a pedestrian crosswalk, or parking on the main street are obstructing the view. Thus in such cases the remedy is clear. A differentiation should be made between sufficient stop-line sight distance and traffic gap acceptance for crossing and turning vehicles. Traffic gap patterns and drivers’ gap acceptance influence minor street delays, which is a criterion for intersection signalization warrants. Basically, the available stop-line sight distance should accommodate at least the minimum gap acceptance for such approach. The procedure for determining the sufficiency of the stop-line sight distance is performed by first determining the location of the sight point, and then the stop-line available non-obstructed view distance (DSANOV) (see Fig. 2). Similarly to determining the available non-obstructed view distance (DANOV) in Section 4.1; the stop-line available non-obstructed view distance (DSANOV) is obtained via a field test or via a map of the intersection with all obstructions greater than 1 m from the elevation of the stop-line. The sight point is determined from the line of sight that is tangent to the obstruction as viewed from the stop-line. Stop-line available non-obstructed view distance (DSANOV) is the distance from the point of sight along the path of the approaching lane to the conflict point within the intersection (see Fig. 2). The stop-line available non-obstructed view distance (DSANOV) should be compared with the stop-line required non-obstructed view distance (DSRNOV). However, there are two types of stop-line required non-obstructed view distances, namely for crossing and for turning movements.
4.3.1. Crossing movement The stop-line required non-obstructed view distance-crossing (DSRNOV-C) is based on providing sufficient non-obstructed
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distance on the crossing road as seen by a driver stopped at the stop-line. This stop-line non-obstructed view distance should be far enough to enable a driver at the stop-line to react, accelerate, cross and clear the intersection before the crossing vehicle arrives at the conflict point and it would be at least a set gap headway time from the conflict point. The time for the stopped vehicle to cross and clear the intersection (ts-cc) is calculated by Eq. (6). This equation consists of two components, start-up lost time and the acceleration time needed to cross and clear the intersection. The start-up lost time does not include the decision time; it is only the reaction time after the decision to ‘go’; similar to the start-up lost time for the first vehicle on a traffic signal’s approach when the signal turns green. The start-up lost time for the first five vehicles for a signalized intersection is 1–2 s. Kunzman (1978) estimated the average cumulative start-up lost time to be 1.1 s. Therefore, the default start-up lost time for only one vehicle (the 1st vehicle) for Eq. (6) should be 0.5–1.0 s. The stop-line required non-obstructed view distance-crossing (DSRNOV-C) is calculated by Eq. (7). If the stop-line available non-obstructed view distance (DSANOV) is greater than the stop-line required non-obstructed view distance-crossing (DSRNOV-C), Eq. (8); then the stop-line sight distance is satisfied for the crossing movement: 2ðw þ LÞ 0:5 þ t sl (6) t s-cc ¼ a where ts-cc is the stop-line crossing and clearing time (s), tsl the start-up lost time (s), default 0.5–1.0 s for passengers cars (for trucks values are higher), a the acceleration rate (m/s2), default 3.0 m/s2 for passenger cars (for trucks values may be as low as 1.0 m/s2 or less), w the intersection width from the stop-line (m), default value for the distance from stop-line to the edge of minor road is 4.4 m and drivers’ eyes are located 2.4 m from the front of a passenger car (AASHTO, 2004): DSRNOV-C ¼ ðt s-cc þ t gh ÞV c
(7)
where DSRNOV-C is the stop-line required non-obstructed view distance-crossing (m): DSANOV XDSRNOV-C
(8)
where DSANOV is the stop-line available non-obstructed view distance (m), and DSRNOV-C the stop-line required non-obstructed view distance-crossing (m). 4.3.2. Turning movement The stop-line required non-obstructed view Distance-turning (DSRNOV-T) is based on providing a distance along the main road, where a vehicle approaching from the main (crossing) road is far enough to allow the stopped vehicle at the stop-line to turn left or right and accelerate to a speed ‘close’ to the speed limit of the ‘turned to’ main road before vehicles traveling on the main road reach within a set time/distance headway gap from the turned vehicle. The time for the stopped vehicle to turn and reach a ‘set’ speed (for AASHTO, 2004, it is set at 70% of the main road speed limit) consist of start-up lost time and acceleration time (ts-t), which is calculated by Eq. (9). The start-up lost time for left-turns should be more than for right-turns; since the former needs the operator to check both sides of the road even after the ‘go’ decision. The stop-line required non-obstructed view distanceturning (DSRNOV-T) is calculated by Eq. (10). If the stop-line available non-obstructed view distance (DSANOV) is greater than the stop-line required non-obstructed view distance-turning (DSRNOV-T), Eq. (11); then the stop-line sight distance is satisfied for the turning movement: pV c þ t sl (9) t s-t ¼ a
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where ts-t is the stop-line turning and acceleration time (s), p the percentage of main road speed in decimal form (that main road vehicles are expected to reduce the speed to accommodate turning vehicles), suggested default 70% (0.7), tsl the start-up lost time (s), default 0.5 s for right-turn and 1.0 s for left-turn, for passenger cars: DSRNOV-T ¼ ðt s-t þ t gh ÞV c
(10)
where DSRNOV-T is the stop-line required non-obstructed view distance-turning (m): (11)
DSANOV XDSRNOV-T
where DSRNOV-T is the stop-line required non-obstructed view distance-turning (m), and DSANOV the stop-line available nonobstructed view distance (m). The stop-line sight distance would be satisfied for a four leg intersection by satisfying both of Eqs. (8) and (11) for both sides’ approaches, or by comparing the higher of DSRNOV-C or DSRNOV-T to DSANOV for both sides of the crossing road. For a stop-line at a T-intersection; only turning movement requirement would be necessary. In order to satisfy Eqs. (8) or (11), simply, obstructions should be removed to increase the stop-line available non-obstructed view distance (DSANOV) to become greater than the required nonobstructed view distance-crossing or/and turning. If this is not possible, reduction of main road speed limit, as a speed zone approaching the intersection, is an option in order to reduce the DSRNOV-C or/and DSRNOV-T to become lower than the DSANOV. Finally, if Eqs. (8) or (11) could not be satisfied by any mean possible (or it particularly requires a reduction of the speed of the main road drastically, which would not be reasonable); then this would be a typical hidden entrance/exist and it should not be used specifically for vehicles existing to cross or turn into the main
road. Vehicles turning right or left into such road or entrance would involve vehicles slowing or stopping (especially left-turns without a left-turn bay) on the main road. However, this case should have satisfactory sight distance based on the appropriate geometric design of roads, where the available sight distance should be greater than the minimum required sight distance throughout designed roads.
4.4. Summary and comparisons with AASHTO (2004) The ISD guidelines and equations in this research were developed from the basic laws of motion. Table 2 provides a summary of the equations used for the various ISD and intersection traffic controls. Fig. (3) shows that values for ISD for ‘no control’ intersection approaches from this research are closely correlated to AASHTO (2004) when using the same assumptions, especially vehicles approaching ‘no traffic control’ intersections slow to 50% of midblock running speeds. Also, using the AASHTO (2004) assumption of vehicles slowing down to 60% of their midblock speeds when approaching a Yield-sign approach resulted in similar high correlations as can be seen in Fig. 4. Finally, Fig. 5 provides comparisons of ISD for Stop-sign approaches for right-turn, left-turn, and crossing maneuvers. While the crossing maneuvers results of this research are very close to that of AASHTO (2004); both turning maneuvers for this research provide somewhat lower ISD for low design speeds and somewhat higher ISD for higher design speeds. This is mainly due to use of time gap instead of distance gap for safety clearance. For speeds less than 50 km/h (30 mph) for the main road, the equations developed from the laws of motion provide safe results for the assumptions of 2 s gap and reduction of main road vehicles’ speeds to 70%. But the distance gap may be short
Table 2 Intersections sight distances requirements. Type of sight distance
Type of applicable traffic control
Requirements in equation form
Sign visibility
Yield-sign and Stop-sign
ASD4MRSSD MRSSD ¼ Vstr ¼ V2s /2a72gG where ASD is the available sight distance (m), MRSSD the minimum required stopping sight distance (m), Vs the velocity or ‘set’ speed for side or traffic controlled road (m/s), tr the driver’s perception–reaction time (s), default 2.5 s, g the gravitational constant (9.81 m/s2), a the deceleration rate (m/s2), default 3.4 m/s2, and G the grade in decimal form (e.g., 1% ¼ 0.01)
Approach sight distance
Yield-sign and ‘no traffic control’
DANOVXDRNOV DRNOV ¼ (tcc+tgh)Vc tcc ¼ (MRSSD+w+L)/Vs where DANOV is the available non-obstructed view distance (m), DRNOV the required non-obstructed view distance (m), tcc the crossing and clearing time (s), tgh the gap headway time (s), Vc the velocity or ‘set’ speed for crossing or main road (m/ s), w the intersection width (m) and L the design vehicle length (m), default 6 m (for passenger cars only)
Stop-line sight distance
Stop-sign and some traffic signals’ minor approaches (e.g., flashing phase)
(a) Crossing movement DSANOVXDSRNOV-C DSRNOV-C ¼ (ts-cc+tgh)Vc ts-cc ¼ [(2(w+L)/a]0.5+tsl where DSANOV is the stop-line available non-obstructed view distance (m), DSRNOV-C the stop-line required non-obstructed view distance-crossing (m), ts-cc the stop-line crossing and clearing time (s), tsl the start-up lost time (s), default 0.5–1.0 s for passengers cars (for trucks values are higher), a the acceleration rate (m/s2), default 3.0 m/s2 for passenger cars (for trucks values may be as low as 1.0 m/s2 or less), w the intersection width from the stop-line (m), default values for distance from stop-line to edge of minor road is 4.4 m and drivers’ eye are 2.4 m from front of passenger cars (AASHTO 2004), tgh the gap headway time (s), tsl the start-up lost time (s), default 1.0 s (b) Turning movement DSANOVXDSRNOV-T DSRNOVT ¼ (tst+tgh)Vc ts-t ¼ (pVc+a)+tsl where DSRNOV-T is the stop-line required non-obstructed view distance-turning (m), DSANOV the stop-line available nonobstructed view distance (m), ts-t the stop-line turning and acceleration time (s), p the percentage of main road speed in decimal form (that main road vehicles are expected to reduce the speed to accommodate turning vehicles), suggested default 70% (0.7), tsl the start-up lost time (s), default 0.5 s for right-turn and 1.0 s for left-turn, for passenger cars
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100 Crossing road speed (km/h)
Approach speed (km/h)
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80
60
40
20
80
60 AASHTO 2004 Left Dsrnov-left
40
AASHTO 2004 right & crossing
20
AASHTO 2004 Drnov (50% approach speed)
Dsrnov-right Dsrnov-crossing
0 0.0
0 0
50
100
149
150
Crossing road ISD (m)
50.0
150.0 200.0 100.0 Crossing road ISD (m)
250.0
300.0
Fig. 5. Comparisons of ISD for Stop-sign control, right-turn, left-turn, and crossing maneuvers.
Fig. 3. Comparisons of ISD for ‘no traffic control’ intersections.
for main road vehicles to accommodate turning vehicles from the minor street, and the reduction of approach speed to a Yield-sign or ‘no traffic control’ approaches.
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Crossing road speed (km/h)
100 5. Conclusion
80
60
40 AASHTO 2004 20
Drnov (60% approach speed)
0 0
50
150 100 Crossing road ISD (m)
200
250
Fig. 4. Comparisons of ISD for Yield-sign approaches of a minor road speed of 50 km/h.
especially due to psychological and human expectation reasons; thus a minimum ISD for stop-sign approaches may be augmented (e.g., 45 m). The difference between the research results for higher design values for turning movements is not substantial up to 100 km/h; but at higher speeds, it is not practical to have a stopcontrolled approach on a highway with a design speed of expressways. Variations due to changes in grades or design vehicles are impeded within various equations for ISD in this research. The percent grade is incorporated in the MRSSD equation; while the design vehicle may be addressed via vehicle length, and acceleration and deceleration rates in the various ISD equations. Other variations that depend on the drivers’ characteristics and attitudes may be addressed, such as start-up lost time, the reduction speed
The importance of intersection sight distance has been a common knowledge among traffic engineers and educators, but the requirements and procedures for satisfying intersection sight distances are neither uniform nor well known in practice. The main benefit of this research that it provides ISD based on the laws of motions and the equations are flexible to changes, especially assumptions; thus traffic engineers obtain results from clear scientific equations, which can be easily defended. The proposed ISD guidelines explicitly cover the various variables of significance and provide the procedures and the underlying assumptions. Intersecting roads need not be straight, level or perpendicular to each other to easily follow the proposed intersection sight distance requirements. Thus comparisons, changes for various conditions, and sensitivity analysis are possible. The default values are obtained from various research, studies, and jurisdictions’ guidelines and practices. However, they can be easily changed and their sensitivity can be easily measured. Further studies to obtain more relevant default values for specific conditions and regions are recommended. References American Association of State Highway and Transportation Officials (AASHTO), 1994. American Association of State Highway and Transportation Officials (AASHTO), 2004. Awadallah, F., 1988. Prediction of the service life of warning signs. Public Roads 51 (4). Gattis, J., Low, S., 1999. Gap acceptance at atypical stop-controlled intersections. Journal of Transportation Engineering 125 (3), 201–207. Harwood, D., Mason, J., Brydia, R., Joubert, H., Lamm, R., Psarianos, B., 1995. International sight distance design practices. In: Proceedings of the International Symposium on Highway Geometric Design Practices, Boston, MA, USA.
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Harwood, W., Mason, J., Brydia, R., 1999. Design policies for sight distance at stopcontrolled intersections based on gap acceptance. Transportation Research Part A: Policy and Practice 33 (3/4), 199–216. Highway Capacity Manual (HCM), Special Report 209, 2000. Transportation Research Board, National Research Council, Washington, DC, USA. Kunzman, W., 1978. Another look at signalized intersection capacity. Institute of Transportation Engineers (ITE) Journal 48 (8). Mahmassani, H., Sheffi, Y., 1981. Using gap sequences to estimate gap acceptance functions. Transportation Research 15B, 143–148.
Manual of Uniform Traffic Control Devices (MUTCD) for Streets and Highways, 2003. Federal Highway Administration, US Department of Transportation, Washington, DC, USA Nehate, G., Rys, M., 2006. 3D calculations of stopping-sight distance from GPS data. Journal of Transportation Engineering 132 (9), 691–698. Neudorff, L., 1985. Gap acceptance criteria for signal warrants. Institute of Transportation Engineers (ITE) Journal 55 (2). Schnell, T., Aktan, F., Li, C., 2004. Traffic sign luminance requirements of nighttime drivers of symbolic signs, Transportation Research Record 1862.