- Email: [email protected]
Restricting intersection visibility to reduce approach speeds
Accident Analysis and Prevention 35 (2003) 817–823
Short communication
Restricting intersection visibility to reduce approach speeds Samuel G. Charlton a,b,∗ b
a Department of Psychology, Waikato University, Private Bag 3105, Hamilton, New Zealand Transport Engineering Research New Zealand Limited, PO Box 97846, South Auckland Mail Centre, Auckland, New Zealand
Received 20 February 2002; received in revised form 20 April 2002; accepted 26 April 2002
Abstract This paper reports the field test of a visual restriction treatment for a rural intersection with a high rate of injury crashes. A human factors analysis of the asymmetric pattern of crashes at the site suggested that most of the crashes were the result of anticipatory decision-making occasioned by visual characteristics of the eastbound approach to the intersection. The field test examined the effectiveness of a visual restriction treatment directed at eliminating drivers’ anticipatory decision-making. The treatment consisted of a hessian screen erected along the eastbound approach to the intersection beginning 125 m prior to intersection and ending 25 m prior to intersection. Over 2 days of testing, approximately 300 drivers’ reactions at the intersection were observed and their responses to a brief survey recorded. The test indicated a 23% reduction in the 80th percentile and mean approach speeds and elimination of all approach speeds over 57 km/h following introduction of the treatment. Survey results showed that the treatment was visually acceptable to the majority of drivers using the intersection and did not affect its perceived safety. Follow-on analyses compared speed data before the treatment, and 2, 21, and 37 weeks after installation of the treatment. These analyses showed that approach speeds remained low; 30% lower than pre-treatment speeds for both the 80th percentile and the average approach speeds. Of perhaps the greatest significance, no crash resulting in serious injury or death has occurred at the intersection since installation of the treatment to the present time. © 2003 Elsevier Science Ltd. All rights reserved. Keywords: Visual restriction; Driver decisions; Intersection crashes; Speed reduction; Field test
1. Introduction A rural intersection located along State Highway 27 on the north island of New Zealand (known as the Tahuna Paeroa intersection) is a staggered “T” intersection; the eastbound and westbound approaches leading to the intersection are regulated with STOP signs and possess clear sight lines for the northbound and southbound lanes of the highway. Of particular interest were the rate and asymmetric pattern of crashes at the site. Twenty-four crashes occurred at the site in 5 years (1995 through 1999) of which 23 were crossing or turning type crashes. Analysis of traffic crash reports showed that 91% of those 23 crashes occurred when drivers crossing the intersection from the eastbound approach collided with vehicles on the highway. All of the crashes occurred during daylight hours, 87% of the crashes occurring between the hours of 7:00 a.m. and 5:00 p.m. and 83% of the drivers involved were from the immediate area or towns in the surrounding district. A human factors test programme was commissioned by the State ∗ Tel.: +64-7-846-2889; fax: +64-7-856-2158. E-mail address: [email protected] (S.G. Charlton).
Highway 27 Crash Reduction Team on behalf of Transit New Zealand to identify design and operational problems with the intersection leading to its high crash rate. The human factors analysis employed a SITE analysis of traffic crash reports and observations of traffic at the intersection. Broadly speaking, the SITE methodology places human factors testing within a context of situation, individual, task, and effectiveness issues (Charlton, 1996, 2002). The goal of the SITE analysis method is to ensure a comprehensive human factors test design by providing a systematic structure for selecting and employing test and evaluation measures and, most importantly, an interpretive context for understanding human factors data and explaining their significance to others. In the case of the Tahuna Paeroa intersection, the SITE analysis suggested that a key driver error mode was due to anticipatory decision-making. The extent to which the preponderance of crashes was associated with the eastbound approach only (i.e. the asymmetric pattern of crashes) provided a key datum for this conclusion. The primary difference between the eastbound and westbound approaches was a very broad clear sight angle associated with only one of the approaches (the eastbound) allowing drivers an unrestricted view of cross-traffic on the highway well in
0001-4575/03/$ – see front matter © 2003 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 1 - 4 5 7 5 ( 0 2 ) 0 0 0 5 2 - 0
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S.G. Charlton / Accident Analysis and Prevention 35 (2003) 817–823
advance of the intersection. This factor, coupled with observations that the stop/go times of many drivers at the intersection were extremely short (averaging 1.5 s, too short to include adequate scanning, decision-making, and response execution) supported the notion that drivers were scanning the cross traffic and deciding on a go/no-go response as much as 100 m before the intersection. Thus, some portion of the crashes appeared to be the result of driver decision-making becoming anticipatory. This analysis of driver behaviour at the intersection suggested that it might be possible to attenuate drivers’ anticipatory decisions and increase the dwell time of vehicles crossing the intersection by reducing clear sight angles of the highway prior to intersection. The outcome of this analysis, the recommendation to improve safety by reducing clear sight distances at the intersection, was contrary to the inclinations and conventions of the highway engineering community. Road design standards have traditionally recommended increasing clear sight distances at intersections to improve safety. Two key studies support this approach; David and Norman (1975) and Hanna et al. (1976) reported that increasing the sight radius at intersections is typically associated with a decrease in crash rates. The findings from these studies were subsequently included in many textbooks, design manuals, and have been used as the basis for roadway modelling efforts (Vogt and Bared, 1998). The decision to field test a safety intervention that actually reduced clear sight distances was thus undertaken with considerable discussion and caution. Nonetheless, the highway engineers and roading authorities agreed to a field test of the treatment, and depending upon its ability to meet certain effectiveness requirements (described in Section 2), allow it to remain as a temporary intervention until a permanent change to the intersection (e.g. a rural roundabout) could be implemented.
2. Method The field test was designed as a before/after comparison of driver behaviour at the intersection. The test focused on drivers crossing the highway from the eastbound approach before and after the introduction of a visual restriction treatment directed at eliminating the anticipatory decision-making. The test was conducted at the intersection site under daytime conditions with approximately 3 h of data collected in the “before” or baseline configuration and approximately 3 h of data collected in the “after” or post-treatment configuration. Prior to the execution of the test, the investigators and the roading authority agreed on several key measures of effectiveness and acceptability with which to judge the success of the intervention (and thus whether to keep it standing following the test). The first measure of effectiveness was the approach speed of eastbound vehicles measured 25 m prior to the intersection. For this measure it was agreed that the treatment would be judged effective if it could meet a requirement of a 10%
reduction in the 80th percentile speed. A second measure of effectiveness was drivers’ traffic detection rates; the percent of drivers correctly reporting the presence and location of a target vehicle (or other vehicle) present in an unmarked area ranging from 250 m to the north and 250 m to the south of the intersection. It was agreed that the treatment would be judged effective if it could meet a requirement of a 10% increase in detection rate. Drivers’ subjective reactions to the treatment were also of interest and were assessed with two measures: driver ratings of the acceptability of the approach treatment (measured with a 5-point equal interval bipolar rating scale ranging from “completely acceptable” to “completely unacceptable”) and driver ratings of the overall safety of the intersection (measured on a 5-point equal interval bipolar rating scale ranging from “very safe” to “very unsafe”). The baseline data were collected on 19 February 2001. Visibility at the site was very good and 161 drivers were stopped between the hours of 8:30 and 11:30. During data collection, a target vehicle (a black 1964 EH Holden Special Station Sedan) was parked on state highway roadside 150 m to the north of the intersection for the first 1.5 h data collection block and 150 m to the south of the intersection during the second 1.5 h block. Two observers were positioned in an inconspicuous location behind a berm on the northeast corner of the intersection to record the speed of approaching eastbound vehicles (with a radar speed gun), their registration numbers as they arrived at the intersection, intersection stopping times, and any other traffic present on the state highway at the time. Eastbound traffic crossing the intersection was then stopped and surveyed at a site approximately 500 m east of the intersection (and not visible from the intersection due to a rise in the road elevation). Each driver passing the survey site was stopped with the assistance of a police officer and was informed that a brief roadside safety survey was being conducted. Drivers were told that their participation was voluntary and then were asked the survey questions by a survey administrator who recorded the drivers’ answers along with the first three letters/digits of the registration tag for later collation with the intersection data (vehicle registration information was subsequently removed and replaced by participant numbers to preserve the anonymity of drivers involved). After answering the survey questions the drivers were thanked for their time and presented with a complimentary candy bar. Immediately following completion of the baseline data collection, the visual restriction treatment was installed. The treatment consisted of a hessian screen erected on the northern side of the eastern approach to the intersection beginning 125 m prior to intersection and ending 25 m prior to intersection. The screen consisted of 30% knitted shade cloth1 100 m in length extending to a height of 2.1 m above the road edge (shown in Fig. 1). The post-treatment data collection was conducted 3 weeks later, on 12 March 2001. 1
The 30% value refers to the shade factor or opacity of the screen.
S.G. Charlton / Accident Analysis and Prevention 35 (2003) 817–823
819
Fig. 1. Screen treatment at eastbound approach to intersection.
Fig. 2. Approach speeds before and after installation of the treatment.
During this phase, 141 drivers were stopped between the hours of 9:30 and 12:30. The target vehicle placements, intersection observers, and intercept survey were the same as during baseline data collection. The time of day and weather conditions during the post-treatment phase was matched to the baseline phase as closely as practicable.
3. Results Fig. 2 shows the approach speeds of eastbound vehicles measured 25 m prior to the intersection before and after installation of the visual restriction treatment. The data indicated a 23.4% reduction in the 80th percentile speed2 , easily meeting the requirement of a 10% reduction. The 2
This statistic includes non-platooned vehicles only.
data also indicated a 10.95 km/h reduction in 85% percentile speeds (from 49.95 to 39 km/h), a 23.7% reduction in mean speed (from 37.98 to 29.22 km/h), and elimination of all approach speeds over 57 km/h. A one-way analysis of variance indicated that the decrease in approach speeds was statistically reliable; F(1,178) = 28.71, p < 0.01. Fig. 3 shows the results for traffic detection rates. The percent of drivers correctly reporting the presence and location of the target vehicle increased from 16.8% (27 out of 161 drivers) prior to the treatment to 31.9% (45 out of 141 drivers). Once again, the measure exceeded the requirement of a 10% increase in detection rate and the change was statistically reliable; F(1,233) = 6.85, p < 0.01. Drivers’ ratings of the acceptability of the treatment are shown in Fig. 4. As can be seen in the figure, when asked if they “noticed anything new or different about the approach to the intersection,” only 56% of the drivers reported having
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S.G. Charlton / Accident Analysis and Prevention 35 (2003) 817–823
Fig. 3. Traffic detection rates before and after installation of the treatment.
Fig. 4. Percent of drivers reporting having noticed the treatment and their ratings of its acceptability before and after installation of the treatment.
noticed the screen. Of those drivers that noticed the screen, 48% rated the screen as “completely acceptable” (“5” on the 5-point acceptability scale). Only one driver rated the screen as “completely unacceptable”. Fig. 5 shows the ratings of the intersection safety. The median rating of the overall safety of the intersection was “3” for all drivers. There was no difference in the distribution of the ratings prior to and after the treatment.
4. Follow-on testing Follow-on testing was based on speed count data collected collaterally with, and subsequent to, the field-testing. Speed
counts were taken with tube counters placed 20 m prior to the intersection on the eastbound approach. Speed count data were collected in four 7-day blocks prior to installation of the treatment (during the week of 3 February 2001), 2 weeks after installation (week of 31 March), 21 weeks after installation (week of 10 August), and 37 weeks after installation (week of 29 November). The purpose of the speed counts was to provide a means of judging the long-term efficacy of the treatment by collecting approach speed data with a common methodology. Fig. 6 compares the speed count data collected on equivalent days and times to the field test data (Monday mornings between 9:00 and 12:00), and shows that the mean, median, 80th and 85th percentile speeds decreased immediately
S.G. Charlton / Accident Analysis and Prevention 35 (2003) 817–823
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Fig. 5. Ratings of intersection safety before and after installation of the treatment.
following installation of the treatment and remained suppressed over the 37 weeks since installation: 85th percentile speeds prior to the treatment were 56.07, 45.67 km/h after the treatment, 39.67 km/h 21 weeks later, and 38.36 km/h 37 weeks later (a 30% decrease). The average approach speeds of eastbound traffic showed a nearly identical 30% drop, from 40.33 km/h prior to the treatment to 28 km/h 37 weeks later (S.D. of 13 and 10.25 km/h respectively). Finally, the median approach speeds showed the same trend, albeit to a lesser magnitude (an 18.9% reduction from 38.11 to
30.89 km/h). Shown in the right-hand panel of Fig. 6 is the distribution of approach speeds during Monday mornings. Immediately apparent in is the almost complete elimination of approach speeds in the 50–60 km/h range in the 37 weeks following installation of the treatment. The figure also shows a progressive reduction of approach speeds in the 40–50 km/h range. To further judge the efficacy of the treatment in reducing approach speeds, Fig. 7 compares approach speeds across all times of day and all days of the week. As can be seen in
Fig. 6. Approach speeds on Monday mornings before and 2, 21, and 37 weeks following installation of the treatment.
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Fig. 7. Approach speeds throughout the week before and 2, 21, and 37 weeks following installation of the treatment.
the figure, a nearly identical pattern of results to that found for Monday mornings was obtained at all times of the day and days of the week.
5. Discussion Based on the results of the field test, the treatment was agreed to be effective and was allowed to remain at the intersection site until a permanent intervention, currently envisioned as a rural roundabout, can be implemented. Looking at the follow-on results it can be seen that the approach speeds at week 21 were even lower than those obtained 2 weeks after the treatment installation. Why approach speeds would decline beyond that observed immediately after implementation of the approach treatment was not readily apparent. One possible reason was a substantially higher traffic flow in early August (14,750 eastbound vehicles per week as compared to counts of 9000 vehicles pre-treatment and 7500 immediately post-treatment), possibly resulting in shorter headway distances (platooning of vehicles) and thus, the probability of lower approach speeds measured by the tube counters. To investigate this possibility further, an analysis of the approach speeds at off-peak hours when traffic flows were low (Saturday and Sunday mornings between 7:00 and 10:00, averaging 98 and 52 vehicles per hour, respectively) was conducted. This analysis found the same pattern of decline; approach speeds at week 21 appreciably lower than those at week 2. Further, traffic flows returned to pre-treatment levels by week 37 without any corresponding increase in approach speeds. Thus, the reduced approach speeds appeared to be due to the screen treatment rather than any direct effect of traffic density. An alternative interpretation of the decrease in approach speeds between weeks 2 and 21 may be based on changes to the opacity of the screen.
Visual inspection of the screen in the months following installation revealed that the screen was not at taut as when originally installed and that the shade cloth itself appeared to have frayed slightly as a result of wind and weather. These changes in the screen took place in the first 6–12 weeks following installation, the resulting increase in opacity corresponding with the continued decrease in approach speeds. No further fraying or changes in screen opacity were noted thereafter, and approach speeds apparently stabilised at that point. It is clear that installation of the treatment was associated with a significant decrease in approach speeds, the near elimination of approach speeds in the 50–60 km/h range, and increased traffic detection rates. The survey results showed that the treatment was visually acceptable to the large majority of drivers using the intersection and did not affect the perceived safety of the intersection. Of perhaps the greatest significance, no crash resulting in serious injury or death has occurred at the intersection since installation of the treatment to the present time. Finally, the results support the philosophy that drivers can and do adjust their driving behaviour to suit road and traffic conditions and that road designers can manipulate road user behaviour to benefit safety.
Acknowledgements Thanks are due to Brett Alley and Rebecca Luther of Transport Engineering Research New Zealand Ltd., John Herbert and Cameron Inder of the State Highway 27 Crash Reduction Team, and John Grummitt of Transit New Zealand for their support and assistance. Thanks are also due to the anonymous reviewers of this manuscript for their thoughtful and helpful comments and suggestions.
S.G. Charlton / Accident Analysis and Prevention 35 (2003) 817–823
References Charlton, S.G., 1996. SITE: an integrated approach to human factors testing. In: O’Brien, T.G., Charlton, S.G. (Eds.), Handbook of Human Factors Testing and Evaluation. Lawrence Erlbaum Associates, Hillsdale, NJ, pp. 27–40. Charlton, S.G., 2002. Selecting measures for human factors tests. In: Charlton, S.G., O’Brien, T.G. (Eds.), Handbook of Human Factors Testing and Evaluation, 2nd Edition. Lawrence Erlbaum Associates, Hillsdale, NJ, pp. 37–54.
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David, N., Norman, J.R., 1975. Motor Vehicle Accidents in Relation to Geometric and Traffic Features of Highway Intersections. Vol. II. FHWA-RD-76-129. Federal Highway Administration, Washington, DC. Hanna, J.T., Flynn, T.E., Webb, L.T., 1976. Characteristics of Intersection Accidents in Rural Municipalities. Transportation Research Record 601. Transportation Research Board, Washington, DC. Vogt, A., Bared, J.G., 1998. Accident Models for Two-Lane Rural Roads: Segments and Intersections. FHWA-RD-98-133. Federal Highway Administration, Washington, DC.
Short communication
Restricting intersection visibility to reduce approach speeds Samuel G. Charlton a,b,∗ b
a Department of Psychology, Waikato University, Private Bag 3105, Hamilton, New Zealand Transport Engineering Research New Zealand Limited, PO Box 97846, South Auckland Mail Centre, Auckland, New Zealand
Received 20 February 2002; received in revised form 20 April 2002; accepted 26 April 2002
Abstract This paper reports the field test of a visual restriction treatment for a rural intersection with a high rate of injury crashes. A human factors analysis of the asymmetric pattern of crashes at the site suggested that most of the crashes were the result of anticipatory decision-making occasioned by visual characteristics of the eastbound approach to the intersection. The field test examined the effectiveness of a visual restriction treatment directed at eliminating drivers’ anticipatory decision-making. The treatment consisted of a hessian screen erected along the eastbound approach to the intersection beginning 125 m prior to intersection and ending 25 m prior to intersection. Over 2 days of testing, approximately 300 drivers’ reactions at the intersection were observed and their responses to a brief survey recorded. The test indicated a 23% reduction in the 80th percentile and mean approach speeds and elimination of all approach speeds over 57 km/h following introduction of the treatment. Survey results showed that the treatment was visually acceptable to the majority of drivers using the intersection and did not affect its perceived safety. Follow-on analyses compared speed data before the treatment, and 2, 21, and 37 weeks after installation of the treatment. These analyses showed that approach speeds remained low; 30% lower than pre-treatment speeds for both the 80th percentile and the average approach speeds. Of perhaps the greatest significance, no crash resulting in serious injury or death has occurred at the intersection since installation of the treatment to the present time. © 2003 Elsevier Science Ltd. All rights reserved. Keywords: Visual restriction; Driver decisions; Intersection crashes; Speed reduction; Field test
1. Introduction A rural intersection located along State Highway 27 on the north island of New Zealand (known as the Tahuna Paeroa intersection) is a staggered “T” intersection; the eastbound and westbound approaches leading to the intersection are regulated with STOP signs and possess clear sight lines for the northbound and southbound lanes of the highway. Of particular interest were the rate and asymmetric pattern of crashes at the site. Twenty-four crashes occurred at the site in 5 years (1995 through 1999) of which 23 were crossing or turning type crashes. Analysis of traffic crash reports showed that 91% of those 23 crashes occurred when drivers crossing the intersection from the eastbound approach collided with vehicles on the highway. All of the crashes occurred during daylight hours, 87% of the crashes occurring between the hours of 7:00 a.m. and 5:00 p.m. and 83% of the drivers involved were from the immediate area or towns in the surrounding district. A human factors test programme was commissioned by the State ∗ Tel.: +64-7-846-2889; fax: +64-7-856-2158. E-mail address: [email protected] (S.G. Charlton).
Highway 27 Crash Reduction Team on behalf of Transit New Zealand to identify design and operational problems with the intersection leading to its high crash rate. The human factors analysis employed a SITE analysis of traffic crash reports and observations of traffic at the intersection. Broadly speaking, the SITE methodology places human factors testing within a context of situation, individual, task, and effectiveness issues (Charlton, 1996, 2002). The goal of the SITE analysis method is to ensure a comprehensive human factors test design by providing a systematic structure for selecting and employing test and evaluation measures and, most importantly, an interpretive context for understanding human factors data and explaining their significance to others. In the case of the Tahuna Paeroa intersection, the SITE analysis suggested that a key driver error mode was due to anticipatory decision-making. The extent to which the preponderance of crashes was associated with the eastbound approach only (i.e. the asymmetric pattern of crashes) provided a key datum for this conclusion. The primary difference between the eastbound and westbound approaches was a very broad clear sight angle associated with only one of the approaches (the eastbound) allowing drivers an unrestricted view of cross-traffic on the highway well in
0001-4575/03/$ – see front matter © 2003 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 1 - 4 5 7 5 ( 0 2 ) 0 0 0 5 2 - 0
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S.G. Charlton / Accident Analysis and Prevention 35 (2003) 817–823
advance of the intersection. This factor, coupled with observations that the stop/go times of many drivers at the intersection were extremely short (averaging 1.5 s, too short to include adequate scanning, decision-making, and response execution) supported the notion that drivers were scanning the cross traffic and deciding on a go/no-go response as much as 100 m before the intersection. Thus, some portion of the crashes appeared to be the result of driver decision-making becoming anticipatory. This analysis of driver behaviour at the intersection suggested that it might be possible to attenuate drivers’ anticipatory decisions and increase the dwell time of vehicles crossing the intersection by reducing clear sight angles of the highway prior to intersection. The outcome of this analysis, the recommendation to improve safety by reducing clear sight distances at the intersection, was contrary to the inclinations and conventions of the highway engineering community. Road design standards have traditionally recommended increasing clear sight distances at intersections to improve safety. Two key studies support this approach; David and Norman (1975) and Hanna et al. (1976) reported that increasing the sight radius at intersections is typically associated with a decrease in crash rates. The findings from these studies were subsequently included in many textbooks, design manuals, and have been used as the basis for roadway modelling efforts (Vogt and Bared, 1998). The decision to field test a safety intervention that actually reduced clear sight distances was thus undertaken with considerable discussion and caution. Nonetheless, the highway engineers and roading authorities agreed to a field test of the treatment, and depending upon its ability to meet certain effectiveness requirements (described in Section 2), allow it to remain as a temporary intervention until a permanent change to the intersection (e.g. a rural roundabout) could be implemented.
2. Method The field test was designed as a before/after comparison of driver behaviour at the intersection. The test focused on drivers crossing the highway from the eastbound approach before and after the introduction of a visual restriction treatment directed at eliminating the anticipatory decision-making. The test was conducted at the intersection site under daytime conditions with approximately 3 h of data collected in the “before” or baseline configuration and approximately 3 h of data collected in the “after” or post-treatment configuration. Prior to the execution of the test, the investigators and the roading authority agreed on several key measures of effectiveness and acceptability with which to judge the success of the intervention (and thus whether to keep it standing following the test). The first measure of effectiveness was the approach speed of eastbound vehicles measured 25 m prior to the intersection. For this measure it was agreed that the treatment would be judged effective if it could meet a requirement of a 10%
reduction in the 80th percentile speed. A second measure of effectiveness was drivers’ traffic detection rates; the percent of drivers correctly reporting the presence and location of a target vehicle (or other vehicle) present in an unmarked area ranging from 250 m to the north and 250 m to the south of the intersection. It was agreed that the treatment would be judged effective if it could meet a requirement of a 10% increase in detection rate. Drivers’ subjective reactions to the treatment were also of interest and were assessed with two measures: driver ratings of the acceptability of the approach treatment (measured with a 5-point equal interval bipolar rating scale ranging from “completely acceptable” to “completely unacceptable”) and driver ratings of the overall safety of the intersection (measured on a 5-point equal interval bipolar rating scale ranging from “very safe” to “very unsafe”). The baseline data were collected on 19 February 2001. Visibility at the site was very good and 161 drivers were stopped between the hours of 8:30 and 11:30. During data collection, a target vehicle (a black 1964 EH Holden Special Station Sedan) was parked on state highway roadside 150 m to the north of the intersection for the first 1.5 h data collection block and 150 m to the south of the intersection during the second 1.5 h block. Two observers were positioned in an inconspicuous location behind a berm on the northeast corner of the intersection to record the speed of approaching eastbound vehicles (with a radar speed gun), their registration numbers as they arrived at the intersection, intersection stopping times, and any other traffic present on the state highway at the time. Eastbound traffic crossing the intersection was then stopped and surveyed at a site approximately 500 m east of the intersection (and not visible from the intersection due to a rise in the road elevation). Each driver passing the survey site was stopped with the assistance of a police officer and was informed that a brief roadside safety survey was being conducted. Drivers were told that their participation was voluntary and then were asked the survey questions by a survey administrator who recorded the drivers’ answers along with the first three letters/digits of the registration tag for later collation with the intersection data (vehicle registration information was subsequently removed and replaced by participant numbers to preserve the anonymity of drivers involved). After answering the survey questions the drivers were thanked for their time and presented with a complimentary candy bar. Immediately following completion of the baseline data collection, the visual restriction treatment was installed. The treatment consisted of a hessian screen erected on the northern side of the eastern approach to the intersection beginning 125 m prior to intersection and ending 25 m prior to intersection. The screen consisted of 30% knitted shade cloth1 100 m in length extending to a height of 2.1 m above the road edge (shown in Fig. 1). The post-treatment data collection was conducted 3 weeks later, on 12 March 2001. 1
The 30% value refers to the shade factor or opacity of the screen.
S.G. Charlton / Accident Analysis and Prevention 35 (2003) 817–823
819
Fig. 1. Screen treatment at eastbound approach to intersection.
Fig. 2. Approach speeds before and after installation of the treatment.
During this phase, 141 drivers were stopped between the hours of 9:30 and 12:30. The target vehicle placements, intersection observers, and intercept survey were the same as during baseline data collection. The time of day and weather conditions during the post-treatment phase was matched to the baseline phase as closely as practicable.
3. Results Fig. 2 shows the approach speeds of eastbound vehicles measured 25 m prior to the intersection before and after installation of the visual restriction treatment. The data indicated a 23.4% reduction in the 80th percentile speed2 , easily meeting the requirement of a 10% reduction. The 2
This statistic includes non-platooned vehicles only.
data also indicated a 10.95 km/h reduction in 85% percentile speeds (from 49.95 to 39 km/h), a 23.7% reduction in mean speed (from 37.98 to 29.22 km/h), and elimination of all approach speeds over 57 km/h. A one-way analysis of variance indicated that the decrease in approach speeds was statistically reliable; F(1,178) = 28.71, p < 0.01. Fig. 3 shows the results for traffic detection rates. The percent of drivers correctly reporting the presence and location of the target vehicle increased from 16.8% (27 out of 161 drivers) prior to the treatment to 31.9% (45 out of 141 drivers). Once again, the measure exceeded the requirement of a 10% increase in detection rate and the change was statistically reliable; F(1,233) = 6.85, p < 0.01. Drivers’ ratings of the acceptability of the treatment are shown in Fig. 4. As can be seen in the figure, when asked if they “noticed anything new or different about the approach to the intersection,” only 56% of the drivers reported having
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S.G. Charlton / Accident Analysis and Prevention 35 (2003) 817–823
Fig. 3. Traffic detection rates before and after installation of the treatment.
Fig. 4. Percent of drivers reporting having noticed the treatment and their ratings of its acceptability before and after installation of the treatment.
noticed the screen. Of those drivers that noticed the screen, 48% rated the screen as “completely acceptable” (“5” on the 5-point acceptability scale). Only one driver rated the screen as “completely unacceptable”. Fig. 5 shows the ratings of the intersection safety. The median rating of the overall safety of the intersection was “3” for all drivers. There was no difference in the distribution of the ratings prior to and after the treatment.
4. Follow-on testing Follow-on testing was based on speed count data collected collaterally with, and subsequent to, the field-testing. Speed
counts were taken with tube counters placed 20 m prior to the intersection on the eastbound approach. Speed count data were collected in four 7-day blocks prior to installation of the treatment (during the week of 3 February 2001), 2 weeks after installation (week of 31 March), 21 weeks after installation (week of 10 August), and 37 weeks after installation (week of 29 November). The purpose of the speed counts was to provide a means of judging the long-term efficacy of the treatment by collecting approach speed data with a common methodology. Fig. 6 compares the speed count data collected on equivalent days and times to the field test data (Monday mornings between 9:00 and 12:00), and shows that the mean, median, 80th and 85th percentile speeds decreased immediately
S.G. Charlton / Accident Analysis and Prevention 35 (2003) 817–823
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Fig. 5. Ratings of intersection safety before and after installation of the treatment.
following installation of the treatment and remained suppressed over the 37 weeks since installation: 85th percentile speeds prior to the treatment were 56.07, 45.67 km/h after the treatment, 39.67 km/h 21 weeks later, and 38.36 km/h 37 weeks later (a 30% decrease). The average approach speeds of eastbound traffic showed a nearly identical 30% drop, from 40.33 km/h prior to the treatment to 28 km/h 37 weeks later (S.D. of 13 and 10.25 km/h respectively). Finally, the median approach speeds showed the same trend, albeit to a lesser magnitude (an 18.9% reduction from 38.11 to
30.89 km/h). Shown in the right-hand panel of Fig. 6 is the distribution of approach speeds during Monday mornings. Immediately apparent in is the almost complete elimination of approach speeds in the 50–60 km/h range in the 37 weeks following installation of the treatment. The figure also shows a progressive reduction of approach speeds in the 40–50 km/h range. To further judge the efficacy of the treatment in reducing approach speeds, Fig. 7 compares approach speeds across all times of day and all days of the week. As can be seen in
Fig. 6. Approach speeds on Monday mornings before and 2, 21, and 37 weeks following installation of the treatment.
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Fig. 7. Approach speeds throughout the week before and 2, 21, and 37 weeks following installation of the treatment.
the figure, a nearly identical pattern of results to that found for Monday mornings was obtained at all times of the day and days of the week.
5. Discussion Based on the results of the field test, the treatment was agreed to be effective and was allowed to remain at the intersection site until a permanent intervention, currently envisioned as a rural roundabout, can be implemented. Looking at the follow-on results it can be seen that the approach speeds at week 21 were even lower than those obtained 2 weeks after the treatment installation. Why approach speeds would decline beyond that observed immediately after implementation of the approach treatment was not readily apparent. One possible reason was a substantially higher traffic flow in early August (14,750 eastbound vehicles per week as compared to counts of 9000 vehicles pre-treatment and 7500 immediately post-treatment), possibly resulting in shorter headway distances (platooning of vehicles) and thus, the probability of lower approach speeds measured by the tube counters. To investigate this possibility further, an analysis of the approach speeds at off-peak hours when traffic flows were low (Saturday and Sunday mornings between 7:00 and 10:00, averaging 98 and 52 vehicles per hour, respectively) was conducted. This analysis found the same pattern of decline; approach speeds at week 21 appreciably lower than those at week 2. Further, traffic flows returned to pre-treatment levels by week 37 without any corresponding increase in approach speeds. Thus, the reduced approach speeds appeared to be due to the screen treatment rather than any direct effect of traffic density. An alternative interpretation of the decrease in approach speeds between weeks 2 and 21 may be based on changes to the opacity of the screen.
Visual inspection of the screen in the months following installation revealed that the screen was not at taut as when originally installed and that the shade cloth itself appeared to have frayed slightly as a result of wind and weather. These changes in the screen took place in the first 6–12 weeks following installation, the resulting increase in opacity corresponding with the continued decrease in approach speeds. No further fraying or changes in screen opacity were noted thereafter, and approach speeds apparently stabilised at that point. It is clear that installation of the treatment was associated with a significant decrease in approach speeds, the near elimination of approach speeds in the 50–60 km/h range, and increased traffic detection rates. The survey results showed that the treatment was visually acceptable to the large majority of drivers using the intersection and did not affect the perceived safety of the intersection. Of perhaps the greatest significance, no crash resulting in serious injury or death has occurred at the intersection since installation of the treatment to the present time. Finally, the results support the philosophy that drivers can and do adjust their driving behaviour to suit road and traffic conditions and that road designers can manipulate road user behaviour to benefit safety.
Acknowledgements Thanks are due to Brett Alley and Rebecca Luther of Transport Engineering Research New Zealand Ltd., John Herbert and Cameron Inder of the State Highway 27 Crash Reduction Team, and John Grummitt of Transit New Zealand for their support and assistance. Thanks are also due to the anonymous reviewers of this manuscript for their thoughtful and helpful comments and suggestions.
S.G. Charlton / Accident Analysis and Prevention 35 (2003) 817–823
References Charlton, S.G., 1996. SITE: an integrated approach to human factors testing. In: O’Brien, T.G., Charlton, S.G. (Eds.), Handbook of Human Factors Testing and Evaluation. Lawrence Erlbaum Associates, Hillsdale, NJ, pp. 27–40. Charlton, S.G., 2002. Selecting measures for human factors tests. In: Charlton, S.G., O’Brien, T.G. (Eds.), Handbook of Human Factors Testing and Evaluation, 2nd Edition. Lawrence Erlbaum Associates, Hillsdale, NJ, pp. 37–54.
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