- Email: [email protected]
Sidewalk potential trip points: A method for characterizing walkways
ARTICLE IN PRESS
International Journal of Industrial Ergonomics 36 (2006) 1031–1035 www.elsevier.com/locate/ergon
Sidewalk potential trip points: A method for characterizing walkways T.J. Ayresa,, R. Kelkarb a
Human factors consultant, 101 Kensington Road, Kensington, CA 94707, USA InSciTech Inc., 185 Berry Street, Suite 3700, San Francisco, CA 94107, USA
b
Available online 7 November 2006
Abstract Most outdoor and many indoor walking environments present abrupt elevation changes of 1.25 cm (0.5 in) or more. Such bumps can disrupt walking as well as travel on small wheels (e.g., skates or skateboards), yet serious accidents from tripping are reasonably infrequent given how much walking is done. Data are presented from walkway profile surveys and factors affecting human ambulatory behavior. Relevance to industry Falls on the same level produce a substantial portion of workplace injuries. Improved understanding of the role of perception and expectation in such falls can lead to improved strategies for preventing injuries. r 2006 Elsevier B.V. All rights reserved. Keywords: Walking; Falls; Trips; Skates; Sidewalks
1. Introduction Walking is easiest and safest on flat, smooth, nonslippery, well-lit surfaces, with stable footwear, but much of our walking is done in less benign conditions. Particularly outdoors, but also indoors, humans confront numerous elevation changes, either leading to a new level (such as a curb) or immediately returning to the prior level (such as crossing a stick on a sidewalk). Failure to negotiate elevation changes while walking can lead to falls, sometimes causing serious and/or fatal injuries. Nevertheless, it is also evident that most elevation changes and other walkway features are passed without incident. Falling is widely recognized as an important safety problem. In the United States, falls play a major role in deaths (as the second-leading cause of unintentional-injury fatalities in 2000, with over 13,000 deaths) and in non-fatal injuries (as the leading cause of hospital-emergency-room treated injuries in 2001, with nearly 8,000,000 injuries) (National Safety Council, 2003). Falls are a particular concern for the elderly, for whom they are the leading Corresponding author. Fax: +1 510 528 4941.
E-mail address: [email protected] (T.J. Ayres). 0169-8141/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ergon.2006.09.004
cause of unintentional-injury death (National Safety Council, 2003). In the workplace, during 2004, falls accounted for nearly 800 deaths (in US private industry) and over 250,000 injuries (US Bureau of Labor Statistics). From a biomechanical perspective, walking is a coordinated movement pattern involving two main phases: stance and swing. Stance is the weight-bearing aspect of gait, and consists of single-support and double-support periods, related to whether one foot is or both feet are in contact with the ground at the time. Stance begins with heel-toground contact and continues to the point of toe-off on the same foot. The swing phase, conversely, begins with toe-off and continues to the point of heel-to-ground contact on the same foot. Note that the single-support stance phase for one limb corresponds to the swing phase for the contralateral limb. During pre-swing, i.e., immediately before toe-off, the ankle plantarflexes as the heel comes off the ground and the lower limb is unloaded. The knee also flexes during pre-swing to initiate the maximum knee flexion (approximately 601) associated with the swing phase. During the initial swing phase, the knee continues to flex and the ankle plantarflexion decreases to neutral, both providing mechanisms to increase toe-to-ground clearance during
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mid-swing. The foot also pronates during initial swing for additional ground clearance. During mid-swing, the critical phase for tripping occurs as the lower limb swings forward with the ankle essentially at 01 of dorsiflexion, and with the knee extending from its flexed position to 01 of flexion. Eng et al. (1994), describe the ‘‘late swing’’ trip as being a greater threat to the equilibrium of the body as the center of mass has passed the stance leg and the time for corrective action is significantly reduced. There are several factors that lead to potential trips including inherent movement variability associated with human movement leading to toe-stubs (Young et al., 1996), clinical conditions such as foot drop characterized by weak or non-functional ankle dorsiflexors, and decreased ground clearance due to potential trip points. This paper is confined primarily to a discussion of small elevation rises, rather than drops. From a mechanical perspective, even a very small rise can act as a trip point; a toe stub can perturb gait and lead to a stumble and possible injury either by falling or by the exertion of avoiding a fall. Ground clearance during the swing phase can be substantially less than 1.3 cm (0.5 in) (e.g., Sloan and Kraemer, 1987; Turnbow, 1998), although minimum clearance occurs when the swinging foot is passing the stance foot and is therefore less likely to cause trouble. The Life Safety Code (NFPA 101, 2000), like ASTM F1637-95, calls for beveling any abrupt change in elevation greater than 0.25 in in an egress path; a change of 0.5 in or greater is to be ramped or else treated as a stair. Codes and recommendations for US sidewalks frequently call for some remedial treatment (e.g., grinding or replacement) when elevation changes by more than 1.3 cm (0.5 in) at sidewalk joints. Thus, there is congruence between standards and safe practice in this regard. Walkway elevation changes may also contribute to falls by people using roller skates (traditional or in-line) or skateboards. In 2000, there were over 200,000 emergencyroom treated injuries associated with the use of these products in the United States (NEISS, 2000). Nearly all of these incidents involve falling; the precipitating event may be collision such as with a fixed object, a person, or a vehicle, but at least some of these falls are caused by contacting surface irregularity. 2. Skate wheels Interestingly, the same minimum rise that can cause a toe-stub stumble may also disrupt the stability of someone traveling on small-diameter wheels. Roller skates (including in-line skates) and skateboards generally use wheels with diameters of 7.6 cm (3 in). Mathematical analysis indicates that a 1.1 cm (0.43 in) abrupt rise is just high enough to displace the velocity vector of 3-in. diameter wheels by 451; this height (which is approximately 30% of the wheel radius) is a transition between bumps that would be minimally disruptive and those that would be more likely to require active adjustment by the skater. For the
purposes of this paper, features such as wheel suspension, tire deformation, and lip deformation are considered negligible or absent. Tests have been conducted with weighted skates rolling into vertical rises, and the results are in agreement with mathematical analysis. Child roller skates with 5.4 cm (2.1 in) diameter wheels were rolled down a wooden ramp into elevation changes of 0.39, 0.74, 1.06, and 1.29 cm. The ratios of the elevation changes to the wheel radius were 0.145, 0.274, 0.393, and 0.478, respectively. Masses were added to change the weight distribution on the skates, causing the rear wheels to be loaded proportionally more than the front wheels (compared to unloaded skates). For the smallest elevation change (0.39 cm/0.15 in), when approach speeds exceeded 1 mph, the skates were able to climb the lip. This suggests that even in the smallest of elevation changes there needs to be sufficient kinetic energy to provide for the increase in potential energy associated with the change in elevation. The next highest elevation change (0.74 cm/0.29 in) provided a transition between climb/no-climb conditions. For lower momentum cases (i.e., lower speeds and/or smaller added masses), the skates were unable to traverse the change in elevation, whereas for higher speeds and/or added masses, the skates were just able to climb the lip. This elevation change (0.74 cm) is close to the 0.79 cm height that would create the 451 angle between the horizontal and the radius from the wheel center to the leading edge of the lip. At the higher lip heights (1.06 cm/0.42 in and, 1.29 cm/0.51 in), the skates were unable to traverse the change in elevations regardless of the approach speed or the added masses. There are active maneuvers that can be taken to negotiate changes of elevation including those with rises greater than 30% of the wheel radius. These maneuvers could include raising the entire foot (in the case of roller skates) or skateboard, or could involve a more subtle redistribution of the skater’s weight onto the rear wheels, thereby partially or completely unloading the front wheels and reducing the potential energy associated with getting the front wheels over the lip. Once the front wheels are over the lip, the skater’s weight could then be shifted forward allowing the rear wheels to traverse the elevation change, although elevation changes are not as disruptive for rear wheels as for front wheels. However, this active maneuvering requires that the skater is aware of and recognizes the change of elevation. 3. Walkway profiles Walkers and other walkway users encounter potential trip points frequently on a daily basis. In order to characterize typical walking environments, walkway profile surveys were performed. While traveling along a sidewalk or walkway, all elevation changes (rises or descents) of 1.3 cm (0.5 in) or greater are measured and noted. The example shown in Fig. 1 is for a 1.6 km (1 mi) route in Palo Alto, CA, involving a residential area and several blocks of
ARTICLE IN PRESS T.J. Ayres, R. Kelkar / International Journal of Industrial Ergonomics 36 (2006) 1031–1035 35
Count per mile
30 25 20 15 10 5 0 0.500 0.625 0.750 0.875 1.000 1.125 1.250 1.375 1.500 >1.5 curbs Elevation change (in)
Fig. 1. Elevation changes on residential/commercial sidewalk.
stores. Including four curbs, there were almost 100 abrupt elevation changes greater than 1.3 cm (0.5 in), or nearly 50 potential trip points (rises) in each direction in just this short walk. A walked route in a residential area that was surveyed in San Jose, CA, was found to have 18 potential trip points (rises) per direction per 1.6 km (1 mi); in addition, various temporary obstacles were noted, including sticks and rocks exceeding the minimum size of interest. Another site in San Jose was found to have 12 potential trip points per 1.6 km per direction (three of at least 2.5 cm [1 in]), plus several temporary obstacles. On the other hand, at a local shopping mall, no abrupt rises of 1.3 cm (0.5 in) or more were found in a normal walking route through the parking lot, through the mall, or through a large department store, other than curbs, planter boxes, and the pedestals of some displays. 4. Discussion Walkway profile surveys offer a quantitative basis for characterizing and comparing walking environments. For the present discussion, data such as those in Fig. 1 reveal that even apparently benign environments ought to be fraught with peril, for walkers as well as skaters and skateboarders, if the presence of numerous potential trip points constitutes a hazardous condition. Abrupt rises that are high enough to destabilize a pedestrian appear to be common, at least in residential sidewalks. This observation provides a context for the large numbers of fall-related injuries and deaths cited earlier: although there are many injurious falls, the risk of falls (e.g., falls per million exposures to possible fall-inducing conditions) may be quite low. Surveys suggest that the average US resident walks over 100 miles each year (FHWA, 2004; NPTS, 1990), which would mean that each person may cross thousands of potential trip points annually without serious incident. Falls (of all kinds, including from one level to another) in public places
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accounted for less than 7000 fatalities in the US in 1999 (National Safety Council, 2000); common experience suggests that injuries rarely result from tripping on sidewalk irregularities. Even when all hospital-treated falls on the same level are considered, it is clear that the average pedestrian would need to walk for hundreds of years before being injured by tripping. Thus, the risk of injurious fall when a person confronts a potential trip point must be very low, and requires that the lowest ground clearance phase during leg swing occurs at the very same location as the change of elevation. Furthermore, since the lowest ground clearance phase during leg swing occurs as the leg passes alongside the stance phase of the contralateral limb, it is likely that the change in elevation has provided some vestibular/tactile stimulus to the individual and may have effectively raised the height of the body, rendering the change in elevation less obstructive. There are two important and interrelated implications. First, common pedestrian experience includes dealing with potential trip points on a frequent basis. It follows that elevation changes of even 2.5 cm (1 in) or more are not unexpected, at least in common sidewalk environments. There also appear to be systematic differences between walking environments, e.g., between residential sidewalks and shopping malls; to the extent that pedestrians are aware of such differences, they may use expectancies to modify their behavior. Second, the fact that the average person safely passes over thousands of potential trip points each year indicates that people are able to pick up sufficient visual preview information to adjust their gait and prevent stumbling (e.g., Leclercq, 1999; Ludwig et al.). Although obstacles can be successfully negotiated if noticed just one step in advance (Patla, 1991), potential trip points need to be seen approximately 3 m (10 ft) ahead to negotiate without gait disturbance (Adams, 1997; Chen et al., 1991) or error (Zohar, 1978). Given that these potential trip points may be as small as 1.3 cm (0.5 in.) or less (subtending a visual angle of less than 0.251 at that distance), it follows that pedestrians must rely on visual scanning of the walkway ahead, as well as detection of contours or salient features in peripheral vision. This must be even more true for people who are traveling at higher speeds than typical pedestrians, whether running or using skates or skateboards. There is some evidence of different information-gathering strategies for outdoor vs. indoor walking (e.g., less looking to the side while outdoors), and for walking on stairs vs. on flat surfaces (e.g., more looking down while negotiating stairs) (Vargas-Martin and Peli, 2001). This is similar to findings of systematic and expected changes in gaze direction by drivers depending on environment (e.g., curves vs. straight roads) and task (e.g., following a lead car vs. driving without traffic) (see reviews in Green, 2001). There is also evidence consistent with the suggestion that elderly pedestrians compensate for age-related changes and control their risk of falling on uneven surfaces such as stairs (Ayres et al., 1998).
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Pedestrians also need to be aware of potential slip hazards in their environment. Subjects have been found to be reasonably good at perceiving the slipperiness of walking surfaces, using visual cues (in preview) as well as feedback from walking (Cohen & Cohen, 1994a, b). Systematic changes in gait, instrumental in reducing the likelihood and severity of a slip, have been observed when subjects approach a walking surface that they expect to be slippery (Cham et al., 2002). To the extent that pedestrians are aware of or expect differences between walking environments with respect to the frequency of potential walking hazards (that may cause trips or slips), behavioral adjustments in gait and/or eyemovement patterns are likely. Such accommodation would be consistent with a more general match between behavior and environment, as mediated by perception of environmental affordance or what is personally reasonable to attempt (Ayres et al., 1998). Future research will examine whether eye-movement strategies or gait characteristics reveal environmental adjustment to local conditions, as measured with walkway profile surveys. One type of accommodation has already been demonstrated: pedestrians slow down sooner for obstacles (such as steps or buckets) under low illumination than under day conditions (Adams, 1997). Assuming that pedestrians generally gather sufficient visual information about the walking environment, what can account for occasional tripping over elevation changes that should have been visible? At least three somewhat interrelated mechanisms are likely to contribute to these falls. First, gaze direction is concentrated directly (although not exclusively) forward; small elevation changes or obstacles that are slightly to the side of the intended walking path are less likely to be seen (Zohar, 1978), and could disturb walking if a pedestrian changes course even slightly. Second, pedestrians can be distracted, either by an event (such as someone calling their name) or by their own activity (e.g., lost in thought), and therefore fail to monitor the area ahead, especially if they perceive the walking environment to be generally benign. Pedestrians may fail to respond to unexpected walkway features, whether within the fixated visual field or slightly to the side; this would be an example of change blindness (Simons and Chabris, 1999). Pedestrians also may fail to note or negotiate walkway irregularities due to intoxication, fatigue, or other attentional impairments, or if their view of the walking surface ahead is blocked by objects they are carrying. Third, it is unlikely for people to understand that even minor elevation changes occasionally can cause walking disturbances resulting from movement variability (Young et al., 1996), just as people fail to appreciate their susceptibility to change blindness (Levin et al., 2000). Despite evidence that people are fairly good at judging the relative riskiness of various activities, there are systematic biases that may be relevant to sidewalk travel, including underestimating the risk of familiar activities (Zimolong,
1985) and an optimism bias regarding personal susceptibility (Weinstein, 1984). It may be that the separate contributions of severity and likelihood need to be considered; a taller elevation rise such as a curb is more likely to trip a pedestrian, but a trip caused by a 2.5 cm (1 in) toe catch may be just as injurious. All of these proposals give a role to perceived affordance: pedestrians are likely to be less vigilant regarding potential trip points in environments that are perceived as easily walked. This perspective can provide a starting point for evaluating proposed interventions (Ayres et al., 2000). As long as perceived affordance matches reality (i.e., as long as the frequency and/or severity of potential trip points remains reasonably constant over the path of travel), it may be difficult to influence pedestrians to pay more attention to their path than they would naturally devote. Behavioral interventions to prevent samelevel falls at potential trip points are more likely to be effective when perceived affordance is not veridical, such as for potential trip points that are difficult to see, more difficult to negotiate than they appear, or out of character in their environment. Assessment tools such as the walkway profile surveys proposed here may help to identify sites for interventions to prevent falls. In general, outliers or unusual obstacles deserve special consideration. For example, a colored strip at the top edge of a step down is more likely to provide useful information and to redirect pedestrian attention when the step is unexpected than when it is part of a set of stairs whose presence is redundantly indicated by other cues. Similarly, adding illumination in an otherwise dimly lit walkway is more likely to help prevent falls in an area where a potential trip point appears without precedent than in an area characterized by numerous small irregularities. References Adams, C.J., 1997. An investigation of navigation processes in human locomotion behavior. Masters Thesis, Virginia Polytechnic Institute and State University. ASTM F1637-95, 1996. Standard practice for safe walking surfaces. American Society for Testing and Materials. Ayres, T.J., Wood, C.T., Schmidt, R.A., McCarthy, R.L., 1998. Risk perception and behavioral choice. International Journal of Cognitive Ergonomics 2 (1–2), 35–52. Ayres, T.J., Arndt, S.R., Young, D., Humphrey, D., 1998. Product-related risk of falling among the elderly. Proceedings of the Silicon Valley Ergonomics Conference and Exposition, 190–193. Ayres, T.J., Wood, C.T., Schmidt, R.A., Young, D., Murray, J., 2000. Affordance perception and safety intervention. Proceedings of the 44th Annual Meeting of the Human Factors and Ergonomics Society 6, 51–54. Cham, R., Moyer, B., Redfern, M.S., 2002. Effect of having a-priori knowledge of the floor’s contaminant condition on the biomechanics of slips. Proceedings of the Human Factors and Ergonomics Society 46th Annual Meeting, 1181–1185. Chen, H.C., Ashton-Miller, J.A., Alexander, N.B., Schulz, A.B., 1991. Stepping over obstacles: gait patterns of healthy young and old adults. Journal of Gerontological Medicine and Science 46 (6), M196–M203.
ARTICLE IN PRESS T.J. Ayres, R. Kelkar / International Journal of Industrial Ergonomics 36 (2006) 1031–1035 Cohen, H.H., Cohen, D.M., 1994a. Psychophysical assessment of the perceived slipperiness of floor tile surfaces in a laboratory setting. Journal of Safety Research 25 (1), 19–26. Cohen, H.H., Cohen, D.M., 1994b. Perceptions of walking surface slipperiness under realistic conditions utilizing a slipperiness rating scale. Journal of Safety Research 25 (1), 27–31. Eng, J.J., Winter, D.A., Patla, A.E., 1994. Strategies for recovery from a trip in early and late swing during human walking. Experimental Brain Research 102 (2), 339–349. FHWA. 2004. National bicycling and walking study–ten year status report. Federal Highway Administration. Green, P., 2001. Where do drivers look while driving (and for how long)? In: Dewar, J.E., Olson, P.L. (Eds.), Human factors and traffic safety. Lawyers and Judges Publishing, Tucson, pp. 77–110. Leclercq, S., 1999. Prevention of same level falls: a more global appreciation of this type of accident. Journal of Safety Research 30 (2), 103–112. Levin, D.T., Momen, N., Drivdahl, S.B., Simons, D.J., 2000. Change blindness: the metacognitive error of overestimating change-detection ability. Visual Cognition 7 (1/2/3), 397–412. Ludwig, O., Mohl, B., Dillmann, U. Human walking stability and reaction to disturbances. Universitat des Saarlandes. National Safety Council, 2000. Injury Facts 2000 Edition. Itasca, IL. National Safety Council, 2003. Injury Facts 2003 Edition. Itasca, IL. NEISS. National Electronic Injury Surveillance System, US Consumer Product Safety Commission. NFPA 101, 2000. Life safety code. National Fire Protection Association.
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NPTS. National Personal Transportation Safety, 1990. Results described in FHWA Course on Bicycle and Pedestrian Transportation, Lesson 2: Bicycling and Walking in the United States Today. Patla, A.E., 1991. Visual control of human locomotion. In: Patla, AE. (Ed.), Adaptability of human gait. Elsevier Science Publishers, Amsterdam. Simons, D.J., Chabris, C.F., 1999. Gorillas in our midst: sustained inattentional blindness for dynamic events. Perception 28, 1059–1074. Sloan, G.D., Kraemer, R., 1987. Walkway airspace mapping and stubtoe height. Presented to the 31st Annual Meeting of the Human Factors Society. Turnbow, C.E., 1998. Slip and fall practice. James Publishing Group. US Bureau of Labor Statistics. /www.bls.gov/iif/home.htm/#NewsS. Vargas-Martin, F., Peli, E., 2001. Eye movements with peripheral visual field loss while walking. Ophthalmology. And Vision. Science. (ARVO Suppl.) 42, s858. Weinstein, N.D., 1984. Why it won’t happen to me: perceptions of risk factors and susceptibility. Health Psychology 3, 431–457. Young, D.E., Ayres, T.J., Schmidt, R.A., Bjelajac, V.M., 1996. Inherent movement variability as a cause of stair accidents. proceedings of the silicon valley ergonomics conference and exposition, 297–301. Zimolong, B., 1985. Hazard perception and risk estimation in accident causation. In: Eberts, R.E., Eberts, C.G. (Eds.), Trends in ergonomics and human factors II. Elsevier. Zohar, D., 1978. Why do we bump into things while walking? Human Factors 20 (6), 671–679.
International Journal of Industrial Ergonomics 36 (2006) 1031–1035 www.elsevier.com/locate/ergon
Sidewalk potential trip points: A method for characterizing walkways T.J. Ayresa,, R. Kelkarb a
Human factors consultant, 101 Kensington Road, Kensington, CA 94707, USA InSciTech Inc., 185 Berry Street, Suite 3700, San Francisco, CA 94107, USA
b
Available online 7 November 2006
Abstract Most outdoor and many indoor walking environments present abrupt elevation changes of 1.25 cm (0.5 in) or more. Such bumps can disrupt walking as well as travel on small wheels (e.g., skates or skateboards), yet serious accidents from tripping are reasonably infrequent given how much walking is done. Data are presented from walkway profile surveys and factors affecting human ambulatory behavior. Relevance to industry Falls on the same level produce a substantial portion of workplace injuries. Improved understanding of the role of perception and expectation in such falls can lead to improved strategies for preventing injuries. r 2006 Elsevier B.V. All rights reserved. Keywords: Walking; Falls; Trips; Skates; Sidewalks
1. Introduction Walking is easiest and safest on flat, smooth, nonslippery, well-lit surfaces, with stable footwear, but much of our walking is done in less benign conditions. Particularly outdoors, but also indoors, humans confront numerous elevation changes, either leading to a new level (such as a curb) or immediately returning to the prior level (such as crossing a stick on a sidewalk). Failure to negotiate elevation changes while walking can lead to falls, sometimes causing serious and/or fatal injuries. Nevertheless, it is also evident that most elevation changes and other walkway features are passed without incident. Falling is widely recognized as an important safety problem. In the United States, falls play a major role in deaths (as the second-leading cause of unintentional-injury fatalities in 2000, with over 13,000 deaths) and in non-fatal injuries (as the leading cause of hospital-emergency-room treated injuries in 2001, with nearly 8,000,000 injuries) (National Safety Council, 2003). Falls are a particular concern for the elderly, for whom they are the leading Corresponding author. Fax: +1 510 528 4941.
E-mail address: [email protected] (T.J. Ayres). 0169-8141/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ergon.2006.09.004
cause of unintentional-injury death (National Safety Council, 2003). In the workplace, during 2004, falls accounted for nearly 800 deaths (in US private industry) and over 250,000 injuries (US Bureau of Labor Statistics). From a biomechanical perspective, walking is a coordinated movement pattern involving two main phases: stance and swing. Stance is the weight-bearing aspect of gait, and consists of single-support and double-support periods, related to whether one foot is or both feet are in contact with the ground at the time. Stance begins with heel-toground contact and continues to the point of toe-off on the same foot. The swing phase, conversely, begins with toe-off and continues to the point of heel-to-ground contact on the same foot. Note that the single-support stance phase for one limb corresponds to the swing phase for the contralateral limb. During pre-swing, i.e., immediately before toe-off, the ankle plantarflexes as the heel comes off the ground and the lower limb is unloaded. The knee also flexes during pre-swing to initiate the maximum knee flexion (approximately 601) associated with the swing phase. During the initial swing phase, the knee continues to flex and the ankle plantarflexion decreases to neutral, both providing mechanisms to increase toe-to-ground clearance during
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mid-swing. The foot also pronates during initial swing for additional ground clearance. During mid-swing, the critical phase for tripping occurs as the lower limb swings forward with the ankle essentially at 01 of dorsiflexion, and with the knee extending from its flexed position to 01 of flexion. Eng et al. (1994), describe the ‘‘late swing’’ trip as being a greater threat to the equilibrium of the body as the center of mass has passed the stance leg and the time for corrective action is significantly reduced. There are several factors that lead to potential trips including inherent movement variability associated with human movement leading to toe-stubs (Young et al., 1996), clinical conditions such as foot drop characterized by weak or non-functional ankle dorsiflexors, and decreased ground clearance due to potential trip points. This paper is confined primarily to a discussion of small elevation rises, rather than drops. From a mechanical perspective, even a very small rise can act as a trip point; a toe stub can perturb gait and lead to a stumble and possible injury either by falling or by the exertion of avoiding a fall. Ground clearance during the swing phase can be substantially less than 1.3 cm (0.5 in) (e.g., Sloan and Kraemer, 1987; Turnbow, 1998), although minimum clearance occurs when the swinging foot is passing the stance foot and is therefore less likely to cause trouble. The Life Safety Code (NFPA 101, 2000), like ASTM F1637-95, calls for beveling any abrupt change in elevation greater than 0.25 in in an egress path; a change of 0.5 in or greater is to be ramped or else treated as a stair. Codes and recommendations for US sidewalks frequently call for some remedial treatment (e.g., grinding or replacement) when elevation changes by more than 1.3 cm (0.5 in) at sidewalk joints. Thus, there is congruence between standards and safe practice in this regard. Walkway elevation changes may also contribute to falls by people using roller skates (traditional or in-line) or skateboards. In 2000, there were over 200,000 emergencyroom treated injuries associated with the use of these products in the United States (NEISS, 2000). Nearly all of these incidents involve falling; the precipitating event may be collision such as with a fixed object, a person, or a vehicle, but at least some of these falls are caused by contacting surface irregularity. 2. Skate wheels Interestingly, the same minimum rise that can cause a toe-stub stumble may also disrupt the stability of someone traveling on small-diameter wheels. Roller skates (including in-line skates) and skateboards generally use wheels with diameters of 7.6 cm (3 in). Mathematical analysis indicates that a 1.1 cm (0.43 in) abrupt rise is just high enough to displace the velocity vector of 3-in. diameter wheels by 451; this height (which is approximately 30% of the wheel radius) is a transition between bumps that would be minimally disruptive and those that would be more likely to require active adjustment by the skater. For the
purposes of this paper, features such as wheel suspension, tire deformation, and lip deformation are considered negligible or absent. Tests have been conducted with weighted skates rolling into vertical rises, and the results are in agreement with mathematical analysis. Child roller skates with 5.4 cm (2.1 in) diameter wheels were rolled down a wooden ramp into elevation changes of 0.39, 0.74, 1.06, and 1.29 cm. The ratios of the elevation changes to the wheel radius were 0.145, 0.274, 0.393, and 0.478, respectively. Masses were added to change the weight distribution on the skates, causing the rear wheels to be loaded proportionally more than the front wheels (compared to unloaded skates). For the smallest elevation change (0.39 cm/0.15 in), when approach speeds exceeded 1 mph, the skates were able to climb the lip. This suggests that even in the smallest of elevation changes there needs to be sufficient kinetic energy to provide for the increase in potential energy associated with the change in elevation. The next highest elevation change (0.74 cm/0.29 in) provided a transition between climb/no-climb conditions. For lower momentum cases (i.e., lower speeds and/or smaller added masses), the skates were unable to traverse the change in elevation, whereas for higher speeds and/or added masses, the skates were just able to climb the lip. This elevation change (0.74 cm) is close to the 0.79 cm height that would create the 451 angle between the horizontal and the radius from the wheel center to the leading edge of the lip. At the higher lip heights (1.06 cm/0.42 in and, 1.29 cm/0.51 in), the skates were unable to traverse the change in elevations regardless of the approach speed or the added masses. There are active maneuvers that can be taken to negotiate changes of elevation including those with rises greater than 30% of the wheel radius. These maneuvers could include raising the entire foot (in the case of roller skates) or skateboard, or could involve a more subtle redistribution of the skater’s weight onto the rear wheels, thereby partially or completely unloading the front wheels and reducing the potential energy associated with getting the front wheels over the lip. Once the front wheels are over the lip, the skater’s weight could then be shifted forward allowing the rear wheels to traverse the elevation change, although elevation changes are not as disruptive for rear wheels as for front wheels. However, this active maneuvering requires that the skater is aware of and recognizes the change of elevation. 3. Walkway profiles Walkers and other walkway users encounter potential trip points frequently on a daily basis. In order to characterize typical walking environments, walkway profile surveys were performed. While traveling along a sidewalk or walkway, all elevation changes (rises or descents) of 1.3 cm (0.5 in) or greater are measured and noted. The example shown in Fig. 1 is for a 1.6 km (1 mi) route in Palo Alto, CA, involving a residential area and several blocks of
ARTICLE IN PRESS T.J. Ayres, R. Kelkar / International Journal of Industrial Ergonomics 36 (2006) 1031–1035 35
Count per mile
30 25 20 15 10 5 0 0.500 0.625 0.750 0.875 1.000 1.125 1.250 1.375 1.500 >1.5 curbs Elevation change (in)
Fig. 1. Elevation changes on residential/commercial sidewalk.
stores. Including four curbs, there were almost 100 abrupt elevation changes greater than 1.3 cm (0.5 in), or nearly 50 potential trip points (rises) in each direction in just this short walk. A walked route in a residential area that was surveyed in San Jose, CA, was found to have 18 potential trip points (rises) per direction per 1.6 km (1 mi); in addition, various temporary obstacles were noted, including sticks and rocks exceeding the minimum size of interest. Another site in San Jose was found to have 12 potential trip points per 1.6 km per direction (three of at least 2.5 cm [1 in]), plus several temporary obstacles. On the other hand, at a local shopping mall, no abrupt rises of 1.3 cm (0.5 in) or more were found in a normal walking route through the parking lot, through the mall, or through a large department store, other than curbs, planter boxes, and the pedestals of some displays. 4. Discussion Walkway profile surveys offer a quantitative basis for characterizing and comparing walking environments. For the present discussion, data such as those in Fig. 1 reveal that even apparently benign environments ought to be fraught with peril, for walkers as well as skaters and skateboarders, if the presence of numerous potential trip points constitutes a hazardous condition. Abrupt rises that are high enough to destabilize a pedestrian appear to be common, at least in residential sidewalks. This observation provides a context for the large numbers of fall-related injuries and deaths cited earlier: although there are many injurious falls, the risk of falls (e.g., falls per million exposures to possible fall-inducing conditions) may be quite low. Surveys suggest that the average US resident walks over 100 miles each year (FHWA, 2004; NPTS, 1990), which would mean that each person may cross thousands of potential trip points annually without serious incident. Falls (of all kinds, including from one level to another) in public places
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accounted for less than 7000 fatalities in the US in 1999 (National Safety Council, 2000); common experience suggests that injuries rarely result from tripping on sidewalk irregularities. Even when all hospital-treated falls on the same level are considered, it is clear that the average pedestrian would need to walk for hundreds of years before being injured by tripping. Thus, the risk of injurious fall when a person confronts a potential trip point must be very low, and requires that the lowest ground clearance phase during leg swing occurs at the very same location as the change of elevation. Furthermore, since the lowest ground clearance phase during leg swing occurs as the leg passes alongside the stance phase of the contralateral limb, it is likely that the change in elevation has provided some vestibular/tactile stimulus to the individual and may have effectively raised the height of the body, rendering the change in elevation less obstructive. There are two important and interrelated implications. First, common pedestrian experience includes dealing with potential trip points on a frequent basis. It follows that elevation changes of even 2.5 cm (1 in) or more are not unexpected, at least in common sidewalk environments. There also appear to be systematic differences between walking environments, e.g., between residential sidewalks and shopping malls; to the extent that pedestrians are aware of such differences, they may use expectancies to modify their behavior. Second, the fact that the average person safely passes over thousands of potential trip points each year indicates that people are able to pick up sufficient visual preview information to adjust their gait and prevent stumbling (e.g., Leclercq, 1999; Ludwig et al.). Although obstacles can be successfully negotiated if noticed just one step in advance (Patla, 1991), potential trip points need to be seen approximately 3 m (10 ft) ahead to negotiate without gait disturbance (Adams, 1997; Chen et al., 1991) or error (Zohar, 1978). Given that these potential trip points may be as small as 1.3 cm (0.5 in.) or less (subtending a visual angle of less than 0.251 at that distance), it follows that pedestrians must rely on visual scanning of the walkway ahead, as well as detection of contours or salient features in peripheral vision. This must be even more true for people who are traveling at higher speeds than typical pedestrians, whether running or using skates or skateboards. There is some evidence of different information-gathering strategies for outdoor vs. indoor walking (e.g., less looking to the side while outdoors), and for walking on stairs vs. on flat surfaces (e.g., more looking down while negotiating stairs) (Vargas-Martin and Peli, 2001). This is similar to findings of systematic and expected changes in gaze direction by drivers depending on environment (e.g., curves vs. straight roads) and task (e.g., following a lead car vs. driving without traffic) (see reviews in Green, 2001). There is also evidence consistent with the suggestion that elderly pedestrians compensate for age-related changes and control their risk of falling on uneven surfaces such as stairs (Ayres et al., 1998).
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Pedestrians also need to be aware of potential slip hazards in their environment. Subjects have been found to be reasonably good at perceiving the slipperiness of walking surfaces, using visual cues (in preview) as well as feedback from walking (Cohen & Cohen, 1994a, b). Systematic changes in gait, instrumental in reducing the likelihood and severity of a slip, have been observed when subjects approach a walking surface that they expect to be slippery (Cham et al., 2002). To the extent that pedestrians are aware of or expect differences between walking environments with respect to the frequency of potential walking hazards (that may cause trips or slips), behavioral adjustments in gait and/or eyemovement patterns are likely. Such accommodation would be consistent with a more general match between behavior and environment, as mediated by perception of environmental affordance or what is personally reasonable to attempt (Ayres et al., 1998). Future research will examine whether eye-movement strategies or gait characteristics reveal environmental adjustment to local conditions, as measured with walkway profile surveys. One type of accommodation has already been demonstrated: pedestrians slow down sooner for obstacles (such as steps or buckets) under low illumination than under day conditions (Adams, 1997). Assuming that pedestrians generally gather sufficient visual information about the walking environment, what can account for occasional tripping over elevation changes that should have been visible? At least three somewhat interrelated mechanisms are likely to contribute to these falls. First, gaze direction is concentrated directly (although not exclusively) forward; small elevation changes or obstacles that are slightly to the side of the intended walking path are less likely to be seen (Zohar, 1978), and could disturb walking if a pedestrian changes course even slightly. Second, pedestrians can be distracted, either by an event (such as someone calling their name) or by their own activity (e.g., lost in thought), and therefore fail to monitor the area ahead, especially if they perceive the walking environment to be generally benign. Pedestrians may fail to respond to unexpected walkway features, whether within the fixated visual field or slightly to the side; this would be an example of change blindness (Simons and Chabris, 1999). Pedestrians also may fail to note or negotiate walkway irregularities due to intoxication, fatigue, or other attentional impairments, or if their view of the walking surface ahead is blocked by objects they are carrying. Third, it is unlikely for people to understand that even minor elevation changes occasionally can cause walking disturbances resulting from movement variability (Young et al., 1996), just as people fail to appreciate their susceptibility to change blindness (Levin et al., 2000). Despite evidence that people are fairly good at judging the relative riskiness of various activities, there are systematic biases that may be relevant to sidewalk travel, including underestimating the risk of familiar activities (Zimolong,
1985) and an optimism bias regarding personal susceptibility (Weinstein, 1984). It may be that the separate contributions of severity and likelihood need to be considered; a taller elevation rise such as a curb is more likely to trip a pedestrian, but a trip caused by a 2.5 cm (1 in) toe catch may be just as injurious. All of these proposals give a role to perceived affordance: pedestrians are likely to be less vigilant regarding potential trip points in environments that are perceived as easily walked. This perspective can provide a starting point for evaluating proposed interventions (Ayres et al., 2000). As long as perceived affordance matches reality (i.e., as long as the frequency and/or severity of potential trip points remains reasonably constant over the path of travel), it may be difficult to influence pedestrians to pay more attention to their path than they would naturally devote. Behavioral interventions to prevent samelevel falls at potential trip points are more likely to be effective when perceived affordance is not veridical, such as for potential trip points that are difficult to see, more difficult to negotiate than they appear, or out of character in their environment. Assessment tools such as the walkway profile surveys proposed here may help to identify sites for interventions to prevent falls. In general, outliers or unusual obstacles deserve special consideration. For example, a colored strip at the top edge of a step down is more likely to provide useful information and to redirect pedestrian attention when the step is unexpected than when it is part of a set of stairs whose presence is redundantly indicated by other cues. Similarly, adding illumination in an otherwise dimly lit walkway is more likely to help prevent falls in an area where a potential trip point appears without precedent than in an area characterized by numerous small irregularities. References Adams, C.J., 1997. An investigation of navigation processes in human locomotion behavior. Masters Thesis, Virginia Polytechnic Institute and State University. ASTM F1637-95, 1996. Standard practice for safe walking surfaces. American Society for Testing and Materials. Ayres, T.J., Wood, C.T., Schmidt, R.A., McCarthy, R.L., 1998. Risk perception and behavioral choice. International Journal of Cognitive Ergonomics 2 (1–2), 35–52. Ayres, T.J., Arndt, S.R., Young, D., Humphrey, D., 1998. Product-related risk of falling among the elderly. Proceedings of the Silicon Valley Ergonomics Conference and Exposition, 190–193. Ayres, T.J., Wood, C.T., Schmidt, R.A., Young, D., Murray, J., 2000. Affordance perception and safety intervention. Proceedings of the 44th Annual Meeting of the Human Factors and Ergonomics Society 6, 51–54. Cham, R., Moyer, B., Redfern, M.S., 2002. Effect of having a-priori knowledge of the floor’s contaminant condition on the biomechanics of slips. Proceedings of the Human Factors and Ergonomics Society 46th Annual Meeting, 1181–1185. Chen, H.C., Ashton-Miller, J.A., Alexander, N.B., Schulz, A.B., 1991. Stepping over obstacles: gait patterns of healthy young and old adults. Journal of Gerontological Medicine and Science 46 (6), M196–M203.
ARTICLE IN PRESS T.J. Ayres, R. Kelkar / International Journal of Industrial Ergonomics 36 (2006) 1031–1035 Cohen, H.H., Cohen, D.M., 1994a. Psychophysical assessment of the perceived slipperiness of floor tile surfaces in a laboratory setting. Journal of Safety Research 25 (1), 19–26. Cohen, H.H., Cohen, D.M., 1994b. Perceptions of walking surface slipperiness under realistic conditions utilizing a slipperiness rating scale. Journal of Safety Research 25 (1), 27–31. Eng, J.J., Winter, D.A., Patla, A.E., 1994. Strategies for recovery from a trip in early and late swing during human walking. Experimental Brain Research 102 (2), 339–349. FHWA. 2004. National bicycling and walking study–ten year status report. Federal Highway Administration. Green, P., 2001. Where do drivers look while driving (and for how long)? In: Dewar, J.E., Olson, P.L. (Eds.), Human factors and traffic safety. Lawyers and Judges Publishing, Tucson, pp. 77–110. Leclercq, S., 1999. Prevention of same level falls: a more global appreciation of this type of accident. Journal of Safety Research 30 (2), 103–112. Levin, D.T., Momen, N., Drivdahl, S.B., Simons, D.J., 2000. Change blindness: the metacognitive error of overestimating change-detection ability. Visual Cognition 7 (1/2/3), 397–412. Ludwig, O., Mohl, B., Dillmann, U. Human walking stability and reaction to disturbances. Universitat des Saarlandes. National Safety Council, 2000. Injury Facts 2000 Edition. Itasca, IL. National Safety Council, 2003. Injury Facts 2003 Edition. Itasca, IL. NEISS. National Electronic Injury Surveillance System, US Consumer Product Safety Commission. NFPA 101, 2000. Life safety code. National Fire Protection Association.
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