Cyclists and Pedestrians vs. Cars: Cars Win! A Human Factors Perspective

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1 Illinois Association of Defense Trial Counsel Springfield, Illinois IDC Quarterly Volume 22, Number 3 ( ) Feature Article By: Farheen S. Khan, PhD, David M. Cades, PhD, and David A. Krauss, PhD Exponent Failure Analysis Associates Cyclists and Pedestrians vs. Cars: Cars Win! A Human Factors Perspective Abstract: Why is there such a large number of pedestrian and bicycle-related motor vehicle accidents? Evidence suggests that one of the leading causes of these types of accidents has to do with unexpected incursions into the motor vehicle s path of travel. This article explores some of the human factors issues related to the investigation of such accidents. Crash! The Accident In 2009, 4,092 pedestrians and 630 bicyclists were killed in traffic crashes, accounting for more than 14 percent of all traffic fatalities in the United States, with another 59,000 pedestrians and 51,000 bicyclists reported to have been injured as a result of traffic crashes. Interestingly, about three-fourths of these crashes occurred at non-intersections. Recently, scientific research has focused on understanding the nature and causes of pedestrian and bicycle accidents. The Federal Highway Administration (FHWA) reports the following major reasons for pedestrian and bicycle accidents: Location: more than three-fourths of the accidents occurred at non-intersections and roadways without crosswalks. Alcohol involvement: a large number of pedestrians were found intoxicated as compared to the motorists involved in the accidents. Alcohol involvement was found to be a major cause of bicyclist-related accidents as well. Conspicuity: more pedestrian incidents occur at night indicating conspicuity of pedestrians might be an issue. Several studies of pedestrian behavior that have categorized crash types and accident locations have identified dart-outs or sudden incursion into the path of travel of the motor vehicle as one of the top causes of crashes involving pedestrians and motor vehicles (Model Pedestrian User s Manual, 1987; Knoblauch, 1975; Knoblauch, 1977; and Knoblauch, 1978). Bicycle ride-outs have also been identified as a common type of bicycle-motor-vehicle accident (FHWA, 1995). Page 1 of 6

2 Preparation What s that over there? Driver errors contribute significantly to many transportation accidents with distraction and failure to monitor the roadway ahead as the most common contributing factors (Dingus et al., 2006). Most human factors vehicle accident investigations focus on the driver, investigating factors such as driver perception and response time (PRT), distraction, fatigue, etc. However, as the FHWA research suggests, when investigating pedestrian/bicycle vs. motor vehicle accidents, there are additional issues specific to pedestrians and cyclists that must be considered in order to reach comprehensive conclusions. For example, in car vs. pedestrian or cyclist dart-out incidents, it is important to consider how PRT would be affected by the unexpected nature of the event. PRT is considered to be the time required for a driver to detect something in their path (detection), interpret its meaning (identification), determine the proper course of action (decision), and then initiate that action (response) (Olson, et al., 2010). Various empirical examinations of PRT have estimated typical PRT values to be approximately seconds. However, these values can be affected by several factors, including conspicuity, context, location of the hazard in the driver s visual field and expectancy (Olson, et al., 2010; Green, 2000; Olson, 1989). As it relates to dart-out incidents, driver expectancy may have a significant effect on the driver s PRT that is, unexpected events require more time to respond. Investigation The Basics Perception-Reaction Time The detection stage of driver PRT begins when the target of interest (e.g., brake lights, or an obstacle in the road) enters the driver s field of view, and ends when the driver detects the presence of that target. Once the target has been detected, it has to be identified as a hazard. Sufficient visual acuity is necessary to identify the object and determine whether action needs to be taken to avoid it or if it can be ignored (e.g., a stopped vehicle on the road or a harmless piece of debris). Once an object has been identified, the driver must decide how to respond. Response choices vary and depend on the operator s condition, the vehicle s dynamics, and environmental conditions. For example, a response alternative could be to ignore a stationary object or person (i.e., do nothing, if the driver determined during the identification phase that the object is not a threat), alert another driver or pedestrian by activating an audible sound (horn) or visual cue (flashing lights), or perform an evasive maneuver, such as braking and/or steering to avoid an obstacle or collision. As the number of response alternatives increases, so will the driver s PRT (Wickens & Hollands, 2000). After deciding how to respond to the object, the driver must translate those decisions into a plan and execute that response. The processing time that this stage requires includes the time it takes for the brain to send signals to the muscles and for those muscles to respond with appropriate timing and intensity. Once the driver has begun to execute his or her response (e.g., turned the steering wheel, or applied pressure to the brake pedal), the driver s PRT is complete (Olson, 1989). PRT takes into account only the time needed for the driver s mental processing and execution of movement to occur, but does not include the time it takes for the vehicle to respond to the driver s action. Conspicuity Conspicuity is defined as the characteristics of an object or condition which determines whether the object or condition will catch the attention of an observer who does not expect it (Olson, et al., 2010). Conspicuity is affected by a number of visual characteristics, including color, pattern, shading, texture, brightness, contrast, uniqueness, motion, size and relative location between an object and the background against which it is viewed. The critical factor in assessing conspicuity is the contrast of an object relative to its surroundings. Page 2 of 6

3 Objects tend to be more conspicuous if they are significantly brighter or darker than their background and are closer to the center of a person s field of view (Olson, et al., 2010). However, a conspicuous stimulus does not automatically and unintentionally attract attention (Theeuwes, 1991). Thus, an environmental stimulus (e.g., the small retro-reflector on a pedestrian s shirt or on a bicycle pedal) will only be considered useful information to the driver if he/she can identify it as meaningful to the driving task (Muttart et al., 2005). As a consequence, an ambiguous or unidentifiable stimulus will require more effortful and extensive information processing, and result in increased driver PRT. Expectation Violations of driver expectations, such as ride- or dart-outs of pedestrians or cyclists, can cause significant delays in identification of uncommon objects or situations on the road. Expectations have a large effect on the speed and accuracy of driver information processing and influence driver PRT in emergency situations. Specifically, driver PRTs are increased, in some cases by as much as a factor of two, for drivers encountering an unexpected obstacle compared to drivers who have been alerted that an obstacle will be encountered (Olson & Sivak, 1986; Green, 2000; Roper, 1938). Drivers also have specific expectations about the behavior of other drivers they encounter on the road. Research shows that based on their prior experiences on the road, drivers come to expect common roadway occurrences (e.g., slow or stopped vehicles at highway onramps) and do not expect uncommon roadway occurrences (e.g., drivers proceeding through red lights without stopping). With relatively little experience, drivers become faster at detecting common (i.e., expected) things on the road and slower at detecting uncommon (i.e., unexpected) things. Thus, violations of driver expectancy tend to increase PRTs (Triggs & Harris, 1982). To illustrate, Hole and Tyrell (1995) asked subjects to search for motorcycles within traffic scenes. Responses to motorcycles with unlit headlights decreased as the likelihood of motorcycles with lit headlights increased, indicating that subjects quickly adjusted their search strategy for detection of the more common stimulus, to the decrement of detecting the less common stimulus in the environment. Alcohol In 2006, of the 4,353 pedestrians killed due to alcohol involvement, 37% had BACs above.08% (NHTSA, 2006). Clayton et al. (1977) (cited in Blomberg et al., 1979) reported that 70% of the injured/killed pedestrians in their investigation were found to have a BAC of.08% or higher and that the risk of a fatal pedestrian accident rises sharply at BACs above 0.12%. Alcohol appears to produce an increase in errors associated with the pedestrian s selection of appropriate crossing locations and his or her evaluation during the crossing maneuver of what must be done to avoid the accident (Blomberg et al., 1979). Alcohol consumption has been known to lead to impaired cognitive functions such as motor skills, reaction time, visual attention (Mackay, Tiplady, & Scholey, 2002; Rohrbaugh et al., 1988) and motor coordination (Tagawa, et al., 2000). Blomberg et al. (1979) found that two categories greatly influenced alcoholrelated pedestrian crashes: psychomotor impairment and risk taking. Psychomotor impairment accounted for a quarter of the pedestrian-related crashes in the study. Typical cases involved pedestrians who staggered into oncoming traffic even when the vehicle was in full view or stopped. The authors also found that in nearly half the cases in their study, in which the pedestrians BAC was 0.05% or higher, the pedestrian showed increased risk taking as characterized by taking unwarranted chances in traffic such as darting out at intersections. The authors concluded that such behavior would have been less likely were it not for the judgment-impairing effects of alcohol. Page 3 of 6

4 Conclusion What can we do? A motorist would only be able to perceive a cyclist or a pedestrian as a hazard after they enter the roadway. However, if the pedestrian or cyclist entered the roadway where there was no intersection, no crosswalk and no indication the incursion were about to occur, the motorist s ability to respond appropriately and in time is greatly reduced. Pedestrians and cyclists in these incidents often have not taken measures to increase their conspicuity in dark environments. In the absence of such measures, there is no impetus for an attentive driver to perceive and respond to unexpected hazards. Put simply, a reasonably attentive driver should not divert his vision away from the path of travel to look in an area where it is unlikely that a hazard would appear. This is especially true when there is no nearby intersection. In these types of situations, a driver may actually create additional hazards by diverting attention away from areas where activity and hazards are more likely (e.g., crosswalks, roadway in front of or behind the driver). At night, pedestrians and cyclists are typically in a better position to detect an approaching vehicle because approaching headlights are a high-contrast, moving, expected i.e., conspicuous target. Cyclists and pedestrians are required, and expected, to obey the rules of the road just as vehicle operators, including obeying traffic signs, signals, and lane markings. Cyclists can increase their visibility to drivers when riding at night by equipping their bicycle, equipment and clothing with appropriate lighting, reflectors and retroreflective markings. Pedestrians and bicyclists have to be aware of and accommodate oncoming traffic, especially in low-light conditions when it might be harder for a driver to detect them just as importantly as drivers need to keep a vigilant watch of their path of travel. References Blomberg, R.D., et al., (1979). A comparison of alcohol involvement in pedestrians and pedestrian casualties. U.S. Department of Transportation, Contract No. DOT HS Clayton, A.B., et al., (1977). A controlled study of the role of alcohol in fatal adult pedestrian accidents. Great Britain Transport and Road Research Laboratory, Supplementary Report No Dingus, T.A., Klauer, et al., The 100-car naturalistic driving study: phase II results of the 100-car field experiment, DOT HS , Washington, DC. FHWA (1995). Pedestrian and bicycle crash types of the early 1990 s. Publication Number: FHWA-RD FHWA (2003). A review of pedestrian safety research in the United States and abroad. Publication Number: FHWA-RD Green, M. (2000). How Long Does It Take to Stop? Methodological Analysis of Driver Perception-Brake Times. Transportation Human Factors, 2(3). Hole, G., & Tyrrell, L. (1995). The influence of perceptual set on the detection of motorcyclists using daytime headlights. Ergonomics, 38(7), Knoblauch, R.L. Causative Factors and Countermeasures for Rural and Suburban Pedestrian Accidents: Accident Data Collection and Analysis, Report No. DOT-HS , National Highway Traffic Safety Administration, June Page 4 of 6

5 Knoblauch, R.L., Urban Pedestrian Accident Countermeasures Experimental Evaluation, National Highway Traffic Safety Administration, DOT-HS , February Knoblauch, R.L., al., Pedestrian Accidents Occurring on Freeways: An Investigation of Causative Factors. Accident Data Collection and Analysis, Report Nos. FHWA-RD /171, Federal Highway Administration, Mackay, M., et al., (2002). Interactions between alcohol and caffeine in relation to psychomotor speed and accuracy. Human Psychopharmacology: Clinical & Experimental, 17, Model Pedestrian Safety Program. User s Manual Report No. FHWA-IP-78-6, Federal Highway Administration, 1987 edition. Muttart, J. W., et al., (2005). Relationship Between Relative Velocity Detection and Driver Response Times in Vehicle Following Situations. SAE Technical Paper, National Highway Traffic Safety Administration (2006). Traffic safety facts 2006 data: Pedestrians. Retrieved from the World Wide Web on April 21, 2008 from NHTSA (2003). Pedestrian roadway fatalities. DOT HS NHTSA (2009). Traffic safety facts: Bicyclists and other cyclists. DOT HS NHTSA (2009). Traffic safety facts: Pedestrians. DOT HS Olson, P.L. (1989). Driver perception response time. SAE Technical Papers, Olson, P. L., et al., (2010). Forensic aspects of driver perception and response (3rd ed.). Tuscon, AZ: Lawyers & Judges Publishing Company, Inc. Olson, P.L., & Sivak, M. (1986). Perception-response time to unexpected roadway hazards. Human Factors: The Journal of the Human Factors and Ergonomics Society, 28(1), Rohrbaugh, J.W., et al., (1988). Alcohol intoxication reduces visual sustained attention. Psychopharmacology, 96, Roper, V. J., & Howard, E. A. (1938). Seeing with Motor Car Headlamps. Transactions of the Illumination Engineering Society, 33, Tagawa, M., Kano, et al., (2000). Relationship between effects of alcohol on psychomotor performances and blood alcohol. Japanese Journal of Pharmacology, 83, Theeuwes, J. (1991). Visual selection: exogenous and endogenous control. In A. G. Gale, I. D. Brown, Applied Vision Association (Great Britain), C. M. Haslegrave, I. Moorhead, Ergonomics Society (Great Britain), & Association of Optometrists (Great Britain) (Eds.), Vision in Vehicles - III (pp ). Triggs, T. J., Harris, W. G., Psychology, M. U. D. of, Group, M. U. H. F., & Safety, A. O. of R. (1982). Reaction time of drivers to road stimuli. Human Factors Group, Department of Psychology, Monash University. Page 5 of 6

6 Wickens, C., & Hollands, J. (2000). Engineering Psychology and Human Performance (3rd ed.). Prentice Hall. About the Authors Dr. Farheen S. Khan is a Manager in Exponent s Human Factors practice. Her work focuses on applying human factors principles to understanding human error and risk-taking behavior. She has also conducted psychoacoustic analysis of sound in an environment (fire alarms, backup alarms, train horns, equipment/industrial noise, etc) with respect to understanding what an individual within that environment could have heard. Dr. Khan has performed investigations to determine exposure to physical hazards from a wide range of consumer products, industrial tools, and heavy construction machinery, and has applied her knowledge to the assessment of incidents involving these products, as well as to evaluate warnings and instructions for these products. Dr. David M. Cades is a Scientist in Exponent s Human Factors practice. He has expertise in the testing and analysis of how interruptions and distractions affect human performance, including driving, commercial aviation, healthcare, offices and classrooms. He also has experience in investigating aspects of human behavior and performance and the role they play in accident analysis. He has applied this knowledge to see how distractions and interruptions can cause errors or missed steps that lead to more serious accidents. Dr. David A. Krauss is a Principal Scientist in Exponent s Human Factors practice. He has specialized knowledge in human perception and cognition, reaction time, attention, the effects of lighting conditions on vision, and how stress affects behavior. He has investigated accidents associated with industrial safety, motor vehicles, and consumer products, among others. Dr. Krauss routinely applies his training and experience in the fields of cognitive psychology and human factors to study human behavior in a wide array of situations and environments. About the IDC The Illinois Association Defense Trial Counsel (IDC) is the premier association of attorneys in Illinois who devote a substantial portion their practice to the representation of business, corporate, insurance, professional and other individual defendants in civil litigation. For more information on the IDC, visit us on the web at Statements or expression of opinions in this publication are those of the authors and not necessarily those of the association. IDC Quarterly, Volume 22, Number Illinois Association of Defense Trial Counsel. All Rights Reserved. Reproduction in whole or in part without permission is prohibited. Illinois Association of Defense Trial Counsel, PO Box 588, Rochester, IL , , idc@iadtc.org Page 6 of 6