Failure to Maintain a Proper Lookout: Driver Perception Reaction Time
This article is merely a resource. This expert has retired.
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On the Interstate highway at night, vehicle A collides with the rear of stalled vehicle B. Driver A is ticketed by the investigating state trooper. Later in court, Judge Smith accepts the ACDA (Assured Clear Distance Ahead) rule argument and finds driver A guilty for failure to avoid the accident. Later, injured driver B files action against A, and eventually receives an award for significant damages. Such an outcome, unfortunate in the mind of this writer, is commonly attributed to an historical, simplistic, and archaic understanding by the legal and judicial professions of the nature of human abilities vis a vis the driving task. Put simply, the common argument of "failure to maintain a proper lookout" assumes that both a driver's vision and attention are focused at all times during the events leading up to the point of collision with B. Certainly, anyone who operates an automobile knows that the driving task includes many elements beyond attention to the roadway ahead, e.g. monitoring of instrument panel, rear and side-view mirrors, traffic signs and signals, and objects peripheral to the vehicle and roadway itself (other on-road and off-road vehicles, pedestrians) - as well as lane-tracking (steering) and speed control.
The basic concept which needs clarification here is that of "Perception Reaction Time" (PRT). This concept takes into account a series of elemental times associated with the following: stimulus (object) activation of sensory receptors; neural transmission of impulses to the brain (occipital cortex in the case of vision); recognition (perceptual processing); and transmission of motor nerve impulses to the muscles for their activation. PRT thus is measured from the moment a driver detects an object in his visual field to the initiation of a response. The American Association of State Highway and Transportation Officials (AASHTO) recommends a PRT of 2.50 seconds for computations of Stopping Sight Distance (SSD). For example, at 60 mph, the brake reaction distance is 220.5 feet (2.5 x 88'/sec.); this is added to the vehicle braking distance on level of 345.5 feet to yield an SSD total of 566 feet. Under ideal conditions, PRT can range between 0.75 and 1.50 seconds, but under less than ideal conditions, values can be higher, especially at night. For an "unexpected event," a PRT value of 3.5 seconds has been recommended. AASHTO acknowledges that a PRT of 2.50 is not adequate"... for the most complex conditions encountered by the driver."
AASHTO has introduced a second standard, Decision Sight Distance (DSD), to address situations where complex or instantaneous decisions are made, "...when information is difficult to perceive or when unexpected or unusual maneuvers are required," or when the roadway environment is visually cluttered. Going beyond SSD, DSD values reflect: (a) times to recognize a hazard; (b) problem solving and decision-making; and (c) avoidance maneuvering. DSD values at 60 mph, for example, range from a low of 610 feet on a rural road to 1280 feet on an urban road with required speed, path, and direction changes. The latter value reflects a DSD time of 14.5 seconds. Compare this figure with a PRT of 2.5 seconds! Unfortunately, the idea that a driver can respond within 2.5 seconds under any circumstances seems to be accepted by most courts despite scientific evidence to the contrary.
Again, note that the above discussions of PRT, SSD, and DSD do not address other visual activities of the driver. Considering those activities, for an entry ramp to a limited access highway at night, the scenario might be as follows: monitor lane; glance to left mirror for merge; monitor lane; glance to right roadway and read "lane ends" sign; monitor lane; glance to left mirror for merge; monitor lane; glance to left mirror for merge; monitor merge lines; monitor lane. Assuming that the access ramp involves a distance of 1000 feet, and acceleration is from 30 to 50 mph over this distance, the above ten visual activities demand over 15 seconds of time. Are not these driver behaviors those of a reasonable person? But suppose at 200 feet, your headlights reveal in your path an outline of a disabled vehicle with no lights. With 2.5 seconds to go, might you avoid collision? No, I opine. The questions of headlamp illumination and visibility distances cannot be addressed here in detail, but suffice it to say that current headlamp designs are unlikely to provide illumination for detection much beyond 200 to 300 feet.
Reliable data do exist for a number of the visual activities described: changes in visual fixations; glance times; dwell times; head movements involved in lane changes and in checking mirrors; and eye movement times.
Apart from the prerequisite of attention, there are additional factors which determine whether in fact a driver will detect an object in his visual field. Objects must be discernible - not just visible. Good conspicuity, contrast, and brightness help. Flash rates for emergency vehicles should be five times higher than the SAE standard of 60 to 120 flashes per minute to ensure their detection.
Yet, even the best driver can err because of human shortcomings. Drivers are susceptible to roadside distractions. The concept of "expectancy," critical to the design of highways and traffic devices, provides us with warnings of hazards ahead, e.g. the obscured stop light on the other side of the underpass; but don't we also expect other drivers to act as reasonable persons, and for the road ahead of us to be safe - to be free of danger? When expectancies are violated, PRT's will be longer, and the risks to drivers are increased.
Finally, I note how photographs taken from a point of collision at 100, 200, 300 feet, etc. and shown to a jury commonly depict unfairly (unscientifically) what a driver "should have seen" (during those few seconds), since jurors are given ample time to study a static scene in detail and to "see" things our reasonable person would never have seen.
This article discusses issues of general interest and does not give any specific legal or business advice pertaining to any specific circumstances. Before acting upon any of its information, you should obtain appropriate advice from a lawyer or other qualified professional.
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