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The time that it takes a driver to perceive and respond to an unexpected situation is of interest in many instances of litigation resulting from motor vehicle crashes.

Studies have been conducted which examine and measure perception - response times to various stimuli. Interestingly, it is apparent from this work that "perception - response time" is a highly variable quantity.



Indeed, situations arise in which motorists fail to perceive and respond to hazard at all. A number of collisions occur in which motorists during night hours and in bad weather, run their vehicles into the sides and backs of tractor semi-trailers which they have failed to detect until it was too late.

While side underrides represent only a modest proportion of these accidents, they are generally spectacular, and usually incapacitating or fatal injuries precipitate a great deal of investigation and litigation. Another interesting characteristic of side underrides is that the tractor is often fully across the roadway with its headlights facing approaching traffic while its trailer is straddling the oncoming lane. In many cases, the automobile does not leave skid marks prior to impact.

Why then, do motorists continue to drive rapidly into these trailers when their visual capacities are diminished during night hours and in bad weather? The minimal visual clues give the driver sufficient input to steer the vehicle and the motorist then relies on these clues and ignores the loss of their spatial resolution, stereoscopic depth perception, and contrast discrimination, which yields greater reaction time.



The following scenario may help account for this behaviour.

The motorist is probably driving along looking at the area illuminated by their vehicle's' headlights (approximately 22 feet ahead). Since their perception of a set of headlights down the road may or may not give clues as to that vehicle's size or movement, the driver continues onward.

Faulty depth perception and contrast sensitivity are factors at work during this period. As the vehicle approaches the truck and semi-trailer now across the road, the glare factor comes into play. The small low candle power side marker lights and/or reflectors are washed out by the headlights due to glare, loss of contrast sensitivity, and the oncoming driver's own tendency to look at the headlights of the truck. The maximum point of glare which obscures the trailer and the trailer side markers occurs at a point up to 165 feet from the truck. It the driver of the oncoming vehicle does not look away to the right edge of the pavement, the period of adaptation and recovery from the glare source will cause the driver to drive straight into the trailer, because the driver's eyes will not have recovered sufficiently to discriminate the side markers. All of this falls into the category of increased reaction time.



Increasingly, the evidence suggests that despite operating a vehicle prudently at night, certain hazards can arise and go undetected by a reasonably attentive driver. Often accidents of this nature imply driver fatigue, error or inattentiveness, which, in some cases, may be an unfair judgement.



Recent studies have shown that the use of reflective tape on trailers substantially reduces the number of these trailer underride collisions. The National Highway Safety Administration (NHSA) in the United States has recently published a rule marking in the Federal Register which requires trailer manufacturers to place reflective tape on the back and sides of all trailers manufactured for American truckers to enhance their conspicuity. The rule takes effect December 1, 1993.

Overall, NHSA predicts that enhanced visibility will reduce by 15%, accidents in which trailers are struck in the side or the rear. The estimated cost of applying this reflective material to a trailer is between $125 and $150 U.S. per trailer.

Transport Canada is currently considering similar amendments to its Motor Vehicle Safety Regulations. These amendments are anticipated to take effect by mid 1995.

This article is a synopsis of a paper presented at the Canadian Society for forensic Sciences Annual Conference, Ottawa, 1990. An update of the original paper is available on request.

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