After an accident, witness sometimes give distance estimates in car lengths. Kineticorp engineers wanted to measure the accuracy of witness distance estimates. Volunteers gave distance estimates as they were driving their car. We used a range finder to measure the actual distance allowing us to compare their estimates to the actual distance. This gave us a better sense of the accuracy or inaccuracy of a witness distance estimate.
Accident reconstructionists generally agree that eye witness estimates of distance are often inaccurate. Robins , for instance, observes that “in spite of the many converging cues available to humans to judge the separation distances and speeds of other objects, we perform poorly at such tasks.” Similarly, Olson  observes that “witnesses’ judgements of distance in the usual measuring units are likely to be unreliable.” Despite the limitations of humans to judge distance, witnesses and drivers involved in car accidents are often asked to use the unit of a “car length” to estimate how far they were from another vehicle at some point prior to a collision. There seems to be some presumption that reformulating the question about distance in terms of “car lengths” somehow improves a witnesses’ or drivers’ ability to judge distance. Law enforcement officers and attorneys, in turn, sometimes use a witnesses’ or drivers’ distance estimates in terms of car lengths to reach conclusions about why an accident occurred or to evaluate whether or not a driver should have been able to avoid an accident. Accident reconstructionists also sometimes use these distance estimates to perform calculations related to following distance or accident avoidance.
Contact Neal Carter for more information regarding this research and publication: [email protected]