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A necessary component of any crash avoidance or stopping distance calculation is the perception-response time. Courts have excluded the testimony of experts who used generalized average response times. Throughout history, reaction time research demonstrated that response times changed based upon probability of the event and other factors.  This research will examine how drivers’ response times change based upon the comparative probability of the event.  The goal of the research is to determine the average and distribution of response times for several common path intrusion events.

After an analysis of the related literature, four primary factors were found to be associated with drivers’ response times when responding to path intrusion events.  1. Comparative probability of the event was correlated with the response time. 2. Measured response times were longer when the onset (i.e., start of the measurement) was earlier in the event, and shorter when the onset was later in the event 3. The difference between brake response time (i.e., when braking occurred) and perception-response time (i.e., when hard braking or sharp steering began) was approximately 0.3 seconds. 4. For events other than very common and very rare, the coefficient of variation, which is the standard deviation divided by the mean was within 0.4 second.  There was a wider distribution of response times for shorter BRT events such as cut-offs, sudden appearance from behind obstacles, and longer BRT events such as head-ons (SD/Mean = 0.54).

This paper will identify common mistakes made by crash investigators and then describe a classical scientific approach that meets the requirements of any court and eliminates bias from the analysis. The classical approach entails comparing the pre-impact response of a driver against the range of typical driver and rider responses in research when faced with a similar response scenario.