Applying an accurate estimate of the delay to the expected reaction of a driver to a collision situation is an extremely important part of a collision reconstruction. Documenting driver responses to traffic signals can provide useful research data for that purpose.

Gorski Consulting has been involved in a two-year study of driver reactions to the activation of a green traffic signal. In that time 1010 observations were made, predominantly in the City of London, Ontario, Canada. The observations were made using a dash camera that was activated while stopped behind vehicles at a traffic signal as in the example shown above. The video segments were analysed to determine when the green signal became activated and how long of a delay occurred before the brake light was extinguished on a stopped vehicle.

The chart shown below is a frequency count of the number of observations, at 1/10 second intervals, from zero to 1.2 seconds delay. The “P/R” refers to “Perception Response” delay.

The above chart accounts for only 799 of the 1010 observations. The remaining observations fell outside of the chart’s range and that is an important finding.

In 236 of the 1010 observations the response delay was equal to or greater than 1.0 seconds. Conversely, in 234 of the 1010 observations the response delay was less than 0.4 seconds.

Stated another way, about 63 percent of the responses were between 0.4 and 1.2 seconds. The remaining 37 percent were either shorter or longer than this range.

It can be fairly certain that a typical driver should not be able to release a brake pedal in less than 0.4 seconds after the activation of a green signal. Therefore the 236 observations where the delay was shorter than 0.4 seconds suggests that about 24 percent of observed drivers were likely releasing their brake pedal before the traffic signal turned green. In fact the actual percentage was larger because in a number of instances vehicles were observed to be crawling forward even though the brake lights were still activated. Thus drivers were releasing their brake pedals just slightly but not enough to extinguish the brake lights.