The Delaware-DOT reported that the amber-light timing is usually three or four seconds, but this can change with traffic flow and the time of day. Virginia-DOT informed me the amber light timing is based on traffic speed; i.e., traffic that moves at 55 mph will result in amber-light 4-second intervals; at 35 mph in three-second intervals. New York State-DOT amber-light timing is based on the same data. The amber light is usually illuminated for four seconds, but traffic speed and road conditions such as curves, hills, obstructed vision, and inclement weather conditions have different dynamics for large trucks than four-wheel passenger vehicles. These factors also must be considered in the analysis of traffic that enters and exits intersections.