Yellow lights typically last for 1 second for every ten miles an hour of the posted speed limit. So if the posted speed limit at the intersection is 40 mph the yellow light will typically last for 4 seconds.

Several years ago I received a ticket for running a red light. I went back to time it and the light lasted for 3 seconds. I thought this was a bit too short. I then began to do research at that time. A national organization for traffic engineers recommended that lights be set for 5 seconds. I contacted the local traffic engineers office. They told me that they were in the process of setting signals around town for 5 seconds. In addition they told me that they were also in the process of setting signals so that the light for the opposing traffic would not turn green for two seconds after the light turns red. Reason, some folks jump the signals and actully go before the light turns green. I can not find any information on federal law regarding the length of timining on yellow lights