Comment Re:Law and Regulation? (Score 1) 433
Technically speaking, it should be based on observed speed of people traveling on the road. However, standards have been weakened over time such that yellow light timing can be based on the speed limit rather than real-world speeds.
The 1994 ITE "Determining Vehicle Signal Change and Clearance Interval" states:
When the percentage of vehicles that entered on a red indication exceeds that which is locally acceptable, the yellow change interval may be lengthened (or shortened) until the percentage conforms to local standards, or enforcement can be used instead.
There's a better analysis of how signal timing standards have been changed in the link.