[tialaramex]

A sensible way for things to work is that traffic engineers interested in safety and flowing traffic work out a rule for how yellow lights work, and that's the rule for the entire country.

Usually your goal is that the yellow lasts long enough that any vehicle which isn't speeding and has brakes at least as good as those required by law, will either pass through the junction while it's still yellow or could easily stop before it goes red. Since even though some drivers will be speeding, anyone who isn't driving a way out-of-specification heap of junk has _much_ better brakes than required by law, this gives plenty of slack. The engineers need to know speed limits on approach roads, the size of the junction (remember, pass _through_ the junction, it's no good if your fast vehicle is still in the middle of the junction when it goes red) and slopes (which affect braking) and various other factors.

However the US is not, on the whole, a country where the sensible way of doing things has much sway. I'd be astonished if there's a Federal Law in the US that sets usable requirements for their whole country, and frankly it wouldn't surprise me if individual towns get to have local politicians just pick a number out of a hat for how long the yellow lasts for the one junction in that town.