That part's not really up to the driver, though.

It's the responsibility of the traffic engineers to make sure the yellow light is long enough to accommodate reasonable braking time and distance for all of the traffic that can be on the road, travelling at the posted speed limit. They can lower the speed limit, increase the yellow light duration, or put up a sign limiting the max vehicle weight.

It's not reasonable to expect every driver to know detailed information like yellow light times for every intersection and section of road.

In this particular road and crossing there is a sign with flashing lights indicating "PREPARE TO STOP WHEN FLASHING". It was flashing while the crossing light was still green to indicate that it would turn yellow pretty soon, but the driver still didn't slow down.

Unfortunately there is also a profit motive in yellow light times [1]. This can cloud the decision-making significantly. [1]http://www.chicagotribune.com/news/watchdog/redlight/ct-yell...

In my state, the minimum yellow light length is 4 seconds, but it's obviously not uniformly applied. You can get a device to time the yellow lights from the state if you get a ticket, and then use that info to dispute it. Not much help if there is an accident.

It's a big enough problem that there's a group trying to do something about it.

http://shortyellowlights.com/standards/

EDIT: according to the link, the yellow light for a 55 mph road should last 6 seconds.

Icy conditions are maybe predictable (ok, it's under 0°C outside), but yellow light times (edit: the time between the light turning yellow and the light turning red) are not.

Yellow light turns are predictable in that you know that they'll happen, just not when. So if you have a heavy load or other adverse conditions you should slow down to a speed where you could stop at the correct distance. Software should do an even better job of predicting the weight to speed to stopping distance ratios.

You don't know how long the yellow light will last before turning red. That time is variable from light to light and essentially unpredictable to drivers.

What is a safe speed to drive when you take into account that the yellow light may last from less than a second to 20 seconds?

but the problem is you don't know how long an yellow light will be. They are all different. State to state. Town to town. Hour to hour. No standardization.

Chicago was caught making their yellow lights purposely short on some intersections to make more money.