The biggest problem with knowing whether to cut the engine or not at a red light is knowing how long the light has been red, how long it's going to stay red, and knowing how soon it will change to green. All of that could be handled by technology widely used in China (and maybe elsewhere) and rarely in the US: countdown timers at traffic lights. Coming up to a light that's counting down from 80-79-78 would let you know it's worthwhile to cut the engine, and if you know your car, then you can wait til the timer hits 8-7-6-5 (or whatever your car needs) and then hit the starter, having everything ready to go as the timer hits 2-1-0. At intersections with especially long lights-- and usually heavy traffic to accompany them-- this would promote considerable fuel/CO2/smog reductions. Who decides where to install these timers? I don't know. But if they came on board, then that would go a long way towards reducing idle emissions, saving money, the environment and so forth....

Countdown timers are getting to be more common on pedestrian crosswalk lights. I often watch them (on the cross-street) to know how long I'm going to wait. Also very convenient for attentive cyclists for many reasons.

another noticable improvement that is common in China is the fact that the lights are coordinated and stay green in one direction longer. I noticed in both Taipei and Shanghai that the lights would all be green for north-south for several minutes, then all be green for east-west and this coordination was accomplished simply by making the green-red cycle a little longer as well as some planning for common traffic flow patterns. For high traffic areas in the US, the same approach would be a boon to traffic flow.

While living in Switzerland, we always cut the engine at stoplights. About 5 seconds before the light turned green, the yellow light came on (with the red light still on) indicating that it was time to start up the engine. This seemed to be a common, if not required, practice in many parts of the country.