Should I turn off my car engine at the signal or keep it running? Assume 90 seconds waiting time.?

you definately want to leave your engine on. every start up is crucial to your cars engine. If your looking to save gas... check out this site it gives you 30 ways to save gas.

http://www.howtoadvice.com/SavingGas

For a wait of only 90 seconds, in traffic that's flowing normally (not a jam or stoppage due to contruction, accident, etc.) keep it running, but put it in park and let it idle. If you know you're going to be waiting in one place for several minutes, shut it off. (Just as starting your car every couple of minutes will wear out your starter and drain your battery, regularly idling for long periods of time does more damage than just burning gas unnecessarily.)

Drive with your windows up, as much as possible and whenever the weather is such that you can stand to use the vents on full warm or full cool instead of using the heat or AC, do it.

Keep it running- starting your car takes more gas than running it for 90 seconds.

I suggest to keep it running. Re-starting your car too often might do some damage to your car most especially your engine starter.

I do only because my car likes to over heat, dang radiator. I have always heard it take more gas if you keep restarting you car. That could be myth or fact, I don't know.

keep it running...to "save" gas, you should shut if off in drive-up windows with long lines, run a/c instead of windows down

Keep it running....starting an ignition is hard on cars.