Down sides.
- You annoy me when you don’t get moving quickly when the light changes.
- Added wear on your starter, battery, and alternator.
- Spousal discontent.
Saving gas?
Probably not. It takes more gas to start an engine than while it is running. It will also have to work a little harder to recharge the battery. Clearly, there is some break even point for your specific car where the overall gas consumption will be less, and it is dependent on how long the engine is off. I don’t know where that point is, and it might be as little as the one minute that you are stopped.