You will still need energy to wind the spring. You could maybe use the spring for regenerative braking, this idea has been mentioned here before. The best way to save idle energy is to turn off the engine.

I've thought about this one and it's been mentioned before. Of course we all know that spring motors power wind-up toys, but they don't go very far. I think the most recent mention associated it with winding the spring with wind power. As I see it, the main disadvanges of a spring motor are first that they don't store much energy (won't go far) and they require lots of stiff structure to hold them.

So I occurred to me that a spring motor completely installed in the roadway might beat both of these problems. When a car needs to stop at a normal stopping location (a light for instance), you'll put a spring in the road and the car will deploy a heavy rod from the center of gravity. The spring would stop the car, it would index, then when the light turned green it would release the same energy to begin the acceleration. It would require damping so you weren't launched like a catapult, but the idea might be interesting. Probably work better for a roller coaster or a train than for an auto.