But by the same token there's not a lot of weight to accelerate either, right? So while a moving bike has much less potential energy to recoup than a car, it needs much less to get back going again also.
So IMHO regen on a bike should be as useful as it is on a car, no?
Right, most of the energy is used pushing air out of the way, which you don't get back from regen. Say you ride half a mile before stopping, the time to accelerate to 20mph is pretty small compared to the time you spend cruising at 20mph pushing all that air out of the way.
That's certainly the case once you're at speed (above 20km/hr is about the point where more energy is spent overcoming wind resistenace iirc). But I'm taking about taking off from the lights. Even to get you to 10 km/hr for free (or cheap) would be a bit boost for a commuter.