And more emissions. Idling is pretty efficent, as is driving at a constant speed. Repeatedly stopping or slowing, then accelerating is not. This is also an unintended consequence of "traffic calming" devices e.g. speed bumps or chicanes. People slow down, then hit the gas again which is awful for emissions.

I’ve sometimes pondered if a traffic calming device could be made which would allow vehicles to pass unimpeded if they are at or below the speed limit, but subject to an increasingly large bump if they exceed it. The problem, I suppose is that it must be extremely robust which would make it expensive and potentially more complex than a simple passive bump on the road.