if you aren't running the AC or heater, an electric sitting idle won't use any of its remaining energy reserves unlike an ICE. plus, an electric crawling along in stop and go traffic doesn't use much power either. funny thing for me is that going 70mph chews up battery faster than than when i'm driving only 15mph.
There are quite a few vehicles with stop/start systems, some even non-hybrids, which really helps. It's not exclusive to electrics, and I feel a hybrid is really the best of both worlds.
Getting a bit off topic here, but:
Most companies have settled on ~1.8-1.9L for their hybrids because that's slightly more than just the right amount of engine for carrying the vehicle along completely without hybrid assist at optimal efficiency at highway speeds (which will be at ~2000-2500RPM at 60-80% load), leaving the hybrid system to make them not complete slugs to drive otherwise. One could conceivably use something much smaller, dependent on vehicle size and aerodynamics; a compact ~0.3L turbo "range extender" could push a Tesla along the highway indefinitely, but after a certain point, shrinking the engine means you're having to rev and boost more and you're going to end up moving out of the peak efficiency window. If you rely completely on an electric motor to push a car along, a ~1.5L designed to operate only at a fixed RPM and load could actually be quite small and extremely efficient, since you could get rid of throttle plate, variable valve and timing shenanigans, variable intake and exhaust geometry, etc., and just toggle it on and off when needed. Of course, you lose something in converting from mechanical energy to electrical, and then back again, so there's a case for keeping all of that baggage and just connecting the engine to the wheels sometimes (as they do in the Volt), but I suspect it's probably not worth the added bulk and complexity once the battery technology is there.
Most vehicles hit peak gas mileage at around 35mph, due to a combination of gearing and aerodynamics. Even if you're getting every lat bit of efficiency out of a gasoline engine, there's still aerodynamic and rolling resistance to contend with, which are pretty much fixed energy requirements. Only smaller and sleeker vehicles are going to get too much above what a Prius already gets on the highway. You can sacrifice more ride quality (noise and vibration) for higher tire pressure and different tire compounds to lower rolling resistance, or cut the height and width of the car down to reduce air resistance, but cutting weight isn't really much of a factor once you're actually moving, and highly efficient regenerative braking generally gets a fair amount of that power back. Less battery weight won't help much with efficiency.