Considering that idle for Volt won't be like idle for a normal ICE vehicle I don't imagine heavy traffic or sitting time to affect its range. Consider the amount of torque required for the motor to generate to get the vehicle moving, then the amount needed to say run the gauge clusters, climate control, Nav, its definitely nowhere near. How the vehicle actually handles the power requirements of varying loads is still a mystery, so its speculation.
Also I see you mentioning a two seater, the Volt is a mid-size(?) sedan and with the range extending gas engine it can go as far as you want. From other blogs I've read that the Volt still achieves 50mpg when running exclusively on gasoline (no prior charge) so that's still rather impressive I think at least.