Throw one of these on the back of your car and pulse and glide up to 25mph and then pull over once in a while so as to not impede traffic.
Actually, a lot of highways and streets do not have a lower speed limit. You can go as slow as you want as long as you are not impeding traffic. Of course it may be difficult to pull off hypermiler techniques on a busy street and not be considered impeding traffic.
The type of vehicle will also change the rules. E-bike anyone??