The Prius doesn't do well on the highway. Note that it's EPA sticker is higher for city driving. That is due to two reasons, lack of engine stop/start out of city driving, and loss of electrical power efficiency at higher road speeds.
The GM/Chrysler/Benz/BMW two mode hybrids (only the GM is in production yet - Chrysler is following soon) swap the function of the drive and generator motors at highway speeds to minimize electrical losses. They end up almost even for highway / city driving fuel economy.
An excellent description of how the Prius system works can be found here
http://eahart.com/prius/psd/