Amy Finkelstein, an economist at MIT, has found evidence of what many people suspect - highway tolls increase when enough motorists use electronic tolling (her study puts the critical fraction of EZ-Pass holders at 60% of motorists, and the resulting increase in tolls at 20-40%.) The likely explanation, she finds, is that it is politically easier to raise tolls when fewer people are aware of their value, and EZ-Pass holders don't pay as much attention to the toll prices as people who are forced to hand over cash for every trip (think about people's attitudes at the gas pump, which Dan Ariely analyzes in this July 2008 op-ed for the New York Times and in his book Predictably Irrational). Finkelstein's technical paper is available here.