Answer: People will drive 55mph only if it is in their financial best-interests to do that. Otherwise, they'll continue to drive whatever speed they're driving.
Look, we've been through this before, folks. 55mph didn't work the first time -- people just ignored it, other than to make the creators of radar detectors rich and create an entire movie industry around truckers using CB radios to pinpoint where cops were running radar traps. There's a saying in the military that they teach to fresh NCO's and junior officers, "never give an order that you know will be ignored." It just makes you look stupid. Same deal with laws. If you pass a law that you know will be ignored by the majority of people, it makes you look even more stupid, as well as undemocratic -- such a law, by definition, by ignoring the will of the majority of the people, makes the place look like some kinda Commie hellhole rather than a free nation. Hmm...
Anywho... folks will drive 55mph when the gas they save costs more than the time they save from driving faster. Right now, we're not there yet. Maybe when gas is $10 a gallon, we will be. That's just how it works in the real world, rather than in the fantasy land of lawmakers who think passing a law is the end of the job. A law which cannot be enforced is a law that is useless, and our lawmakers should feel ashamed of themselves for even proposing such nonsense.
-- Badtux the Law Penguin