I came to the U.S. from France, on the Visa Waiver Program. A friend of mine recently joined me here, and is suggesting we do a road trip across the United States. I would so love to join him, but my 90-day stay will be up soon, and I understand that I am not allowed to apply for an extension. What about if I took a trip to Canada, and then came back: Would that start me off with a new 90-day period of stay?
Clever idea, but unfortunately for your road-trip plans, the U.S. government thought of it before you.
If, after entering on the Visa Waiver Program (VWP) you leave the U.S. for "contiguous territory" (Canada or Mexico), the immigration regulations will allow you back in for "the balance of [your] original Visa Waiver Pilot Program admission period." (That's assuming that you're still eligible to be let into the U.S. at all -- that is, that you are otherwise admissible and meet all the same conditions as for your first VWP entry, such as being financially solvent and having a return ticket home, with the exception of the requirement that you arrive on an authorized, signatory carrier.) (See 8 C.F.R. Section 217.3.)
Legally speaking, you will not be seen as having made a true exit or reentry, but only an "authorized departure" under the program.
So, for example, if you have already spent 60 days in the U.S., then leave and return after ten days, you will be allowed another 20 days to finish out your VWP stay. Your ten days outside the U.S. will be counted against the total 90 days.
Let's hope your friend doesn't want to stop the car at too many scenic vistas!