Oh, do I know that scenario! I truly think it's one of the most vexing problems I faced when I was commuting: I didn't want to offend somebody who was obviously favorably disposed toward cyclists, yet I didn't want to break the law--and possibly cause an accident!--by accepting the well-meaning motorist's "gift." I think that out of the dozens of times I faced this dilemma, I may have gone ahead on the wave-through twice, maybe only once, and that was when there was no other car to complicate things.

It's times like those that make you wish you were carrying some pre-printed leaflets for various situations: some titles might be "Thanks but... (what you should know about vehicle right-of-way"; "Hey! We're vehicles, too"; "Paved roads were created for cyclists, not cars" (this last, for those who say, "Get on the sidewalk where you belong!" and is a reminder that the system of paved roads came about, historically, because of cyclists' demands--before the motor car was a twinkle in Daimler's eye, or whoever invented the damn thing).