"Behaving with a degree of propriety has been on the wane for the past several decades."
Amusing answer ;-) I think that trust will be the biggest factor initially, and having 1 Google Car per 10,000 on the roads makes its behavior an oddity and thus a target. Make it 1 in 10, and now it starts changing road behavior in bulk.
It's a bit like these cars that can travel in convoy on highways using computer-controlled adaptive cruise control. Hooray, so long as every car involved supports that technology. It's a bit tricker when you have to mix the old and the new in the same place.