My honest opinion is that it will take gas prices at double of what we have today (7 bucks+) before one can actually notice less vehicles on the roadway. For many, there are many other things in there life to cut back on like eating out, smoking, chewing, drinking, etc. Each one of us who are not independently rich will have to set priorities and make cutbacks where needed.
I still think that the scooter thing will catch on like wild fire in this country at some point. I mean, you can buy scooters 5-600cc that get 50+ mpg and that'll do 100 mph for around 6 grand. If one is commuting to work (not buying groceries or needing to haul anything) the scooter represents a great opportunity to minimize the effect of high gas prices.