Gas could be $8 a gallon and for the most part everybody in the US who has positive cash flow would still be buying exactly the same amount of it. Maybe teenagers would go back to riding bikes.
Never mind just teenagers. For distances of about 5 miles or less, if you're not carrying much cargo, a bike isn't a bad way to get around. That's doubly true in large cities. I make as good or better time on most of my bike trips than you would by car. And I have at times gone 20 miles each way.
Velomobiles and dedicated infrastructure can make human-powered transport practical out to 15-20 miles at average speeds exceeding 30, even 40 mph, with properly streamlined vehicles. In fact, since most car trips are under 20 miles, velomobile can handily replace them. And you can stick in a small electric motor for those unable to keep up with stronger riders.
Upon consideration, $8/gallon gas is probably a good step toward jtr's utopia.
Maybe. My guess is if such prices continue long-term it'll simply mean most people switching from gas cars to electric cars. Although EVs solve my single biggest complaint about cars, namely air pollution, they're still a car which requires wasting lots of space for roads and parking lots. And they'll still kill and maim at the same unacceptable rate as gas cars. Honestly, assuming we had the infrastructure, I'm having a hard time seeing that cars are needed at all for most people. Basically, the best passenger modes for various distances are as follows:
0-1 miles: walk
1-5 miles: bicycle, motorized bicycle/tricycle for those unable to pedal, walk
5-20 miles: velomobile, subway, bus, bike if you're fit enough
20-50 miles: commuter rail, possibly velomobile
50 to 1000 miles: high-speed rail
1000+ miles: evacuated tube maglev
Naturally some of these modes can be combined. For example, if you're traveling intercontinental distances you probably won't have an evacuated tube maglev station next door. It might be something like bike to subway station, take subway to high-speed rail station, take high-speed rail to evacuated tube maglev station. Do the reverse at your destination. Still faster overall than flying. And the other modes are faster overall for their respective distances than what we do today, which is usually drive.
That takes care of passenger transport. Cargo is another thing. Here you'll probably still need trucks to get most cargo the last few miles. For most of the distance though freight rail or water is the most cost-effective way to move cargo. I'm not seeing much need for intercity trucking.
Basically then when you consider everything motor vehicles aren't needed at all for long distances, and neither is long-distance motor vehicle infrastructure. You basically go to a mix of delivery trucks and emergency vehicles, all on local streets only. The Interstate Highway System can be converted to a combination of high-speed rail, with maybe two lanes left for dedicated velomobile tracks for those adventurous enough to take velomobiles on cross-country trips.
Not a utopia by any stretch, but much better and more balanced IMO than what we have today, which is a choice between driving (bad), or flying (worse).