Now not having a car out here in Montana. Well, here in Helena there really isn't any public transportation. Not that people don't try. We just don't have the population density (although when the town started, we had streetcars on two routes!) We have a dial-a-ride minimum capacity bus that mainly moves around the elderly and the disabled. If you are patient and have a few days to spare to work out arrangements with them, you can get to most places you want to go within the city limits of this very small city.
Some younger and some older and fitter folks are bicycle enthusiasts, both city and trail, though we usually have a couple accidents every month and a death or two every year from collisions with autos and trucks. Of course most of those same folks use autos to go to the store or if they want to get out of the town limits or during the really cold icy times of the year. It's at least 90 miles over mountain highways to get to the next sizeable town, and if you want to go in between, say to a friend's or your ranch, you are SOL.
There's no passenger rail service. You would have to ride a Greyhound-type bus service that runs once a day from Helena to Great Falls (last I checked anyways) then wait a couple hours, then bus up to Big Sandy where the Amtrak connects Chicago and Seattle, and wait a few hours there too, if you want to see the wider world.
If you live outside city limits, including the valley where the city is located, or you are going hunting or fishing, well, most places hereabouts you can't get to without a car, unless you have a string of pack animals and a few weeks to spare.
As time goes on, and the economics and fuel changes, I speculate on what will happen around here.
-Some people will consolidate closer and within town. Property values within town, a 6 or so mile radius, will go up. Bicycles will be joined by tricycles and cargo-cycles. There is a business opportunity there for deep pockets and long range vision. They might even get streetcars going again, either pulled by horses (we had lots of horse and buggy rentals here 100 years ago) or run by electric power from the dams about 25 miles away. Downtown Helena, having a hard time finding tenants, will revive.
-People far away on ranches, 20 miles or more away, will return to living most their life on ranches, herding their livestock to the railheads. Only going into town on urgent business or to get supplies every month or two by wagon or pulling a tarp off an old pickup using ethanol they distilled themselves on rutted trails. The rich folk and celebrities who don't really ranch but just hire others to ranch for them and play rancher, they will have to make up their minds to get real, or go home to the cities they came from (if there is any decent place to return to). Their UPS and Fed-Ex vans driving 60 miles round trip to bring them their luxury goods will stop coming.
-The people in between the city limits and the ranches, they also will have to make up their minds. Some will migrate into town and scuffle with everyone else over living space. Some will go to outlying ranches if they have that sort of individual temperament. There will be a lot of empty houses in this "no-man's land" of suburbia, some becoming shelters for the very poor or those with no options, some becoming cattlesheds or storage barns, and that no-man's land that is irrigable and has suitable soil (not a lot of it is, though most can be some kind of haying country) could become farms, gardens, orchards. Of course the water table is lower now, and the weather more chaotic, and the snowpack undependable (and will get more so). The far-sighted should look at crops being grown in the southwest areas, and start experimenting and developing local hybrids.
But before we get there, there will be a lot of social chaos and stresses, violence and drugs of choice, in periods/cycles of readjustment over the next 20-50 years. Some will only relinquish TV and fast food and cars under the same condition as we would relinquish our guns...when you pry them from our cold dead hands. Migrants will come and others will go. And life will go on in some form or fashion. Looking even further down the road, a hundred or more years away, I think the Rocky Mountain states will eventually end up in some form of nomadic pastoralism like in Mongolia or Afghanistan, because that is the kind of country we have, high dry and cold. Of course climate is hard to predict, through nomadic pastoralism is a time-honored way of making a living from a hard-bitten land.
But if the caldera under Yellowstone goes, all bets are off of course :-)