The changing economy of fuel expense is driving many people out of ownership. New car purchases have recently completely collapsed! Bike sales meanwhile have surged wildly. As cars can no longer be afforded, or are retired, or even simply usage reduced, the ratio of bikes and other people powered travel (as well as trains and busses) will increase.

It is not unreasonable to imagine that last year alone saw at least a 1% conversion from car to bike traffic. I've heard numerous news stories relating specific observations in bike sales increases and reduced car traffic growth. I've seen I would guess a tripling of the number of bike related stories in local print including The Reader, New City, and Red Eye. And of course there was the record setting 2008 Chicago Naked Ride, which TRIPLED prior year turnout

I'm not enough of a futurist to predict the exact time table, but for comparison I'll point out that numerous experts beyond reproach have converging estimates of extraordinary climatic changes possibly leading to complete loss of ice caps literally within 50 years. We're talking chunks the size of small New England states falling off every year or two. These are unprecedented times. How trivial in comparison (though chicken vs. egg) will be the passing of the brief auto epoch over the same period.

As a Cassandra I expect blithe dismissal of this vision, but I know what will be and am often right on such meta topics. Over decades of awareness, I've seen countless trends and technologies appear, on the horizon ignored by most, come over years to be norm. From microwaves to multi-touch I've watched the science of such things from academic demos, to in everyone's kitchen and pocket.
With such certainty I now assure you, after thousands of hours of consideration on this specific topic, that IF the species survives (yes, I went there), bikes as well as walking, skates, mass transit, telecommuting and changed expectations will become the new norm. I now take this as a given.
As surely as electricity and radio, computers and satellites, plastics and antibiotics, flight and cars, the seemingly outrageous, absurd, and far fringe has a way of becoming reality quite suddenly. As surely as shoe sizing x-ray machines, smoking and trans-fats the down side of the car fad has become gravely visible. As surely as polio, horse traffic, and even slavery (yes I went there), the norms of now will be absurd memories a hundred years hence.

As individual gasoline powered vehicles increasingly become a personal economic impossibility, I think it's not unreasonable to expect a 1% change per year minimum continuing, hopefully accelerating. Let's imagine the transition over coming decades. Imagine a 10% changeover in ten years. Literally one in ten less cars and many many more happy cyclists. You can imagine that right? Looks like a typical spring/summer day here near the lakefront, and ten years is a while. What's happened to the drivers? They're walking, or biking, or busses, or just staying put, hanging with friends, enjoying on demand videos, and CALLING their parents out of state.
Now imagine a 25% changeover. Not only could it happen, it MUST happen. 20 years is a serious chunk of dynamic urban time. Fully one quarter less drivers, and maybe a simple doubling or tripling of happy cyclists, in 20 years? Easy. Looks like a pleasant European scene, with cleaner air and friendly people. Where are the drivers? They've joined the trend. Vastly changed expectations are everywhere, reinforced in media, pop culture, government programs and youth awareness, and on viral mobile 3d vids.
Now imagine a 99% reduction in internal combustion engine powered vehicles... in under 50 years! Too much? OK, you seem nice, for you, a 75% reduction in 50 years, and the remainder are low emission. THINK how much happens here every decade. In fact, incredibly by the statistics that's actually just a roll-back to a mere 40 years ago. Just give it some enabling changes like improved bikes, free and rental systems, and street use conversions, and geometric growth WILL happen. As responsibility becomes a core human value world-wide, humility will be the new ostentation.
Looking back, the overall rate of conversion TO nearly 90% cars is a modern phenomenum. Remember, the product is only a smidgen over 100. Now IMAGINE 100 years hence... Why not 90% less cars? Aim for perfection, achieve excellence I always say.

Though these visions of low-car decades may seem optimistic, they're reasonable if car retirement begins now, which it has. And I have observed that though humans are poor at long term risk management, people do change when actually nose to nose with crisis.
By a mere half century speculation becomes hazy, BUT I'll tell you this, if we haven't transitioned on the 99% scale off of burning ancient blood of the sun at the altar of wanderlust, than we will have committed species suicide (yes, I went there).

Frankly, I consider it not unreasonable to see the automobile as the largest big business rape of the people in history. A multi-fronted profit maximization cancer, which so knowingly gained from irresponsibility and manipulation, that I don't see it unreasonable to hold the industry accountable for crimes on a global scale. Bailout my ass. But that's another story.