The rapid expansion of American suburbs based on easy highway access took place in the 1950s and 1960s, and largely coincided with the shift from Eisenhower-era Republican conformity toward the Johnson Great Society era, which saw the greatest Democratic majorities in the U.S. since the New Deal.  Those suburban regions, traditionally staunchly Republican (having fled from the inner cities), have themselves been trending more and more Democratic of late.  Indeed, it is now the most fertile ground for Democratic pickups in House and state/local races.

Urban dwellers are intimately aware of the high costs associated with sophisticated infrastructure development and maintenance, and are thus more inclined to be accepting of higher taxes required for those services.  Rural and suburban residents are often ignorant of the degree to which their lifestyles are still dependent upon large government spending.

Many Americans fled the cities in search of lower taxes and "safer" neighborhoods; for more than a generation, the GOP has thrived on law & order and tax reduction (Nixon's "Silent Majority" and Reagan's supply-siders).  In a more general sense, the urban vs. suburban/rural political dynamic has been around for well over a century, both in the U.S. and in Europe, long before automobiles even existed.

At least in the U.S., it would be hard to claim that the cost of car-ownership has become cheaper, certainly not over the past 30 years or so.  The cost of an average vehicle is much larger relative to income now than it was a generation ago (one of the reasons why leasing has become so endemic).  While I can't readily get the figures for as far back as I'd like, the average expenditures for transportation among urban residents rose 74.4% from 1984-2003, while it increased by 123.1% for rural residents.  (Source: BLS data for transportation, type of area, urban and rural.)

We could go on and on.  While there's definitely a degree of correlation between driving and libertarianism, I would doubt very strongly that the former causes the latter.  If, say, a columnist alleged causation between the proportion of darker-skinned people and crime in a community, he/she would rightly be denounced as an unreformed racist.  One can dress it up however one pleases, but it amounts to little more than anecdotal pop psychology.

(If I sound cranky, my apologies.  A large amount of work descended unexpectedly late this afternoon, and I'm dreading the consequences of it for the next couple of weeks.)