AP

We're attracted to the stability of bus and rail fares — not just their lower cost.

Americans made nearly 10.7 billion trips on public transit in 2013, the country's highest ridership point in more than half a century. The mark is largely the result of increased investments: bigger systems in some cities, more off-peak service in others. We can and should discuss whether the public costs of these projects matched the ridership return, but the trend is at a minimum in the right direction.

At the end of the day, though, improving trains and buses alone can only attract so many riders. The bigger changes in travel mode won't occur until local governments pair such transit incentives with automobile disincentives. The latter involves removing the social discounts that encourage driving — chief among them, an artificially low gas tax that doesn't cover the cost of our roads.

We catch a glimpse of this potential shift from road to rail whenever gas prices spike. Bradley Lane of the University of Texas at El Paso recently showed that a 10 percent increase in fuel cost led to bumps of 4 percent in bus ridership and 8 percent on rail. That trend even held true in heavily car-reliant cities with traditionally poor transit systems, like Omaha, Des Moines, Kansas City, and Indianapolis (below, the bus bumps):

But gas prices as posted in big numbers high above a filling station aren't the only thing with the power to influence our perceptions about the cost of driving, and thus our travel behaviors. Turns out the volatility of fuel costs matters, too. Every price swing at the pump makes it harder to estimate a reliable cost of a regular commute and form a dependable budget for household car expenses.

And as planning professor Michael Smart of Rutgers shows in a new analysis, gas prices have been nothing if not volatile in recent years:

The solid top line of the chart shows real gas prices (in 2012 money) since 1979. The dotted and light lines, meanwhile, show fluctuations in those prices over 6- and 12-month windows. The massive leap that those lines have made in recent years indicates gas price volatility. In more practical terms, these lines reflect how unreliable the costs of fuel — and thus of driving — have become.

For his research, Smart paired this volatility with surveys on support for public investment in mass transit since 1984. His models revealed a connection between this support (as shown through agreement with a statement that "We are spending too little on mass transportation") and the variance in gas prices. Meanwhile, the models found no such link between support and real gas price.

So it wasn't cost per gallon that got us thinking more positively about public transit, it was the dizzying leaps and dives. When gas volatility was low, respondents displayed a 37 percent likelihood of stating support for transit investment. When volatility was high, that likelihood grew to 46 percent:

What's happening, Smart believes, is that rapid fuel-cost fluctuations inspire support for transit as a means to buffer price shocks in the future. Drivers might not head for the train or the bus at the first sign of wild gas price swings, but they take some comfort in knowing that if the swings ever get too wild, they could. With that in mind, Smart encourages public officials trying to drum up support for transit to highlight the stability of transit fares, not just their low cost.

Now for the caveats. This research tracked stated support for transit investment on a survey — not actual ridership swings, not even a call to a local representative. In that sense, even the highest support figure seems a bit low; to paraphrase the famous Onion headline, just about everyone favors public transit for others.

Still, the larger point about transit incentives and car disincentives remains. Cities can and should invest in balanced transport networks with high-quality alternatives to driving. But until the cost of taking a car becomes much higher or much more unstable, those efforts are likely to go largely unnoticed.

Top image: High school senior, Kaylen Gordon, 18, left, waits for a Metro Rail light train on the Gold Line at Union Station in Los Angeles Monday, March 10, 2014. (AP Photo/Damian Dovarganes)

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