The Infrastructurist links to and summarizes a study in World Transport Policy and Practice by Peter Newman and Jeff Kenworthy positing that driving in the US has hit a peak and is now in decline. The study’s contribution is not the trend, which has already been identified and described in previous research, e.g. by Todd Litman at the Victoria Transport Policy Institute, but some explanations for why it’s happening. Newman and Kenworthy are attributing the decline in driving to six causes:

1. The Marchetti Wall. People budget a constant amount of time for travel, about 60-90 minutes per day (Marchetti’s research suggests 60), and respond to faster travel by living farther from where they work or taking more trips. Research has confirmed this not only within developed countries, but also in rural Africa; for a lit review, read section 2.2 of Gary Barnes’ article on density.

The Marchetti Wall refers to the fact that today’s American cities have sprawled to the maximum range of this constant travel time at automobile speed; therefore, infill is required to add more density. Exurbanization is still happening – the fastest growth in US cities today is in the gentrified center and in the exurbs – but is often low-income, i.e. people are violating the wall’s limit because they can’t afford any better. This is also noted in a 2006 study by Steven Polzin predicting stagnant driving: commute times in the US have been inching up, which suggests current development trends are about to hit a wall.

2. Public transportation growth. Public transit ridership in the US bottomed in the early 1990s and has been increasing faster than population since; for one famous example, New York’s annual subway ridership went from 1 billion in 1990 to about 1.6 billion today.

Newman and Kenworthy posit further that since the empirical relationship between driving and transit ridership is exponential, small increases in transit ridership will produce large decreases in driving; I’m skeptical, since the causality seems to go the other way (very low transit use requires vast sprawl, raising the amount of driving), but the increase in transit traffic can still produce measurable decreases in driving.

3. The reversal of urban sprawl. Standard, unweighted densities of many US urban areas stopped declining or are even rising. Newman and Kenworthy don’t mention this, but some cities outgrow their suburbs – e.g. New York and San Francisco in the US (as usual, going by more accurate ACS data rather than the failed Census), as well as some European cities. Exurbs are still growing quickly, but those are a small proportion of population; they can’t account for much.

4. The aging of cities. As the population is getting older, people are driving less. A related trend, again one not mentioned in the study, is the reduction in the proportion of households with children, making the usual crime and school concerns of the American middle class less relevant. The study does mention empty nesters as a related trend, in point #5.

5. The growth of a culture of urbanism. This is an issue Litman touches on as well – young people are less enthusiastic about driving today than they were in the 1960s, and are getting licenses at lower rates. Newman and Kenworthy cite previous literature about the rise of urban coolness, what I would call the culture of urban romanticism; they compare the popularity of Friends in the 1990s with that of Father Knows Best in the 1940s and 50s.

6. Rising fuel prices. Those make driving more expensive (especially since fuel prices are visibly and directly proportional to the amount of driving, unlike more fixed costs of driving), and reduces the desirability of exurban housing. This is attested in previous studies in the 1970s. Although there’s a return to normal after fuel prices come down, the general consensus is that the oil prices of the future is going to be much higher than that of the 1990s, if perhaps not the $200 per barrel predicted by Matthew Simmons. In other words, the new normal is not the same as the old normal, and this is going to be reflected in reduced driving.

Rising fuel prices are probably the most important of the six. The reason is that even outside urban areas, it’s possible for an auto-oriented region to require much less driving than is standard in suburban America. Raise fuel prices to European levels, and the result could well look like France, where most people outside Greater Paris and Lyon drive, but the distances driven are shorter (13,000 km per car, vs. nearly 20,000 for cars and light trucks in the US), and the cars are much more fuel-efficient. Provincial France is not livable or urban – on the contrary, it resembles suburban America in many ways, complete with hypermarkets; the largest chain, Carrefour, is the second largest retailer in the world after Wal-Mart. Urbanism is for the rich, as is increasingly the trend in the US.

Newman and Kenworthy do not mention the social issues coming from this new urbanization, in which the cities are for the rich. But they do have pointed suggestions to planners for how to deal with a future in which fewer people drive: plan cities and engineer traffic for more pedestrian- and transit-friendliness, do not assume more road capacity will be needed, finance more urban construction, do not treat cars as a perfect proxy for economic growth. In short, do not continue to act as if the regime is still that what’s good for General Motors is good for the USA and vice versa; driving is entering decline, and cars are just one consumer good among many.