A selection of 10 non-Atlantic long pieces on cities, all great reads.

4 August 2012 1 Permalink

Searching for authoritative numbers on how much of urban space is devoted to cars, I found this gem by Manville and Shoup, People, Parking, And Cities. The authors debunk the numbers bandied about by many — two thirds of LA is devoted to car use, etc. — as being undocumented if you follow the trail of citations. They found that Meyer and Gómez-Ibáñez (1983) had proposed an inverse relation between the share of land in streets and the share of land in streets per person, based on 1960 data:

Automobile use does not result in an exceptional percentage of land being given to transportation purposes. Rather, the automobile seems to create exceptional demands for transportation land relative to the number of people in an urban area. Specifically, cities more dependent on the automobile tend to have more street acreage per person but a smaller percentage of total land in streets.

Basically, larger lots leads to low population density, but more importantly, as the car has become dominant in transportation the cities are designed for cars and not for people:

People, Parking, And Cities - Michael Manville and Donald Shoup

Given these results, how can we account for the perception that low-density areas give more of their land to streets? Certainly people tend to associate lower density with increased automobile use, and automobile use with streets. The first of these associa- tions, as we have seen, is more complicated than a simple one- way relationship, but the second may increasingly be true. The association between low density and auto-oriented land use, in other words, may lie less in the share of land given over to streets, and more in the share of streets given over to cars.

The modernist street designs identified by Southworth and Owens (1993) consume less total land area than the dense grids that preceded them, but broad boulevards and cul-de-sacs are also streets whose primary purpose—and perhaps sole purpose—is the swift and safe movement of automobiles. The desire in newer areas to accommodate the car has often led to the removal of other uses from roads and streets. Cul-de-sacs, which force more circuitous routes and have a notoriously limited utility for pedestrians, have been promoted. Intersections, which slow traffic or cause it to stop—but which make streets more amenable to walking—have been minimized. Those intersections that get built are made wider, allowing cars to turn with less deceleration but forcing pedestrians to traverse more road space (Southworth and Ben-Joseph 1996).

Where older intersections often have a curb radius of 3–4 ft, newer intersections flare out: It is not uncommon for zoning laws to call for 15 or 20 ft curb radii. The 9 ft travel lanes of older neighborhoods were replaced in newer developments by 11 and 12 ft lanes, and parking lanes are recommended to be wider still, so through traffic will not be unduly slowed when drivers pulled into or out of spaces. In practice, parking lanes rarely reach their recommended widths, but the standards illustrate a new concern with the street as a territory of the car, rather than as an arena for multiple modes and activities. In some places parking lanes have not been widened but instead prohibited entirely; Century City has banished all its parked cars to off-street garages, and reserves its broad streets for moving automobiles. The end effect is the same. Because curb parking can help make a street feel more human scaled (by encouraging movement on the sidewalks, and by providing a barrier between pedestrians and fast-moving traffic) its removal can amplify the sense that the street is a facility for cars alone.

Manville and Shoup reevaluated the study data that Meyer and Gómez-Ibáñez used, and reaffirmed the basic insights.

High-res →

They wrote:

Our results indicate that the relationship they identified between density, street space, and streets per capita is still valid. The coefficient of correlation between density and lane-miles per square mile was 0.87, while the coefficient of correlation between density and lane miles per 1,000 persons was −0.39. This latter coefficient is weaker than the relationship identified by Meyer and Gómez-Ibáñez, but still negative.

[…]

Columns 4 and 5 of Table 2 show each area’s daily vehicle miles traveled (VMT) per square mile, and VMT per capita. Like our figures for lane mileage, these numbers are derived from the TTI’s database. Given the relationship we have found between street space and density, it is reasonable to expect that VMT interacts with density in a similar manner. Previous research has shown that traffic volumes correlate highly with density: Ross and Dunning (1997), in a report to the Federal Highway Administration, found that traffic volumes rose at 80% of the rate of population change. It may be, however, that density and VMT share the same complicated relationship as density and street space.

Our calculations suggest this is so. For the 20 largest urbanized areas, the coefficient of correlation between population density and VMT per square mile is 0.90, while the coefficient between density and VMT per capita is −0.58. Los Angeles, the densest area, has the highest daily VMT per square mile (128,000), and by a significant margin. It sits in the middle of the pack in terms of VMT per capita. Using all 85 urban areas weakens the relationship only slightly: the coefficient of correlation between density and VMT per square mile falls to 0.86, and the relationship between density and VMT per capita becomes −0.47. Increases in population density reduce the VMT per person but increase the VMT per square mile. In low-density areas each person creates more VMT, but because there are fewer people per square mile the VMT per square mile falls. These findings accord well with the idea that sprawl can reduce congestion, but that it also makes for longer trips.

High levels of VMT per square mile suggest high levels of traffic congestion. For this reason it is not surprising that Los Angeles has such a large VMT per square mile, not only because it reinforces the popular perception that LA has the nation’s worst traffic, but because the region’s relative equality of density (which we discuss in the next section) deprives it of any truly low-density areas that would offer a respite from high congestion levels. We can follow this logic back further into our original seeming paradox: since congestion is properly thought of as competition for scarce road space, areas with high levels of congestion—which is to say dense areas—can be conceived of as lacking in road space, even though they have more of it than less dense areas.

Obviously the problem is not quite that simple. The optimal solution to competition for scarce road space is not more road space, but—as with competition for any scarce resource—prices. In the absence of road pricing, however, it is not uncommon for traffic engineers to state that a congested area has an undersupply of streets. Congestion worsens as population increases because the supply of streets is relatively static, and cannot keep pace with increases in density and VMT if everyone drives everywhere.

So, cities become designed around their streets, and the lower the population density (larger lots) the more time people spend driving in cars, which leads to greater congestion, like LA.

And the result is that cities like LA do in fact dedicate a higher proportion of space to cars.

This means that the rise of autonomous cars — even in places like LA, will lead to strong motivations to increase density, and to reuse space now dedicated to cars that are generally at rest: parking. LA has 24% of its central business district dedicated to parking, for example, leaving aside the underground and multilevel structures allocated to it.

The final table includes a wide variety of cities, including New York, and rationalize parking as a function of jobs in the city:

High-res →

New York has the amazingly low figure of 0.06 parking spaces per job in the downtown area, contrasted with LA’s 0.52: ten times more parking per person in LA than NYC, and LA is — to the authors’ knowledge — the highest percentage on earth.

The authors quote Lewis Mumford, who said

The right to access every building in the city by private motorcar, in an age when everyone owns such a vehicle, is actually the right to destroy the city.

And they close with a recommendation:

Perhaps the simplest and most productive reform of American zoning would be to declare that all existing off-street parking requirements are maximums rather than minimums. The examples of New York and San Francisco suggest that limits on off-street parking can foster many of density’s benefits, and urbanists who admire these cities might urge other places to adopt their approaches to parking. From a different perspective, however, more regulation may not be the best first step. The market can mediate the supply of parking in most urban areas, and despite the planner’s frequent desire to replace a floor with a ceiling, it may be better to simply deregulate parking—to force it on no one and let those who want it pay for it. A market-oriented approach to parking would eliminate cumbersome regulations, remove incentives to drive, and let city planners concentrate on matters that seriously demand their attention.

Or let some innovation like autonomous cars come along, and watch what happens when 70% or more of the cars go away.

21 February 2012 4 Permalink