Study: Shorter Blocks May Be the Key to Cutting Traffic in Small Cities

It's well-established that density and mixed-use development reduce driving. Right? But strategies like those don’t work the same way everywhere, according to new research published in the Journal of Transport and Land Use. While in major cities, denser development is linked to lower rates of driving, researchers found that in smaller cities it might not have much effect at all. The research suggests that for smaller cities, a focus on reducing block sizes and improving street connectivity may be the most effective way to cut down on driving, though the authors caution that more research is needed to draw universal conclusions.

The research team, sponsored by the Federal Highway Administration, sought to drill down and identify how urban characteristics affect driving levels in different types of places. They looked at four different case studies: Seattle, WA; Richmond-Petersburg and Norfolk-Virginia Beach, VA (grouped together as one case study); Baltimore, MD; and Washington, DC. Using travel surveys and land use information, they modeled the impact on vehicle miles traveled (VMT) of five factors: residential density, employment density, mixed-use development, average block size (which they use as a stand-in for “measuring transit/walking friendliness”), and infill development (or distance to city center).

While the authors knew from previous research that these five factors all contributed to reducing VMT, they found that the Virginia regions didn't follow the same patterns as the other three. In the smaller urban areas of Richmond-Petersburg and Norfolk-Virginia Beach, they found, mixed-use development did not have a significant impact on reducing driving.

“This is probably because in smaller urban areas, even those living in neighborhoods with well mixed land development may still need to travel far to reach work and non-work destinations,” the researchers write. “In other words, mixed development areas are less likely to be self-sufficient in smaller urban areas.” Mixing uses proved to be a good way to reduce driving in the larger metros.

These findings would seem to show a major weakness of New Urbanist-style “town centers” developed in otherwise suburban areas. A small walkable area isn’t enough to actually spark a real shift in transportation habits – the urban area has to be big enough that most people’s needs can be satisfied without a car. But lead researcher Lei Zhang said the findings don't warrant that conclusion. "The paper has a small sample size," Zhang said. "I wouldn't want to generalize the results to other places."

Zhang and his team are working on another paper that broadens the scope of their analysis to 20 urban areas. They hope this bigger data set will help planners evaluate land-use plans and how those decisions affect driving rates in different types of places.

Living close to the urban core (the “infill” factor) was also the least significant factor in the Virginia case. Strangely, Zhang and his team observed that people living at the fringes of these smaller cities actually drove less than people in the core -- a finding they attribute to "semi-rural areas near the fringes of the Virginian cities where residents already travel less than their urban center counterparts."

In general, their model suggests that you get more bang for your buck increasing residential density in already-dense areas than in relatively spread out areas. The authors found that increasing residential density by 20 percent in an area with an existing density of 11,400 people per square mile (which is denser than Rio de Janeiro) would net a 16 percent reduction in per-capita VMT – a major improvement. But a 20 percent density bump in the Virginia metros Zhang and his team studied, with just 1,950 persons per square mile, nets juts a 3 percent cut in VMT.

Meanwhile, they projected that in Seattle, roughly doubling residential density (to more than 8,000 people per square mile) would reduce per-capita VMT by 14.27 percent.

So what is the factor that reduces driving in the smaller metros in Virginia? Smaller block sizes. That made the biggest difference in Virginia, which had the largest average block size of all the case studies. In the other metros, changes in block size have a much smaller effect on VMT.

The researchers concluded that employment density isn't a major factor in reducing driving anywhere. It made only a small difference, and only in Seattle and Baltimore. Puzzlingly enough, in Virginia, VMT increases with employment density.

I asked Zhang why employment density didn't play a greater role -- after all, with more density, parking is probably more scarce and transit more abundant. "That's probably true for the case of downtown," he said. "But there are a lot of cities where, if you built a certain employment space, you have to -- by land use regulation and zoning codes -- you have to build a lot of parking spaces as well. And now, in the U.S., on average in major metro areas, only about 10 to 15 percent of jobs are located in traditional downtown CBD areas."

In their conclusions, the authors mention other factors that they don’t delve into with their quantitative analysis, including transit service coverage and quality, and land use decision-making processes. They suggest that there’s a qualitative difference between land use decision-making at the state versus the local level, but they don’t go into detail about why or how it plays out. Zhang says they're leaving those questions for subsequent studies.