A framework for determining where congestion really matters

By Eric Sundquist

These two things are true: 1) Travelers dislike slow traffic, and 2) slow traffic is sometimes an inescapable result of things that people do like—cities with popular destinations. Conventional transportation practice responds well to No. 1, with well-known standards for delay and capacity. Practice has no clear standards to deal with No. 2—what to do in places where speeding up cars amounts to destroying the village in order to save it.

A new study of neighborhoods in Los Angeles, a place with more than a little congestion, helps fill this gap.

The authors, Andrew Mondschein of the University of Virginia and Brian Taylor of UCLA, scan the city to understand where traffic delays are “irritating but relatively benign versus serious and costly.” Using travel-survey data, they examine places where tripmaking is constrained by traffic—indicating lessened economic and quality of life resulting from mobility problems—and compare those areas to places with traffic but no lack of tripmaking—places they term “sweet spots” where the density of destinations is such that residents can meet their needs with shorter trips, by car or other mode, making traffic delays relatively benign.

They find numerous examples of both in LA:

[S]weet spots exist, even in Los Angeles. … [P]laces where individuals tend to make both more than average and shorter than average trips are located throughout the region. In these places, trip-making is higher than the median (4 trips) for survey respondents and average trip lengths are below the median (approximately 6 km); thus accessibility is less tightly linked to mobility. Some of these locations, such as Santa Monica, West Hollywood, and Newport Beach, are among the most well-known and popular areas in Los Angeles. Other locations, however, like Reseda, Whittier, and Garden Grove, are lower-income ethnic enclaves. Conversely, the areas where individuals make fewer, longer trips – an undesirable situation for both individuals and society – include some of the poorest neighborhoods in the region such as Watts, the port areas near Long Beach, and Van Nuys/Pacoima.

We encourage you to go read the paper, but here are the authors’ takeaways:

[R]esidents of some areas enjoy high levels of accessibility and activity participation despite high levels of congestion, while in other areas congestion appears to contribute to lower levels of accessibility, as intuition would suggest. As a result, the picture of Los Angeles as a congested dystopia painted by metropolitan congestion measures like the TTI Mobility Index is misleadingly simplistic; the real story is far more nuanced, and interesting.

Because of congestion’s paradoxical relationship to activity participation, people and firms may be better off in congestion-adapted neighborhoods, and worse off in congested-maladapted neighborhoods – even if the absolute levels of congested delays in both neighborhoods are similar. If so, standard measures of traffic delays – like V/C ratios or Levels of Service – on their own tell us next to nothing about whether the social and economic effects of congestion are benign or pernicious.

We emphatically do not argue that congestion causes more trip-making or that on its own is beneficial, but rather congestion is often an inevitable consequence of vibrant, thriving, agglomerated places. We wholeheartedly agree with many millions of drivers that traffic congestion is frustrating and irritating, but disagree with the premise of metrics like Level of Service that assume traffic delays on their own are necessarily problematic and costly – metrics that have been at the foundation of traffic impact analyses and development debates for decades. Instead, our analysis suggests that, in the right local conditions that promote both increased activity participation and perhaps increased congestion as well, a certain level of increased traffic delay associated with increased development may indirectly foster – or at least not discourage – patterns of trip-making and activity participation that benefit both individuals and society.

Finally, the authors suggest that practice move toward a framework that puts accessibility rather than conventional mobility measures at forefront.

There is a movement afoot, spearheaded by planners seeking to increase the roles of biking and walking in urban travel, to develop more inclusive, multi-modal indicators of street and road performance that do not privilege vehicular movements over other forms of travel…. The efforts of these planners to shift from an analytical focus on vehicular throughput are supported by our findings presented here. Indeed, shifting the units of analysis and focus away from (admittedly maddening) traffic delays on street networks, and onto how land use/transportation systems promote or discourage trip-making and activities in households and by firms will help planners and policy makers deliver greater benefits to travelers who so often find themselves stuck in traffic.

This is an area where SSTI is active, most prominently by assisting the Virginia DOT in developing project-selection criteria around accessibility, and we are happy to share with interested parties.

Eric Sundquist is Director of SSTI.