Learning from TTI

Michael Lewyn's picture

This week, I finally got around to looking at the latest (2009) Texas Transportation Institute study on traffic congestion. (1)

Two facts struck me as interesting.  First, the great congestion surge of the past decade or two is over.  In most large metropolitan areas, congestion (measured as hours lost to congestion per traveler) peaked around 2005, and actually declined in 2005-07.  For example, in Atlanta, hours lost to congestion peaked at 61, and decreased to 57 by 2007.  Congestion increased in only three of the fourteen largest regions (Washington, Detroit and Houston)- and in each of these by only one hour per traveler.

Since increased gas prices probably had some effect on driving (and thus on congestion) I suspect that congestion will continue to decline in 2007-09; however, we will have to wait a year or two for data, at least from TTI. 

Will reduced congestion have any political impact? Probably not.  On the one hand, reduced congestion should make the arguments for new transportation facilities (both roads and transit) less compelling.  On the other hand, the severity of the current recession means that transportation lobbies will argue for roads and transit lines as job creators rather than as congestion reducers.

The second thing that grabbed me is the weakness of any correlation between density and long-run congestion increases.  Some regions became more dense between 1982 (when TTI statistics began) and others became less dense.  Yet both groups experienced comparable congestion increases. 

The only "Top 14" region where density (as measured by population per square mile) increased by over 20 percent was Houston, where congestion doubled (from 29 hours per traveler in 1982 to 56 in 2007).   This may be because Houston's density is still under 2000 people per square mile, the fourth lowest among large regions and too low to lead to increased public transit use.  Both the fastest increase in congestion (Dallas) and the slowest (Phoenix) were in low-density cities where density also increased, though at an even more glacial pace than in Houston.

On the other hand, density nosedived in some older northern cities: by 43 percent in metro Philadephia (from 4083 people per square mile in 1982 to 2329 today) and 35 percent in metro Chicago (from 3726 to 2398).  If low density reduced congestion, surely we would see reduced congestion in those places.  But in fact, congestion in these regions increased at a pace faster than Houston's: from 15 to 41 hours per traveler in Chicago, and from 16 to 38 in Philadelphia.



(1) The data most relevant to this post are as follows:

Data on congestion trends is online at http://mobility.tamu.edu/ums/congestion_data/tables/national/table_4.pdf .

Data on density trends is online at http://mobility.tamu.edu/ums/congestion_data/ .


Michael Lewyn is an assistant professor at Touro Law Center in Long Island.



TTI pro-multi-modality?!

I have delved quite extensively into TTI's 2009 mobility report. As we are well aware, many anti-transit folks use TTI data in order to pave their pro-road argument, but what I find more than quite interesting is that TTI's analysis of their own data contradicts them:

• Providing more options for how a trip is made, the time of travel and the way that transportation service is paid for may be a useful mobility improvement framework for urban areas. For many trips and in many cities, the alternatives for a peak period trip are to travel earlier or later, avoid the trip or travel in congestion. Given the range of choices that Americans enjoy in many other aspects of daily life, these are relatively few and not entirely satisfying options

• In growing areas adding capacity of all types is essential to handle the growing demand and avoid rapidly rising congestion

• Commute trips generally cluster around the most congested peak periods and are from the same origin to the same destination at the same time of day. These factors make commute trips by carpooling, vanpooling, public transit, bicycling and walking more likely

• Peak period public transportation service during congested hours can improve the transportation capacity, provide options for travel mode and allow those without a vehicle to gain access to jobs, school, medical facilities, and other destinations. In the case of public transportation lines that do not intersect roads, the service can be particularly reliable as they are not affected by the collisions and vehicle breakdowns that plague the roadway system and are not as affected by weather, road work, and other unreliability-producing events

• Transit, like ridesharing, park-and-ride lots and high-occupancy vehicle lanes, typically have a greater effect on the congestion statistics in a corridor, rather than across a region. Transit and these other elements “compete” very well with the single-occupant vehicle in serving dense activity centers and congested travel corridors

• (Smart Growth) characteristics can be incorporated into new developments so that new economic development does not generate the same amount of traffic volume as existing developments. Among the tools that can be employed are better management of arterial street access, incorporating bicycle and pedestrian elements, better parking strategies, assessing transportation impact before a development is approved for construction, and encouraging more diverse development patterns

• If a region’s vehicle-miles of travel were to increase by five percent per year, roadway lane-miles would need to increase by five percent each year to maintain the initial congestion level

• This analysis shows that it would be almost impossible to attempt to maintain a constant congestion level with road construction only. Over the past 2 decades, less than 50 percent of the needed mileage was actually added. This means that it would require at least twice the level of current-day road expansion funding to attempt this road construction strategy. An even larger problem would be to find suitable roads that can be widened, or areas where roads can be added, year after year

David Parvo
Most Senior Fellow
The Placemaking Institute

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