"Driven Apart" Drives Right at Transportation Modelling

A report released last week by CEOs for Cities casts a critical eye on the Texas Transportation Instititute's Urban Mobility Report (UMR).  Even if you haven't read the UMR, which ranks congestion in US cities, you know about it since it produces headlines like the one below every year.

The report's conclusions are compelling and disconcerting. Among the key findings, are that the UMR "has completely overlooked the role that variations in travel distances play in driving urban transportation problems" and that "the key tool contained in the Urban Mobility Report--the Travel Time Index--actually penalizes cities that have shorter travel distances and conceals the additional burden caused by longer trips in sprawling metropolitan areas." This is a significant finding because it essentially penalizes Smart Growth policies. The report states:

In the best performing cities – those that have achieved the shortest peak hour travel distances – such as Chicago, Portland and Sacramento, the typical traveler spends 40 fewer hours per year in peak hour travel than the average American. In contrast, in the most sprawling metropolitan areas, such as Nashville, Indianapolis and Raleigh, the average resident spends as much as 240 hours per year in peak period travel because travel distances are so much greater. These data suggest that reducing average trip lengths is a key to reducing the burden of peak period travel. Over the past two decades, for example, Portland Oregon, which has smart land use planning and has invested in alternative transportation, has seen its average trip lengths decline by 20 percent.

Among the other significant problems "Driven Apart" raises regarding the UMR includes:

  1. The UMR does not accurately estimate travel speeds, since the "travel time estimates are based on volume data, not on actually observed travel speeds;"
  2. The UMR exaggerates travel delays that "do not correspond with other independent measures of travel times" so much so that the UMR overstates travel times by 70% nationally. (In Seattle, however, the UMR corresponds almost perfectly with the other independent measures); and,
  3. The UMR overestimates fuel consumption associated with urban travel, in part because the underpinning study is based upon a 29-year-old study using GM cars "of questionable applicability to today's vehicles and to highway speeds." They are not consistent with other recent estimates and the UMR does not account for increased fuel consumption associated with longer trips, leading the study's authors to conclude that "the UMR overstates the cost of congestion by about $49 billion."

Undoubtedly other researchers will find problems with the Driven Apart report, but it offers such a cross-cutting critique of the methods and conclusions of the UMR, that it seems clear that the Texas Transportation Institute will need to re-assess and reconfigure their methodology and findings in years to come.