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LOS and Travel Projections: The Wrong Tools for Planning Our Streets

9:49 AM PST on February 7, 2012

Gary Toth is director of transportation initiatives with the Project for Public Spaces. This post first appeared on PPS’s Placemaking Blog.

Would you use a rototiller to get rid of weeds in a flowerbed? Of course not. You might solve your immediate goal of uprooting the weeds — but oh, my, the collateral damage that you would do.

Yet when we try to eliminate congestion from our urban areas by using decades-old traffic engineering measures and models, we are essentially using a rototiller in a flowerbed. And it’s time to acknowledge that the collateral damage has been too great.

First, an explanation of what I call the “deadly duo”: travel projection models and Levels of Service (LOS) performance metrics.Travel projection models are computer programs that use assumptions about future growth in population, employment, and recreation to estimate how many new cars will be on roads 20 or 30 years into the future.

Models range from quite simplistic to incredibly complex and expensive. Simple models deal primarily with coarse movements of vehicles between cities, while complex models deal with the intricacies of what happens on the fine grid of urban areas. To be truly accurate, growth projection modeling can be expensive. Therefore, absent compelling reason to do otherwise, most growth projections tend to be done using less expensive techniques, which usually lead to overestimates.

Levels of Service (LOS) is a performance metric which flourished during the interstate- and freeway-building era that went from the 1950s to the 1990s. Using a scale of A to F, LOS attempts to create an objective formula to answer a subjective question: How much congestion are we willing to tolerate? As in grade school, “F” is a failing grade and “A” is perfect.

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