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We Spend Too Much Time Trying To Predict Traffic

The amount of time, money, and mental effort spent on auto infrastructure and traffic models in the U.S. is mind boggling.  A recent write up in the Baltimore Brew exemplifies this point.  Even in one of the best infill sites in the country, and in one of the most dense, urban areas in the state, discussions revolve around auto traffic like there was a tsunami headed for Baltimore, and the only thing that matters is getting in and out of development sites as quickly as possible.

Sure, we don’t want grid lock, but Baltimore could actually accommodate a bit more traffic given our history of population loss and disinvestment. It could use more of a lot of things, including density, transit use, and a balanced road network which de-prioritizes automobiles and encourages alternative modes. But this can’t be just a vague concept.  Our public discourse has to reflect a shift in priorities. In development review meetings across the country where 80% of the conversation is spent on traffic and parking,  we are implying the automobile is still king. It’s not.

Assuming constant traffic growth isn't realistic.

A 2006 USDOT study stated: “The United States has reached a critical juncture in terms of national mobility trends and underlying socio-demographic conditions and travel behavior that will result in more moderate rates of annual vehicle miles of travel (VMT).” Why the change? Lots of reasons: Aging populations, denser development and population gains in transit accessible cities, an entire generation that no longer views cars as status symbols or a means to freedom, and a recession which made people look at the true cost of driving.

So even when we try to predict future traffic volumes using really sophisticated models, we often err on the side of amplifying the benefits of adding lanes/roads and parking facilities while minimizing, or outright ignoring, the negative externatilies of expanding auto capacity. Anthony Downs’ “Triple Convergence” theory introduced the concept of induced demand in traffic engineering circles, but I still think many transportation professionals would rather look at streets the way plumbers look at pipes rather than as living arteries and veins of civic life. Recent research in the European Journal of Transport and Infrastructure makes the case better than I can:

The results show lower travel time savings, more adverse environmental impacts and a considerably lower benefit-cost ratio when induced traffic is partly accounted for than when it is ignored. By exaggerating the economic benefits of road capacity increase and underestimating its negative effects, omission of induced traffic can result in over-allocation of public money on road construction and correspondingly less focus on other ways of dealing with congestion and environmental problems in urban areas.

While traffic models are predicated on fluid dynamics equations, human behavior is more complex. And traffic is just that; human behavior based on variables which are difficult to quantify.  How many people will drive to a new office building if VMT and auto ownership rates continue to stagnate or decline? If a new, high quality transit line is built nearby? If  a critical mass of bike commuters encourages lots of new riders? If nearby roadway improvements encourage walking and positive streetlife? The answer is no longer a point on a straight line graph.


I’d also like to mention Mark Gorton’s new project, Rethinking the Automobile.  For those not familiar, Mr. Gorton is the man who brought you OpenPlans, a non-profit focused on open government and livable streets, as well as StreetsBlog, a sustainable transportation and livable communities clearinghouse.

  • Kay3959

    I think we have to make driving less convenient for people instead of enabling single occupancy vehicle trips. This means slower traffic speeds in neighborhoods (check out NYC”s new 20mph neighborhood zones), no more parking garages downtown, and more road diets. Baltimroe has way too many streets designed like highways which encourage high speeds.

  • Barry Childress

    This may be of interest: