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Reports from the Editor

The Other Half of the “Deadly Duo”

Deadly Duo posterIn my last post, "Breaking Our Addiction to Highway Level of Service Standards," I reported on the first half of the "deadly duo" -- and, no, I wasn't referring to rapper Jay-Z. What I was talking about something more important, at least to planners and local officials: level of service standards and travel projections. They're what transportation engineer Gary Toth calls the "deadly duo," since unthinking reliance on them can kill efforts to build strong, vibrant communities.

The first half of the deadly duo, according to Toth, is LOS (that's "level of service" standards for those not familiar with the acronym). Today I want to go over some of his basic arguments about the other half of Toth's deadly duo: travel projections.

As Toth made clear during a recent Project for Public Spaces workshop, traffic projections -- and the models that generate them -- are of critical importance because they set the table for how we plan and design our roadways ... and communities.

We all know the expression, "garbage in," "garbage out," that shorthand reminder that the quality of the results we get from using data depends on the quality of the data we're relying on. That's also true when thinking about traffic models. So what are some of the warning signs?

For Toth, the first thing to watch out for is growth projections, both for population (which he says often result in overestimations) and for traffic. As Toth explains: "Most [traffic models] ignore changing demographics such as the aging of our population, rising energy prices, ... and societal changes. Most assume that our economy will continue to grow at the same rate as it has over the last 30 years."

But there's an even more important thing to watch out for, Toth warns, models have a built-in bias towards continuation of the status quo. "Without direction or a reason to do otherwise, modelers will likely assume that future growth will occur ... as usual segregated and spread out (sprawl) patterns." Why is this critical? "Because research shows that compact mixed use development can reduce travel by 25% or more." Traffic models, Toth continues, don't consider how we can reshape the direction our communities are heading.

traffic graphicWhat's more, by relying on the projections of traffic models that overestimate our need for more or wider roadways, we're making it more difficult to achieve the goal of more walkable, less auto-dependent, communities.

In his article, "K is for Knowledge," noted planning historian Laurence C. Gerckens wrote about the tyranny of projections driven by over-reliance on past trends:

"The second 'flaw' -- or built in bias -- in the 'classic' planning process lay in the fact that it was premised on the projection or continuation of past trends. In other words, past trends became the policy-bases for the comprehensive plan. As a result, plans rarely reflected any vision for positive change. Instead, they reinforced historic patterns. ... 'Modern' planning processes began not with the acquisition of immense amounts of data, but with the creative visioning of alternative futures -– establishing community goals, alternative patterns of development, and the means of their attainment. ... Responsible planning is a creative art using data from the past and knowledge of interrelationships to create new and better communities for the future."

But what can you as a planner or local official do when faced with traffic projections you believe overestimate future travel demand and fail to take into account the community's vision of its future? Ask hard questions, was Toth's reply. "Challenge growth and buildout numbers, and ask if they adjust for walkability and for increased mixed-use ... listen to the answers, and then ask more questions."

“Don't let the model tell you how wide your streets should be ... you tell modelers how wide you want your streets to be, and then have them tell you what that will mean in terms of congestion.”

"Don't let the model tell you how wide your streets should be," Toth continued, "you tell modelers how wide you want your streets to be, and then have them tell you what that will mean in terms of congestion." In other words, remember that you are the policy makers, and the traffic modelers are there to help you understand the impacts of what you want. It's up to you then "to decide what you want to do about any projected congestion" given your community's vision and goals.

What's more, Toth concludes, if the model is cheap or faulty, just don't use it.

[Note: in putting together this post I drew on both my notes of Gary Toth's comments during the workshop and material in a handout he prepared, "Traffic projections and levels of service targets."]