The Deep Mystery of Traffic Congestion
by Hannah Twaddell

Transportation planner Hannah Twaddell explores "The Deep Mystery" of traffic congestion.

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Even after sixteen years in the profession, the way transportation planners talk is still a mystery to me sometimes.

Take the acronym "TDM." It's actually a fairly old term in the transportation lexicon. But, as slang is wont to do, it has begun journeying to new places in the planning world. Local planners now find themselves being asked by everyone from citizens to elected officials about how they are using TDM to deal with traffic congestion. I daresay more than one has wrinkled a brow and thought, "Now what the heck is that? Traffic Diet Methods? Turnaround During Midtrip? Timed Disruptive Measures?"

Our bewildered planner actually isn't too far off with any of those guesses. The official term is Transportation (or Travel) Demand Management, but it does involve -- albeit a bit loosely -- all the strategies mentioned above, and then some.

A typical definition of TDM runs something like this: "Transportation Demand Management (TDM) is a set of planning processes, strategies, and policy decisions that are aimed at relieving congestion and improving efficiencies of the transportation infrastructure."

Sounds impressive, doesn't it? Except it doesn't quite tell me what one does to achieve the aforementioned benefits. Here's another way of putting it:

Let's say "Transportation" means "satisfying a need that compels us to go from here to there;" and "Demand" means "the amount we use our roads and paths." Finally, let's try defining "Management" as "finding the most efficient way." TDM would thus mean something like this: "Finding the most efficient way to use our roads and paths in order to satisfy the needs that compel us to go from here to there."

I'm not sure my language is much more interesting, but it does put the emphasis on the fundamental factor that makes TDM a unique approach to solving congestion. Instead of assuming that ever-increasing traffic is a given and our solution is to engineer more and bigger roads to carry it, a TDM approach asks "What compels us to travel in the first place?" and "What can we do to get those needs met and make the necessary travel more efficient?" It makes us think about the root causes of congestion and find solutions that speak to them. ...

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