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Traffic engineering follies

One thing I really enjoy about Chuck Marohn's writing is when he gets into detailed analysis and critique of engineering practice. Yes, it's a little too wonky for most. But it's important to understand the assumptions and thinking that are the inputs, so we can understand why the outputs are so similar from place to place. You know the saying, garbage in, garbage out. As an engineer himself, it Marohn can write about this well from a first-hand viewpoint. Last week, he wrote such a piece - here's a couple of key excerpts:

The traffic engineer believes:

  1. That people will drive at whatever speed they feel is safe and prudent, AND
  2. That traffic engineers are incapable of influencing speeds through design.

This makes the traffic engineer into one of the world’s most impotent professions. They design driving lanes with consideration of design speed, but never based on desired speed. They then plead helplessness in influencing how fast people drive. It is as if they compartmentalize different parts of their brain and allow no cross communication, despite the obvious tools for good at their disposal.

As a practitioner from the urban design side, I'd say that the mentality he describes above is one of the most frustrating aspects of community planning. At its core it fails to recognize that traffic engineering is really a social science. That statement alone would drive most engineers crazy, but it's the cold, hard truth. Cars are not piloted by robots - they are human beings. Human beings operating vehicles simply do not respond to the same impulses as water flowing through a pipe or robots on an assembly line (thankfully).

And here's a bit more on the exceptional data that we rely on for engineering inputs:

Just so everyone knows how the 85th percentile is calculated: they send a young engineer (I used to do this) out to a place where they want to do a speed study. The young engineer listens to talk radio and writes down the speeds that drivers are traveling over a set interval...say, two hours. After that period of time, the data is reviewed and the speed limit is the speed which only 15% of drivers exceed. So 85% of drivers are driving at that speed or slower.

Donald Shoup has written about the lack of true science behind much traffic engineering data as well. A great piece he wrote about a decade ago is here called Roughly Right or Precisely Wrong. It's only a few pages, but devastating as a critique of the methods engineers use to establish "trip generation" and thus, demand for parking.

Marohn's entire piece is worth the read, as he gets into a specific example. But here's a great quote to leave you with:

So your engineer is right: people will drive what they feel is safe and prudent. Where your engineer is wrong is in believing they are held captive by that reality. The engineer is not powerless. Safety in this situation is not someone else's silo.

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