The Case Against Traffic Projections
December 10, 2015

We should not listen to traffic projections.

The famous graph from the Washington State Department of Transportation.

All around the country, and all around the world, time and time again, traffic projections are proven to be wildly inaccurate. The so-called "science" behind them is filled with so many assumptions and flaws that you could extrapolate any line on a graph and the thing would be just as accurate.

Occasionally, when the projection is close, it is simply because we have spent a ton of money building the capacity to induce the demand to turn the projection into a self-fulling prophecy, and using that as an argument that their projection was correct. In those cases, the traffic increased because we spent a ton of money, not because their projection was correct.

The whole field of projecting traffic was invented by traffic engineers to ensure their field more engineering work. We are basically hiring a bunch of people that are always going to tell us to hire them to do the work they predict we need.

If you let a traffic engineer design a city, it is going to be one built around moving cars. If you let a mall architect design a city, it might end up centred around one big food court in the middle. If you let an Internet engineer design a city, well, uh.. we might have a city with really good Internet connectivity and no retail (except online retail.) You get the picture. A traffic engineer is going to tell you to do what they know how to do the best - more roads, more capacity.

This is not to discredit traffic engineers. We need traffic engineers to design roads. But, we should not pay them just to tell us we need to give them more work. Just ignore traffic projections - call their bluff and go on with life.


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