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Traffic is everyone getting exactly where they want to go, and getting in everyone else’s way on the way there.

In places with not much going on, this rule falls apart. Driving is fast and easy. This is what cruise control was made for. In place that a lot of people want to go, traffic is more stop-and-go, look-and-react. Congested. A pain in the tuchas. Drivers hate driving in this, as it limits their free progress towards wherever they want to go.

Highway engineers design to minimize this congestion wherever it is, with all the tools in their scope. Mostly by claiming more right of way, adding lanes, lengthening signal timings, removing entrances to the highway and even turning roads into divided highways. The first and only metric of the quality of a road is its level of service (A= Great, F= Disaster). How congested is it?

Congestion is the restriction of free flow in traffic. It may also be the greatest force for safety on our roads today. If fewer vehicles are getting up to a killing speed (30 MPH for walkers, 60 MPH for other cars), then fewer cars can do serious damage to each other. Sitting in a long line of traffic going the same speed as a horse-drawn carriage may be frustrating, but it is unlikely to get you hurt.


This same direct relationship between congestion and safety is shown by the pattern of fatalities on our highways. The chance of dying on the highway is over twice (PDF) as high for rural as for urban, more congested roads. People are able to drive free more often in the countryside, and when something goes wrong, it goes very wrong.

Rural roads are not usually congested at all. Level of service A in traffic engineering argot. These are perfect roads, and yet more dangerous by total and average metrics. Perfect roads that connect not much with little else.

What are traffic engineers managing for?