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Don’t shoot the messenger; the 85% problem

Pedro Madruga at Copenhagenize penned an interesting post regarding traffic engineering’s famous (or infamous) 85% rule yesterday. To be clear, I myself enjoy an opportunity to rant against modern American traffic engineering. However, I feel Copenhagenize’s criticisims of the 85% rule were somewhat misdirected.

Just to refresh everyone’s memories: the 85% rule states that the optimal speed limit of a roadway is a speed at which 85% of drivers remain at or under the limit. In essence, this means that we expect 15% of drivers to drive above the speed that a roadway is designed for.

At the risk of mischaracterizing Pedro’s argument, here’s how I understand it: speed limits are set based on the 85% rule, thus speed limits on streets don’t adequately account for other road users. In addition, safety increases as you increase the average speed.

For starters, I think Pedro has misinterpreted the meaning of the Solomon Curve (see above). The X axis is deviation from average speed, not the average speed itself. In other words,  increasing the average speed doesn’t make a roadway safer or more dangerous. It’s the variation in average speed that determines its danger. Here’s a thought experiment to illustrate: imagine two roads, both have the same average speed (50 mph). On Road #1 the speed limit is set at 50mph and it has been designed accordingly. Everyone more or less drives within about 10 mph of the speed limit – some more cautious drivers keep their speed down near 40mph and some speed demons push it to near 60. Road #2 has an arbitrary speed limit of 35 mph, but is designed for travel at 65mph. The law abiding drivers keep around 35 while many others, because of the road’s design characteristics, feel perfectly comfortable cruising along at 70mph.

Which is the more dangerous road? Reason (and Solomon’s curve) tells us that Road #2 is more dangerous because of the high variability in speed. Someone cruising along at 65mph (the design speed) may find themselves having to suddenly brake hard for a pesky law-abiding motorist driving at 35mph.

In a nutshell, speed isn’t dangerous per se, it’s speed differential that causes accidents.

The 85% rule isn’t some magical black box that engineers have devised to trick us into destroying our cities – it is based on observed phenomena. Solomon actually gathered data on car crashes and the curve is the result. Attacks on observable patterns are misguided – we’re shooting the messenger rather than addressing the issue.

To Pedro’s concern about other road users not being properly accounted for: there’s nothing that says an engineer couldn’t factor in pedestrians, bicyclists, turtles or slugs as part of the design of the roadway. The 85% rule doesn’t have to apply to cars alone. When a road is built, they are solving the problem they’ve been asked to solve: design a roadway where 85% of users will travel at or below a given speed. Whether the road’s optimal speed should be 20mph or 75mph is not up to the engineer – that’s the job of the city/state/federal official in charge of the project.

The underlying problem that Pedro and Copenhagenize allude to is in roadway design itself, which is a direct result of the values that we as a society have placed on different modes of travel. That is, for nearly a century our societies have been asking for larger, faster roads and the engineers have delivered. The 85% rule just as easily applies to a well-designed woonerf as it does to Interstate 94 here in Chicago. In either case, it’s the design decisions undertaken by politicians and bureaucrats that determine how users utilize the road. But until the powers that be hear from their constituents that we need to design low-speed roads that are inviting for pedestrians and bicyclists, we’ll continue to see 40mph stroads.

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