Why 30 km/h Zones are important?

February 20, 2014 - 2 minutes read

You discover that we live in cities that are controlled by bizarre and often outdated mathematical theories, models and engineering “solutions” that continue to be used despite the fact that they are of little use to modern cities.

One of them is called The 85th Percentile. It’s a method that cities all over the planet use to determine speed limits. It’s the standard. Nobody questions it. Certainly not the engineers and planners who, for decades, have been served it up and who have swallowed it whole during their studies. Which reminds us of the old traffic engineer joke: Why did the engineer cross the road? Because that’s what they did last year.

The concept is rather simple: the speed limit of a road is set by determining the speed of 85% of cars that go down it. In other words, the speed limit is solely set by the speed of drivers, and this is the basic rule that determines traffic speeds worldwide. Including the street outside your home. Here is a rather telling graph about the danger of speed:


20 MPH/30 KPH seems to be a good compromise, with the number of deaths being very small and the number of uninjured victims being more significant, unlike with higher speeds where things are reversed. Though of course a low speed limit should be the last thing we count on to save lives; cities should be designed for walking, and the infrastructure should be designed to make things safe for everybody, including pedestrians.


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