Why the 85th Percentile Rule still matters

Earlier this month Pedro Madruga at Copenhagenize found a really bad zombie rule that continues to wreak havoc all over the world: the 85th Percentile Rule. This is the procedure used to set a speed limit:

1. Design the road to permit the highest speeds that the budget and the landowners will let you.

2. Measure the speed of drivers and find the 85th percentile – the speed where 85% of drivers are going at that speed or slower. Round to the nearest multiple of 5. That’s the speed limit that you put on your signs.

As Madruga points out, this is a shitty idea, because most pedestrians and cyclists aren’t going to go more than ten miles per hour, never mind stopped cars and buildings. Animals too, of course. This leads you straight into Strong Towns’ famous Conversation with an Engineer.

However, Madruga doesn’t acknowledge that the 85th Percentile Rule does contain two valuable insights. The first is that large differences between the speeds of drivers on a road can be dangerous. If you’ve taken physics, you know that relative speed is what matters in a crash. With pedestrians and cyclists we’re able to ignore that, because the difference between a pedestrian’s speed and zero is so much smaller than that between a driver’s speed and a pedestrian’s. But for drivers and car passengers, it matters.