How I Stopped Worrying And Learned To Love the 85th Percentile

Many alterative transportation advocates are against the 85th percentile rule: http://streetsblog.net/2012/11/16/one-for-the-dustbin-the-85th-percentile-rule-in-traffic-engineering/.

I have to say, that while I’m against speed limits in general, the 85th percentile rule has been useful from a data analysis standpoint.

First of, all let’s talk about the Solomon curve which allegedly proves that the slowest motorist is the most in danger of crashing.

To that, “No!”

This curve seems to only care about rear endings. If you do a mental experiment, you’ll see that the faster you drive, the more the chance that your car goes out of control. This is especially true on a curvy road. Plus, the faster you go, the more likely you’ll need to overtake over vehicles.

Finally, whether or not you crash isn’t that important, but rather how fast you are going when you crash. If we all go sane speeds in controlled areas, we’d be OK with a few crashes every now and then. I got doored twice, hit by two cars and flipped my bicycle, but I didn’t even get a scrapped knee in any of the five crashes. It only made me more alert for the future. And yes, in many of them I was NOT wearing a helmet so there. :P

So why the hell am I against speed limits?

Because it’s one of the fake solutions that make people feel good while doing nothing. Look at how scared we are thinking of getting rid of speed limits. Guess what? They don’t exist now.

How many people brag about how they speed all the time and get away with it. What a capital offense. Since everyone gets away with it all the time, it’s pointless. Plus, usually nobody gets hurt so it’s a victimless crime. Finally, as the police say, they are more concerned with murder and kidnappings to care about dead cyclists. So if some idiots want to race on the freeway no big deal.

Here’s the thing, though.

We can control speed by traffic calming measures. Get rid of the speed limit and we realize that we have to control people’s speed anyway so instead of making a sign, we make a lasting change that actually works. This will free up tons of resources from the police so they can actually walk around our community instead of wasting their time writing tickets which just piss people off.

Plus, if we keep changing the speed limit when people speed it’s the same as not having a speed limit after all.

If people abuse it and race, we can still arrest them for driving too fast for conditions. In fact, we’ll have more resources to focus on this when the police aren’t wasting time on tickets.

But why do I love a rule that I want to abolish?

Because for crash data, it makes it easier to guess how fast the motorist was driving. I don’t know the rules in Australia, but the data on speed limit and death rate is super tight–much, much tighter than any other variable.

Thus, I suggest having the notion of speed zones, which means that instead of making a speed limit, we make a speed target. For example, if we have a single lane, cross walks, and frequent stops, we’d assume that the speed target is 20 MPH. If instead we widen the road, eliminate stops at intersections, and make the scenery boring, we’re looking at motorists going from 50-60 MPH. And so on.

Let’s be realistic. Changing numbers on a sign that people speed past and writing tickets to every thousandth unlucky motorist is NOT going to make people drive at sane speeds.

One Response to “How I Stopped Worrying And Learned To Love the 85th Percentile”

  1. Catching up on bike news while I’ve been otherwise distracted « BikingInLA Says:

    [...] which may be the single most destructive traffic law on the books, to people and communities; though not everyone agrees. People for Bikes notes three companies that support cycling. A 21-year old Perris CA cyclist is [...]

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