Your money and your life?

KXAN investigative reporters suggest an 85 mph speed limit is dangerous. The evidence: the speed limit on the Texas SH 130 toll road was posted 85, there were accidents, and accidents increased as more drivers used the highway. Pure fearmongering. There’s not enough information in the report to judge how safe SH 130 is and whether the speed limit is to blame.

I’ve seen this kind of reporting many times before, mixing absolute and relative quantities and actual speed and speed written on speeding tickets.

Here’s a quote:

On average, drivers speeding in zones marked with an 85-mph sign
travel about 98 mph, according to an analysis of DPS data.

Ninety-eight miles per hour. That’s scary to a reporter. What does it mean?

What it means is, most speeding tickets are for more than 10 mph over the speed limit. The one in ten thousand cars that got tickets were going 98, more or less. I’d rather know how fast the other 99.99% of traffic is. To find that out you’ll need to grab a laser and shoot cars for a half hour. Then you won’t need ticket statistics. You’ll probably find that less than 10% of drivers go over 90.

A speed survey tells us how fast drivers are going. A ticket survey tells us how police are enforcing the speed limit.

A spectacular wreck, or any number of them, does not tell me if the highway is more or less dangerous than other highways with comparable traffic. Without more context, I don’t know if I’m safer driving SH 130 at a speed of my choosing than fighting heavy traffic on I-35.

The report goes on to complain about the standard process for setting speed limits.

A long time ago people observed that traffic naturally chose a safe average speed. Now we call it “the wisdom of crowds.” Drivers collectively have good judgment even if some individuals do not. And thus was born the 85th percentile rule, the principle that we should not be ticketing the average driver. The rule calls for speed limits based on free-flowing traffic unaffected by congestion or threat of enforcement.

That’s what the law says and that’s what the facts say. Real life is sometimes different. When it is considered at all, the 85th percentile rule is often reversed as people lie about traffic speed to get the signs they want.

My town recently posted a radar speed trailer with an illegally low speed limit sign. Town officials know it is illegal. They figure if traffic slows down for the trailer they can use fake speed measurements to get a real speed limit reduction. The city where I used to live ran speed traps before collecting speed data, for the same reason. They never did get that speed limit reduction approved. Somebody tipped off MassHighway. (Success not guaranteed. Another town with more political pull did get a speed limit reduction even though state engineers knew they were being lied to.)

The KXAN reporters understand a little of the 85th percentile, but not the important part. They think speed traps should be used to lower traffic speeds before choosing speed limits:

About eight months after the toll road opened, commissioners approved
a speed study with an 85th percentile test, indicating the 85 mph was
appropriate based on TxDOT’s analysis. But, because the toll road
opened at 85 mph, instead of the standard 70 mph limit, it’s possible
the results of the secondary study were impacted as drivers traveled
at the posted speed.

If police officers had enforced a 70 mph speed limit, that would have adversely affected the speed study. Ideally SH 130 would have opened with no speed limit. Unfortunately, Texas law requires some speed limit. Whether chosen by a good guess or an expert estimate, 85 was the right choice.

And now we get to the real problem. In fact the speed limit was not set by luck or skill. I complained about the speed zoning process a couple years ago and the KXAN team has the same concern. The speed limit was based on how much investors were willing to pay. The company that collects tolls on SH 130 paid $100 million for a high speed limit to draw traffic. Meanwhile, I-40 is stuck at 75 because nobody paid for more. Some of I-35 is even slower to encourage drivers to pay for SH 130 instead. I agree that this disparity should be fixed.

Texas DOT should raise the rest of its rural freeways to 85 without charge.

The opinions expressed in this post belong to the author and do not necessarily represent those of the National Motorists Association or the NMA Foundation. This content is for informational purposes and is not intended as legal advice. No representations are made regarding the accuracy of this post or the included links.

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One Response to “Your money and your life?”

  1. James C. Walker says:

    I got the data from TxDOT showing the actual 85th percentile speeds averaged to 86 mph. The 85 limit is perfect.