A story from Colorado hit my inbox this morning. With fewer speed traps on I-70 near Grand Junction the highway is safer. That’s not what the reporter wants us to believe, but it’s at least as true as the conclusion he drew.
Here are the facts. The road had seen “some pretty graphic accidents” so the city asked the state to take action. Police increased speeding enforcement in 2018. Colorado DOT improved guardrails and installed median cable barriers at the end of 2018. In 2019 the DOT reduced the speed limit from 75 to 70 and police wrote fewer than average speeding tickets.
So in 2019 three things were different: the speed limit went down, median barriers were installed, and police wrote fewer speeding tickets.
The result is… ambiguous. In the three months after the speed limit was reduced there were 8 property damage accidents, 2 injuries, and 1 death. An average three month period before 2019 saw 12, 4, and 0. That is not meaningfully different. Remember the basic statistical guideline I use: if the change is comparable to the square root of the total value, it is not significant. We’re seeing changes that are consistent with random chance.
We can also expect the “after” numbers to go down for two reasons. The after period was April to June, warm weather months. A comparison to a typical April to June would be more useful than comparison to an average including winter weather. And we have reason to suspect a case of regression to the mean. The change was in response to a percieved increase in accidents. Would the rate have returned to normal with no action?
From most likely to least likely the explanations for the change are: (1) random chance, (2) median barriers, (3) reduced ticketing, (4) reduced speed limit. I put ticketing ahead of speed limits because we know drivers respons to enforcement more than they respond to signs. But people writing about transportation policy are conditioned to consider speed limits signs like remote controls on toy cars that will fly off the track if you push too hard.
Western Colorado doesn’t need speed limits. It doesn’t need speed traps. But at least the hands-off approach of police is keeping the speed limits from doing too much damage.
Since I keep calling out insignificant changes you might wonder if traffic control ever makes a difference. Sometimes it does. When MassDOT installed a new traffic signal at a highway ramp in Danvers people were getting hit twice a week. The reported increase in accident rate was 67 to about 100 per year, which is likely to be real. (Change is three or four times the square root.) And there was a good explanation for why the accidents are correctible by traffic control changes. People were getting hit making left turns across traffic when one lane of oncoming traffic stopped and the other didn’t. MassDOT changed the permissive turn to a protected turn, and declared victory without reporting any numbers. But at least it’s possible the evidence in Massachusetts backed them up, while the evidence in Colorado surely does not.
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. A hotel in Grand Junction gave the author the best free breakfast of any he visited in several cross-country road trips.