Motor Mouth: Lower speed limits don’t make for safer roads

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The American Automobile Association recently released its latest Multi-site Examination for the Impact of Changes in Posted Speed Limit on Traffic Safety report, and the only definitive, absolutely categorical statement it could make after five years of study is that if you lower speed limits, motorists will be given more speeding tickets. And, conversely, if you increase the speed limit, the fuzz will issue fewer speeding violations.

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That’s it. There was no conclusive affirmation that raising speed limits increases the frequency of car crashes. Nor was there a spirited defense of lowering limits either, since the real-world crash data on roads that lowered their speed limits came out “inconsistent” on the question of whether the speed reduction also reduced the frequency of incidents. Having read the AAA Foundation for Traffic Safety’s many missives lo these many years, I suspect this one was a tough “write” for the normally conservative, safety-oriented association.

As the study was conducted by the AAA, this wasn’t this some shoddy, short-term research. Not only did the association review all the literature on speed-versus-safety and gather feedback from the traffic engineers who determine appropriate speed limits for American highways and byways, but it also worked with the Insurance Institute for Highway Safety (IIHS) — an organization almost religious in its fervour for lower speed limits — and then conducted rigorous analysis on 12 different rural and urban freeways — six with raised speed limits; six that were lowered — in five different states.

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Using information from the American Department of Transportation, its Federal Highway Administration, and something called the Regional Integrated Transportation Information System, they monitored not only the number of crashes and the speed at which they occurred, but also the traffic volumes at the time of the collision, as well as the changes in travel times the revised speed limits engendered.

And yet still the most they could determine with any certainty was that the effect of raising those speed limits was inconclusive. And by “inconclusive,” I mean the effect of raising — or lowering — speed limits on the frequency of car crashes seemed almost perfectly random. In some cases, raising speed limits increased the number of crashes; in others, they were lowered. Ditto for lowering the limits.

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This randomness persisted regardless of what state the route was in — in other words, none of those “freaky Californians always drive stoned, don’t they?” excuses — and what type of highway was being tested (rural interstate, urban arterial, or collector lane). For instance, in two of the three interstates that raised limits from 65 mph to 70 mph (about 105 km/h to 112 km/h) crashes increased, but in the other, the frequency went down. But, says the AAA, in all three cases, the differences in the number of crashes was “not statistically significant.” Ditto for arterial feeders, where some saw the frequency of incidents go up and others down.

The same applied for lowering speed limits. In almost all cases, reducing speed limits by five miles per hour (about eight kilometres per hour) did not offer any significant decline — or increase — in crash numbers. There was one anomaly worth noting, however. One highway in Oregon — a set of collectors — that reduced posted speed limits by 10 miles per hour actually saw an increase in crash count and rate. Yes, reducing the speed limit from 35 miles per hour to 25 mph actually increased the rate of crashes of all types (both injury and property damage only). It would appear that not even the most dramatic changes that safety advocates propose have the desired effect.

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In fact, as I said off the top, the “only consistent results” the study found was the likelihood of drivers being issued speed-limit violations: “After raising posted speed limits, vehicle speeds were less likely to exceed the speed limits, while they were more likely to exceed the speed limits after lowering speed limits.” This effect seemed universal — all six venues that increased speed limits saw a decrease in traffic tickets, and five of the six (one was not-applicable) that decreased speeds saw an increase in motorists being fined.

(On one set of highway collectors in Oregon) reducing the speed limit from 35 miles per hour to 25 mph actually increased the rate of crashes of all types

So, let’s see: the change in the number of cars crashes vis-à-vis changes in speed limits is, at best, inconclusive. At worst, the two aren’t correlated. Either way, increasing highway speed limits by five miles an hour doesn’t measurably increase the number of crashes, nor does reducing posted limits by the same amount significantly lower them. In other words, altering the speed limits, at least by these small amounts, does not appear to have an appreciable effect of the number of crashes.

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On the other hand, it would seem to have a significant effect on the number of speed violations issued and the revenues generated. Far be it from me to accuse the police and insurance companies from trying to line their own pockets with motorists’ money, but when you have an organization like the AAA telling you reducing or increasing speed limits has little noticeable effect on safety, but a significant effect on revenue, it bears paying attention to who is profiting from the “speed kills” messaging the two so assiduously promote.

That study’s data also puts paid to one other gross misconception about increasing speed limits, namely that people will simply raise their average speeds commensurate with the uptick in speed limits. In fact, that’s not true either. Of the six sites that raised their limits by five miles per hour, the mean speed increase was but three miles per hour, and the 85th-percentile operating speed — the velocity which traffic engineers claim is the safest to travel — went up by just two mph.

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The news gets even worse for all the safety Nazis advocating for reduced speed limits. For one thing, the mean speed on roads with lower limits actually went up, albeit a miniscule 0.4 mph. Worse yet, the only locale where then mean speed decreased significantly — minus 1.5 miles per hour — was the highway where the speed limit was reduced by 10 mph, and crash occurrences increased.

Simply put, if the study has any veracity at all, the specific speed limit of your local highway has very little influence on whether you’ll be in a car crash or not. But it will dramatically affect how generously you will fatten the pockets of your local constabulary and insurance broker. I’m pretty sure that wasn’t the conclusion the AAA was looking for, but it is the one that stands out.

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Author’s note: In another report — Review of Current Practices for Setting Posted Speed Limits — the AAA cited all the factors that go into the setting of speed limits. As you might expect — or perhaps hope — the process is largely well-reasoned. Targeting the 85th-percentile speed is the aim of 88 per cent of the experts in the field, and the majority of agencies had specific policies that governed their decision that were based on land use, crash frequency statistics, and the number of access points, etc. In other words, the process was scientific, or, at least, thoughtful.

The one exception was when a speed limit was reduced. Then the number one reason cited was requests from the public to increase safety. Considering the information gleaned from this latest study, those reductions look like nothing more than political expedience.