Stick to the science
Researcher says FMCSA study that shows 'profound benefit' for speed limiters relied on flawed data

By David Tanner, associate editor

You’d think that a study setting out to show speed-limited trucks are safer than trucks without speed limiters would compare the actual speed of trucks involved in crashes. But an FMCSA study that proclaims a “profound safety benefit” for speed limiters does not.

That’s one of the reasons researcher Steven Johnson, who is listed as one of authors of the FMCSA study published in March, has distanced himself from the study and published his own response to the findings.

“Probably the most critical factor in evaluating the study is the fact that the speed of the truck at the time of the accident was not known,” Johnson wrote in his rebuttal released in July.

He went on to explain that the speed of the vehicle is “obviously critical” to determine whether a speed limiting device could have stopped a collision from occurring.

Johnson, a professor of industrial engineering with the Mack Blackwell Transportation Center at the University of Arkansas, has published studies on speed differentials and highway safety. One notable study commissioned by the U.S. DOT and trucking fleets showed that different speed limits for cars and large trucks on rural interstates can compromise safety.

He says the recent FMCSA study that compared data on speed-limited trucks and non-limited trucks contained a number of flaws.

He says speed-limited trucks that traveled shorter distances were compared on par with non-speed-limited trucks that mainly traveled long-haul. And instead of comparing miles traveled, which is widely used in transportation studies, the researchers were supplied with “accidents per 100 truck years” to make the comparisons.

Johnson said using a timeline instead of miles traveled does not provide enough information about the risk factors for crashes.

“A fleet could have a large number of trucks for a year and travel very few miles on high-speed roads,” Johnson wrote.

“The standard and intuitive measure of exposure for speed-related accidents would be the number of miles traveled on higher speed roadways.”

Johnson noted that one motor carrier accounted for 486 of the 636 so-called “relevant” crashes in the entire non-speed-limited group being studied. He says that figure should have flagged the carrier as an exception to the rule rather than representative of all non-speed-limited trucks.

“If this one fleet is excluded, due to being an outlier and not representative, there is no longer a statistically significant difference between the speed-limited and non-speed-limited (trucks),” he wrote.

Johnson says the study highlights exposure to crash risk of certain types of fleets but does not make a clear case for speed limiters as an industry-wide practice.

Looking at the speed-limiter study as a case in point, Johnson says studies can be “statistically significant” but not necessarily valid.

“For a study to be valid,” he wrote, “it is important that the results are statistically significant; however, statistical significance does not guarantee that a study is meaningful and useful,” Johnson wrote.

Fuel savings questioned
In addition to claims about safety, the FMCSA study claims that speed limiters cause trucks to burn less fuel, therefore producing an economic and environmental benefit.

Johnson says while the study mentions fuel savings as being accepted within the trucking industry, it stops short of using science to prove it.

“Given the fact that there are no actual results reported that address fuel consumption, the reader is left to assume that there was no relationship observed between the two factors,” he wrote.

In previous research, Johnson has noted that fleets do not obtain the reduction in fuel consumption that is generally assumed in the industry. LL

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