Mandatory speed limiters. It's one of those topics on which you see heated debate in this industry, and there doesn't seem to be a clear-cut winner.

At the Commercial Vehicle Safety Alliance's annual meeting this fall, law enforcement officials were reluctant to endorse one safety group's push for mandatory limiters on trucks in the U.S.

Steve Owings is co-founder of Road Safe America, which wants all Class 7 and Class 8 commercial vehicles built after 1990 to have their speeds capped at 65 mph. The group petitioned the National Highway Traffic Safety Administration for the mandate in 2006, and the agency said earlier this year it would explore the possibility and publish a notice of proposed rulemaking next year. The American Trucking Associations is also in favor of speed limiters, but has asked for them to be required in trucks built after 1992.

Carriers such as Schneider and J.B. Hunt already use speed limiters, and support such a mandate. The main opposition has come from the Owner-Operator Independent Drivers Association.

The problem with different speeds

At issue is the expectation that speed limiters would create a speed differential on highways, which is also an issue in some states that post different speed limits for cars and trucks. It is pretty well documented that vehicles traveling either excessively faster or slower than other traffic on the road are likely to cause accidents. However, will that be outweighed by the benefit of slowing trucks down?

At CVSA's meeting, one Grand Prairie, Texas, traffic investigator told Owings that "whether the truck's going 15 mph faster than the car or 15 mph slower than the car, you've still got a speed differential there," reported ATA's Transport Topics newspaper. "You're going to have collisions out there. That is not safe."

Yet Owings contended that the United Kingdom and Australia reported crash reductions after implementing mandatory speed limiter requirements. Canadian provinces Ontario and Quebec also have speed limiter laws, but so far there's been little data produced as to the effect of the regulations.

We decided to ask safety guru Ron Knipling, whose book, "Safety for the Long Haul," is a definitive work on the causes of truck crashes. In the book's section on differential speed limits, he notes that "There is a rationale for differential speed limits, but there are also compelling arguments against them."

Pro and con

"'Pro' arguments center on the operational limitations of trucks and the severity of their crashes," Knipling says in his book. "Trucks have longer stopping distances and are more vulnerable to brake failure and rolling over, suggesting that speeds lower than other traffic would reduce crash risks.

"An 'anti' argument is that differential speed limits increase the variation among vehicle speeds on the highway, whereas uniform speed limits reduce speed variation. Less speed variation among vehicles reduces conflicts, and therefore reduces risk. FHWA studies indicate that crash involvement rates increase for any vehicles traveling significantly faster or slower than surrounding vehicles. The risk of rear-end crashes in particular is reduced on roads with uniform speed limits compared to those with different speed limits for different vehicles."

Knipling reports that a 2004 FHWA Tech Brief reviewed several major studies of states that switched from uniform to differential speed limits, vice-versa, or maintained existing uniform or differential speed limits. The study found mixed and inconclusive effects on crash rates, leaving the safety question unresolved. Both average vehicle speeds and crash risks increased generally over the 10 years of data collection, and these trends far outweighed any effect, positive or negative, of differential speed limits.

"So, you have a countermeasure -- differential speed limits -- with no verifiable benefits. Truckers do bear a cost, however, in reduced productivity and potential groundless liability claims. Four-wheelers striking truck trailers may allege that trailer lighting or conspicuity treatments were deficient. The lack of evidence in favor of differential speed limits is compelling evidence against them. Differential car-truck speed limits seem justifiable only at unusual roadway locations like steep downgrades or work zones."

So, in a nutshell: Speed limiters improve safety by slowing down trucks. The consequent speed differential harms safety by introducing a factor that can be difficult for drivers to manage. Which is more or less dangerous? We don't know.

We'll know where NHTSA comes down on it in December 2012, when the agency is supposed to publish the proposed rule.