Safety in Numbers

| May 29, 2007

Combining differential speeds, basic physics and a wealth of truck crash data results in a mixed message about the wisdom of governing truck speed.

Common sense tells us that the faster vehicles move, the more likely they are to wreck. The greater the speed of a wreck, the worse it will be. Common sense also tells us that vehicles moving together at varying speeds are more likely to wreck than those moving at virtually the same speed.

Which point is more important when it comes to governing truck speed? Regulators now face that question and others as they weigh the merits of truck governing petitions submitted by the American Trucking Associations and Road Safe America.

“Information about the kind of precise effect of speed on crash rates, at the level of resolution needed, is not currently available,” says Dan Blower, a researcher at the University of Michigan’s Transportation Research Institute. He served as a consultant on one of the most important truck safety studies to date, the Large-Truck Crash Causation Study, a project of the Federal Motor Carrier Safety Administration and the National Highway Transportation Safety Administration, released in March 2006.

The Truckload Carriers Association considers the data so inadequate that it opposes the petitions. In its comments, TCA says, “While ATA and Road Safe America have determined that speeds greater than 68 mph are unsafe and detrimental to the motoring public, neither have been able to justify why 68 mph is the barrier at which unsafe speed is obtained.”

For some safety proponents, including Road Safe America co-founder Steven Owings, crash data pales in comparison to the physics of a truck at high speed. Because it’s “the biggest, by far most dangerous vehicle on the road, that can’t see as well as smaller vehicles, can’t maneuver as well as smaller vehicles, and mostly importantly can’t stop anywhere near the same distance as smaller vehicles,” it should be kept to slower speeds, he says.

Following is an analysis of some key speed statistics.

Speeding nearly triples the odds of being involved in a crash.
- AAA Foundation for Traffic Safety

Speeding is a factor in 30 percent of all fatal crashes.
- National Highway Transportation Safety Administration

As opponents of governing note, much of the speed-related crash data is too broad to determine the effectiveness of governing trucks. “Speeding” can be 40 mph in a 30 mph zone, for example. “Too fast for conditions,” a common accident factor, includes driving on wet highways, around tight curves and through construction zones, at speeds that could be high or low, legal or not.

Nevertheless, certain basics about speed can’t be ignored, argue low-speed proponents. On its website, the Insurance Institute for Highway Safety notes that speed increases both the severity and the risk of having accidents in three ways: It reduces driver response time; it increases stopping distance; it increases crash energy exponentially (a 50 percent increase in speed increases energy by 125 percent).

Beyond that, the effect of tinkering with speed limits isn’t always clear. Look, for example, at what happened as Interstate speed limits changed in recent decades.

By March 1974, all states had adopted a 55 mph speed limit, following federal highway funding pressure in response to the oil crisis. “The National Research Council attributed 4,000 fewer fatalities to the decreased speeds in 1974, compared with 1973,” says IIHS.

After the 1995 repeal of the 55 mph national speed limit cap, 31 states had raised speed limits to 70 mph or higher by mid-2006. Yet highway fatality rates per 100 million vehicle miles fell from 1.73 in 1995 to 1.46 in 2005, a 16 percent decline, according to NHTSA data cited in a 2006 Wall Street Journal editorial.

Of those 31 states, “29 saw a decline in the death and injury rate and only two – the Dakotas – have seen fatalities increase,” wrote the Journal. “Two studies, by the National Motorists Association and by the Cato Institute, have compared crash data in states that raised their speed limits with those that didn’t and found no increase in deaths in the higher speed states.”

That 16 percent decline in fatalities, however, pales before the 50 percent decline in fatalities between 1974 and 1995, when the national speed limit was in effect, wrote Judith Stone, president of Advocates for Highway and Auto Safety, in response to the Journal editorial. “Fatalities have increased on roads with speed limits of 65 mph and higher, while fatalities on roads posted at 50 mph and lower have remained stable,” she wrote.

Only 0.2 percent of truck crashes occur at speeds above 70 mph.
- A 1991 National Highway Transportation Safety Administration report to Congress

In the 20 percent of truck-involved crashes where speed was a factor, it exceeded 68 mph.
- American Trucking Associations study of crash data

The problem with the first statistic, a favorite among governing opponents, is it’s based on a period when the nationwide speed limit was capped at 55 mph or, in the case of many rural Interstates, 65 mph. So one would expect wrecks at 70-plus mph to be somewhat rare, says Anne McCartt, senior vice president for research for IIHS. Blower of UMTRI offers a similar, updated analysis. From 1999 to 2003, when speed limits were similar to today’s, 6.7 percent of the trucks involved in fatal wrecks were traveling faster than 68 mph, he says.

As for the second statistic, though speed was not the sole factor, it still correlated with one in five truck crashes. And even when speeding is not cited as a contributing cause of an accident, speeds of 68-plus mph obviously contribute to the severity of an accident, McCartt says.

The 1991 NHTSA report was one of the most ambitious of any that have focused on heavy trucks at speeds above 65 mph. The study supported speed governors for fleets but found insufficient justification to mandate it.

The 2006 Large-Truck Crash Causation Study showed that among the crash causes attributed to trucks, the most common “were driver factors, such as legal drug use, traveling too fast for conditions, [and] unfamiliarity with the roadway.”

Because this and other studies still don’t present enough of the information needed to weigh all the issues involved with governing trucks, better data is needed – and on the way, says Dan Murray, American Transportation Research Institute vice president. ATRI’s survey of carrier speed and safety “will bring more clarification to what is safe speed,” Murray says.

Preliminary results showed 69 percent of respondents govern trucks, at an average governed speed of 69 mph. That’s encouraging, Murray says, because it’s so close to the 68 mph requested in the petitions – though he admits that correlation is primarily based on posted speed limits, not a proven optimal level for safety.

“Vehicles traveling 10 to 15 mph slower than the mean speed of
traffic were much more likely to be involved in accidents than vehicles traveling slightly above the mean speed.”
- Testimony before a Senate committee in 2003 by Julie Cirillo, former head of the Federal Motor Carrier Safety Administration, citing a 1963 study by David Solomon

Eleven states have split truck/car speed limits on rural interstates, most with a 10 mph differential.
- Insurance Institute for Highway Safety

Thanks to state laws and carriers’ use of speed governors, the motoring public has been living with split car/truck speed limits for years. Whether this creates a problem continues to be debated.

Split speeds create safety hazards, says Todd Spencer, executive vice president of the Owner-Operator Independent Drivers Association. Take Ohio, where rural Interstates have 65/55 mph limits.

“All the trucks are congregated in the right-hand lane,” Spencer says. “It makes it harder for people to get on and off the highway.” Consequently, four-wheelers begin speeding or making inappropriate lane changes to work around the solid lanes of trucks. “You actually condition people to drive stupid,” Spencer says.

A 2006 University of Arkansas study comes down largely against speed differentials. “Data from previous studies and simple logic say that a higher number of interactions among vehicles increases the chance that accidents will occur,” says Steven Johnson, a UA industrial engineering professor, though he acknowledges that “lower truck speeds help equalize the stopping distance” between heavy trucks and four-wheelers.

Cirillo’s testimony noted that the controversial relationship between speed limits and safety has spawned many studies. “There has been no evidence to alter Solomon’s original finding that variance from the mean operating speed is a major contributor to accidents,” she said. “Many safety organizations and states, including Ohio, advise drivers to ‘drive with the flow of traffic.’”

Trucks, however, already average 6 mph slower than four-wheelers in highway traffic, Blower says, “so I don’t think there’s any strong evidence in the literature that there would be a negative safety impact” if trucks were limited to 68 mph. That level of governing would probably change the types of crashes – for example, more cars rear-ending trucks – but the bottom line could be fewer severe crashes, he says.

Road Safe America’s Owings disputes the differential speed argument for the same reason, only he estimates the current Interstate differentials are far higher than 6 mph.

Both speed and speed differentials are important factors in accidents, says IIHS, but “the risk of death and severe injury is a direct exponential function of speed, not speed differences.” Also, “Institute research found that lower speed limits for trucks on 65 mph highways reduced the proportion traveling faster than 70 mph without increasing variation among vehicle speeds. The percentage of trucks traveling faster than 70 mph was twice as large in states with uniform 65 mph limits.”

“The solution is not to let trucks go faster and faster to keep up with the passenger vehicles,” says IIHS’s McCartt. “The solution is to make all speeds safer.”

“In 2004, 86 percent of speeding-related fatalities occurred on roads that were not Interstate highways.”
- NHTSA report

Thirty-two states have different speed limits for rural and urban Interstates.
- Insurance Institute for Highway Safety

One of the things that’s always overlooked is where the vast majority of accidents take place,” OOIDA’s Spencer says. “It’s not on roads that have the maximum speed limits.”

Only 14 percent of speeding-related fatalities occur on Interstates, the highways where speed governing would have its main impact because most non-Interstate highways have speed limits below 68 mph. Of that 14 percent, factors other than speeding are responsible for some accidents.

Even so, say McCartt and others, that’s no reason to shy away from speed governing. “The proposed rule should be evaluated more on its merits rather than on what it wouldn’t address,” she says. “Even on some non-Interstate highways, trucks can go at very high speeds.”

Blower suggests considering the data of large carriers with experience running governed and ungoverned trucks on Interstates to see whether governing appears to improve safety.

Given that two-thirds of states have higher speed limits on rural Interstates, the practicality of mandating a nationwide truck speed limit becomes more questionable. Add other variables, such as varying truck sizes and applications, and the issue is even more complicated, Murray says.

“Ultimately the question everyone in government and industry needs to resolve is: Do we promote a level playing field, or recognize that the complexity of the industry takes precedence, and one size does not fit all?” he says.

With more than 15,000 drivers and a long history of governing trucks, Schneider National is a safety experiment unto itself.

Schneider’s owner-operators drove 17 percent of the company’s miles in 2006 with ungoverned trucks, yet “they were involved in 40 percent of our potentially severe crashes,” the company says in its comments filed on the speed governing petition. “Since their exposure is comparable to our company drivers (whose trucks are governed at 65 mph), it’s reasonable to conclude that the most significant variable between the two groups is speed.”

Such experience may well be significant, says Dan Blower, a researcher at the University of Michigan Transportation Research Institute. Still, he cautions that the two driver groups need to be scrutinized for factors other than speeding, such as driver experience levels and types of equipment, that might affect crash rates.

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