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Stable Talk

Posted: August 1, 2014

Finally we have a definitive study on the two key kinds of stability control systems for heavy trucks, a U.S. effort that takes us a step closer to seeing them mandated.

Published last fall by the National Highway Traffic Safety Administration (NHTSA), it’s called “Safety Benefits of Stability Control Systems For Tractor-Semitrailers”.

Whether it’s looking at roll stability control (RSC) or electronic stability control (ESC), which adds understeer/oversteer sensing, they’re both proven to be huge difference makers.

It’s worth pointing out that rollovers occur in only about 13 percent of heavy-truck fatal crash involvements, but they account for 50 percent of truck-occupant fatalities.

NHTSA’s study was conducted by the much respected University of Michigan Transportation Research Institute (UMTRI) under a co-operative agreement between NHTSA and Meritor WABCO Vehicle Control Systems.

NHTSA supplied the money as a grant to Meritor WABCO, which then supplied expertise to make the simulator test hardware work and to act as the link between the equipment manufacturers, while funding UMTRI to do the testing and an awful lot of accident analysis.

This is the most comprehensive report yet on the potential for electronic stability control and roll-stability control technologies to reduce jackknifes, rollovers, and other accidents involving a loss of vehicle control. And it will very likely lead to a mandate soon because the study offered up what may well be a conservative estimate suggesting that RSC could prevent at least 3489 rollover crashes in the U.S. annually, rising to 4659 rollover and other loss-of-control (LOC) crashes with ESC.

“They [NHTSA] are very seriously looking at mandating it on heavy trucks,” says Alan Korn, Meritor WABCO’s director of vehicle dynamics and controls. “I’m guessing they’ll make a decision in 2010, with a likely implementation in 2012 or 2013.” It’s 99.999-percent likely that Canada will follow suit.

Deciding to do it is one thing, but Korn allows that writing the rule will be a challenge. He adds that it will be especially difficult with loss-of-control crashes, partly because it’s extremely difficult to isolate their causes.

A similar European Union mandate is set to take effect in 2011.

The UMTRI study was specifically designed to estimate the potential benefit of the two distinct safety systems, RSC and ESC. The former senses vehicle lateral acceleration in a curve and intervenes to slow the vehicle in accordance with an algorithm. The deceleration interventions are graduated in this order: de-throttling; engine brake; and foundation-brake application. 

TO SWERVE AND PROTECT: ESC can help drivers
maintain control through an emergency manoeuvre.

The ESC system contains all the attributes of the RSC system plus yaw sensing and thus the added capability of seeing and then controlling vehicle understeer and oversteer, which are directly related to loss of control. The loss-of-control intervention strategy uses selective braking of individual wheels on the tractor.

One of the key issues in the study involved the paucity of real-world crash data to work with because stability systems haven’t been around all that long and just aren’t widely used yet. So the study was based on the analysis of independent crash datasets using engineering and statistical techniques to estimate the probable safety benefits of stability control technologies for five-axle tractor-semitrailer vehicles. It’s complicated stuff, to say the least, but the researchers examined two distinct accident databases and isolated crashes that fit certain criteria, namely those that suggested a given crash could have been affected by the use of RSC or ESC.

They also examined the comprehensive records of one un-named for-hire fleet that has used some variation of these technologies in significant numbers for quite a few years. There’s some interesting stuff in there, including the fact that icy roads mean you’re 30 times more likely to see a jackknife. That risk hasn’t been quantified before, and 30 times is a heck of a lot.

We’ll just quote the study itself to give you the basic results, noting that we’re only talking about the U.S. here and the dollar figures are in American currency:

“The findings of the study indicate that stability control systems provide substantial safety benefits for tractor-semitrailers. Assuming that all existing five-axle tractor-semitrailers operating on U.S. roads were fitted with RSC, the expected annual rollover relevant safety benefit is a reduction of 3489 crashes, 106 fatalities, and 4384 injuries.

Alternatively, assuming that all existing five-axle tractor-semitrailers operating on U.S. roads were fitted with ESC, the expected annual combined rollover and directional (yaw) instability relevant safety benefit is a reduction of 4659 crashes, 126 fatalities, and 5909 injuries. Because ESC addresses both rollover and yaw instability crashes and it is more effective in mitigating rollover crashes (through additional braking capabilities over RSC), the net annual expected benefit for an ESC system was found to be greater than for RSC.

“Assuming ESC was fitted to all tractor-semitrailers, savings from rollovers ­prevented by ESC are estimated at $1.527 billion annually, and from LOC crashes prevented at $210 million annually, for a total of $1.738 billion annually. Assuming RSC was fitted to all tractor-semitrailers, savings from rollovers prevented are estimated at $1.409 billion annually, and from LOC crashes prevented at $47 million annually, for a total estimated benefit of $1.456 billion annually.” 

WABCO’s Alan Korn expects a decision on a stability-control
in 2010, with likely implementation in 2012 or 2013.

In the report, UMTRI notes that “…there are events that may occur in other crash types that would benefit from the [RSC and ESC] technologies, but these crashes cannot be identified effectively using coded data.”

Perhaps not surprisingly, the UMTRI examination of accident data revealed a distinct connection between driver age and the probability of either rollover or jackknife in a crash. But the two change at different rates. Younger drivers have “a somewhat higher probability of rolling over… which declines after about age 30 but then rises at the oldest age category, 65 and older.” In fact it rises sharply at age 65, almost equal to the 25-to-29 age range where the probability peaks. With jackknife crashes, the probability is highest — consistently so, and higher than rollovers — between ages 20 and 34. Then it drops steadily to age 65 and rises slightly again.

Interestingly, the evidence seems to suggest that drivers don’t get a firm handle on controlling the jackknife issue until age 50 but they beat the rollover risk earlier, at age 40. Until, that is, they’re 65 when it spikes.

And here’s a few other tidbits of info uncovered in the study: the odds of loss of control are 4.7 times higher on curved roads than on straight roads; a rollover is more likely in dry-surface conditions, while loss of control is more likely on wet surfaces or other road conditions with less friction; the odds of rollover increase as cargo weight increases; the odds of loss of control were 1.9 times greater for tanks compared to vans; and the odds of rollover are greatest at speeds ranging from 40 ot 55 mph.

Bendix supports the study’s findings, of course, and notes that a separate NHTSA study was also released recently, looking at crashes involving single-unit medium and heavy vehicles such as school buses and straight trucks. Bendix tells us that, of the straight-truck crashes where a stability system could have helped, ESC may have mitigated 91 percent of them.

What isn’t clear from the report, because only tandem/tandem rigs were investigated, is the effect of RSC and ESC systems on multi-axle tractor-trailers. A safe assumption is that broadly similar safety improvements would be seen on B-trains and the like but that will only be proven in time.

Another angle to watch is the reaction of insurance companies to the spec’ing of such safety enhancements in a given fleet. Another safe assumption is that premium discounts will be at least considered. Again, only time will tell.

As Allan Korn puts it, “that’s very much a show-me industry. What they want to see over a period of time is accidents going down.”

And it’s more than just likely that they will if the experience of California’s Apex Logistics is any indication, as it probably is. A dry- and liquid-bulk hauler that runs up and down the west coast, including Canada, three quarters of its 250 tractors are now equipped with Meritor WABCO RSC.

That started in January of 2005, at which point they’d been averaging three rollovers a year, but they haven’t had one since. And in the process they’ve cut workers comp, physical damage, and liability insurance costs by 50 percent. 


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