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Status Report, Vol. 47, No. 6 | August 14, 2012 Subscribe

Truck tractors, buses could get standard ESC under NHTSA proposal

Electronic stability control (ESC), a crash avoidance feature required on 2012 and newer model passenger vehicles, could become standard on new large truck tractors and certain large buses if the National Highway Traffic Safety Administration (NHTSA) adopts a proposed rule announced in May. 

Analysis of the real-world experience of ESC indicates the technology is saving lives by helping to prevent rollovers and loss-of-control crashes in cars, minivans, pickups and SUVs (see "ESC benefits keep adding up as feature becomes standard," Sept. 28, 2011). Researchers haven't yet been able to quantify ESC's real-world effect for large trucks.

NHTSA has been conducting extensive research on stability control systems for truck tractors and large buses since 2006. The agency has sponsored studies of crash data to help examine the potential safety benefits, conducted test track studies to understand how the systems respond in different maneuvers, and undertaken simulator evaluations to understand how drivers might use the systems. NHTSA also has evaluated data on dynamic test maneuvers from truck makers and brake suppliers. The agency used this information to develop test maneuvers that define what systems must do to comply with the proposed regulation and to predict the effectiveness of stability control systems.

The agency estimates that ESC on truck tractors could prevent 40 to 56 percent of untripped rollovers and an additional 14 percent of loss-of-control crashes each year. Requiring ESC for all truck tractors and certain buses with a gross vehicle rating of more than 26,000 pounds would prevent as many as 2,329 crashes and as many as 858 injuries, plus save as many as 60 lives annually. NHTSA based its estimates for truck tractors and large buses on an initial target crash population of 10,313 crashes (5,510 rollovers and 4,803 loss-of-control crashes), 327 fatalities (111 rollover and 216 loss of control) and 3,358 injury crashes (2,217 rollover and 1,141 loss of control).

There are two kinds of stability control systems available for truck tractors: ESC and roll stability control. Roll stability control systems automatically intervene if sensors monitoring a truck tractor's lateral acceleration and wheel speed detect a high rollover risk. Tractor-based systems can selectively apply brakes on the tractor's drive axle and the trailer, plus modulate engine power to slow down the truck and keep it upright. Trailers can have roll stability control, too, but trailer systems only control trailer brakes, not the tractor's, and can't reduce engine power. Roll stability control can reduce rollovers but isn't meant to help drivers maintain directional control. That ability is unique to ESC.

ESC incorporates all of the features of roll stability control but with the added benefit of a steering-angle sensor and a yaw sensor to measure the tractor's directional stability. These sensors help mitigate severe oversteer and understeer conditions. If measures exceed specific thresholds, ESC reduces engine power and can selectively apply brakes on multiple tractor axles in order to bring the vehicle back into line, plus apply the trailer brakes as needed to slow down the truck.

"We support NHTSA's decision to require ESC rather than roll stability control," says Anne McCartt, Institute senior vice president for research. "Although it's more costly, ESC affects a wider range of rollover and loss-of-control collisions than roll stability control." 

About 26 percent of 2012 model truck tractors and 80 percent of new buses have ESC, NHTSA estimates. If adopted, the rule would take effect within two years of publication. The agency isn't requiring in-service truck tractors and buses to be retrofitted but seeks comments on a future ESC retrofit mandate.

Single-unit trucks aren't included in the current proposal because NHTSA needs to determine how effective ESC would be for these vehicles. The agency plans a feasibility study and could consider a future ESC requirement. Estimating the technology's potential benefits for single-unit trucks is challenging, NHTSA says. Compared with truck tractors, the single-unit truck fleet varies by model in terms of weight, wheelbase, axles, cargo type and other factors that affect the calibration and performance of stability control systems. Another issue is that because multiple suppliers handle the design and building of each vehicle, chassis suppliers who fit brakes and potentially stability control often don't know the vehicle's ultimate function. NHTSA also notes that stability control systems are more widely available for air-braked vehicles, including truck tractors, than for hydraulic-braked single-unit trucks. Only about 1 percent of new single-unit trucks have these systems now, NHTSA says.

"We commend the agency for moving ahead with an ESC mandate for new truck tractors and large buses independent of its ongoing research on single-unit trucks," McCartt says. "We encourage NHTSA to expedite this research and also to explore the feasibility of a retrofitting requirement so a bigger proportion of the fleet benefits from ESC." 

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