The National Highway Traffic Safety Administration (NHTSA) has issued a notice of proposed rulemaking (NPRM) that would establish a new Federal Motor Vehicle Safety Standard No. 136 to require electronic stability control (ESC) systems on truck tractors and certain buses with a gross vehicle weight rating of greater than 26,000 pounds.
There have been two types of stability control systems developed for heavy vehicles. A roll stability control (RSC) system is designed to prevent rollover by decelerating the vehicle using braking and engine torque control. The other type of stability control system is ESC, which includes all of the functions of an RSC system plus the ability to mitigate severe oversteer or understeer by automatically applying brake force at selected wheel-ends to help maintain directional control of a vehicle.
To date, ESC and RSC systems for heavy vehicles have been developed for air-braked vehicles. Truck tractors and buses covered by this proposed rule make up a large proportion of air-braked heavy vehicles and a large proportion of the heavy vehicles involved in both rollover crashes and total crashes. Based on information NHTSA has received to date, it has tentatively determined that ESC and RSC systems are not available for hydraulic-braked medium or heavy vehicles.
This proposal requires that the ESC system must meet both definitional criteria and performance requirements. NHTSA says it is necessary to include definitional criteria in the proposal and require compliance with them “because developing separate performance tests to cover the wide array of possible operating ranges, roadways, and environmental conditions would be impractical.”
The agency considered two regulatory alternatives. First, the agency considered requiring truck tractors and large buses to be equipped with RSC systems. When compared to this proposal, RSC systems would result in slightly lower cost per equivalent life saved, but would produce net benefits that are lower than the net benefits from this proposal. NHTSA says this is because RSC systems are “less effective at preventing rollover crashes and much less effective at preventing loss-of-control crashes.”
The second alterative considered was requiring trailers to be equipped with RSC systems. However, this alternative “would save fewer than 10 lives at a very high cost per equivalent life saved and would provide negative net benefits.”
NHTSA researchers at VRTC in East Liberty, Ohio, initiated a test program in 2006 to evaluate the performance of stability control systems under controlled conditions on a test track, and to develop objective test procedures and measures of performance that could form the basis of a new FMVSS.
When both tractor- and trailer-based stability control systems were enabled, results were similar to the results of the tractor-based stability control system for the low CG and high CG conditions. Under the LLVW condition, results were similar to the trailer-based RSC system values observed.
The trailer-based RSC system also improved the baseline vehicle’s roll stability in the J-turn maneuver.
Comments on the NPRM are due by August 21, 2012.