NHTSA proposes federal rule requiring electronic stability control systems on large commercial trucks and buses
The US Department of Transportation’s National Highway Traffic Safety Administration (NHTSA) has proposed a new federal motor vehicle safety standard to require electronic stability control (ESC) systems on large commercial trucks, motorcoaches, and other large buses (gross vehicle weight rating of greater than 11,793 kg / 26,000 lbs) for the first time.
ESC systems use engine torque control and computer-controlled braking of individual wheels to assist the driver in maintaining control of the vehicle and maintaining its heading in situations in which the vehicle is becoming roll unstable (i.e., wheel lift potentially leading to rollover) or experiencing loss of control (i.e., deviation from driver’s intended path due to understeer, oversteer, trailer swing or any other yaw motion leading to directional loss of control).
In such situations, notes NHTSA, intervention by the ESC system can assist the driver in maintaining control of the vehicle, thereby preventing fatalities and injuries associated with vehicle rollover or collision. NHTSA research shows the technology could prevent up to 56% of rollover crashes each year—the deadliest among all crash types—and another 14% of loss-of-control crashes.
In the Notice of Proposed Rulemaking (NPRM), NHTSA identifies two types of stability control systems developed for heavy vehicles: roll stability control (RSC) systems designed to prevent rollover by decelerating the vehicle using braking and engine torque control; and ESC.
[ESC] 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 we have received to date, the agency has tentatively determined that ESC and RSC systems are not available for hydraulic-braked medium or heavy vehicles.
A NHTSA research program to determine how available stability control technologies affect crashes involving commercial vehicles found ESC systems to be the most effective tool for reducing the propensity for heavy vehicles to rollover or lose control. As a result of the data analysis research, NHTSA determined that ESC systems can be 28 to 36% effective in reducing first-event untripped rollovers and 14% effective in eliminating loss-of-control crashes caused by severe oversteer or understeer conditions.
NHTSA estimates that in 2012, about 26% of new truck tractors and 80% of new buses affected by this proposed rule will be equipped with ESC systems. A standard requiring ESC on the nation’s large trucks and large buses would prevent up to 2,329 crashes, eliminate an estimated 649 to 858 injuries, and prevent between 49 and 60 fatalities a year, according to NHTSA.
We’ve already seen how effective stability control can be at reducing rollovers in passenger vehicles—the ability for this type of technology to save lives is one reason it is required on cars and light-duty trucks beginning with model year 2012. Now, we’re expanding our efforts to require stability enhancing technology on the many large trucks, motorcoaches, and other large buses on our roadways.—NHTSA Administrator David Strickland
The NPRM requires that the ESC system meet both definitional criteria and performance requirements. The definitional criteria are consistent with those recommended by SAE International and used by the United Nations (UN) Economic Commission for Europe (ECE), and similar to the definition of ESC in FMVSS No. 126, the agency’s stability control standard for light vehicles.
This definition would describe an ESC system as one that would enhance the roll and yaw stability of a vehicle using a computer-controlled system that can receive inputs such as the vehicle’s lateral acceleration and yaw rate, and use the information to apply brakes individually, including trailer brakes, and modulate engine torque. The proposal also requires that the system be able to detect a malfunction and provide a driver with notification of a malfunction by means of a telltale.
NHTSA is proposing to use two test maneuvers for performance testing: the slowly increasing steer (SIS) maneuver and the sine with dwell (SWD) maneuver. The SIS maneuver is a characterization maneuver used to determine the relationship between a vehicle’s steering wheel angle and the lateral acceleration. This test serves both to normalize the severity of the SWD maneuver and to ensure that the system has the ability to reduce engine torque.
Using the steering wheel angle derived from the SIS maneuver, the agency would conduct the sine with dwell maneuver. The SWD test maneuver challenges both roll and yaw stability by subjecting the vehicle to a sinusoidal input.
While many truck tractors and large buses can currently be ordered with this technology, the proposed standard would require ESC systems as standard equipment on these types of vehicles. As proposed, the rule would take effect between two and four years after the standard is finalized, depending on the type of vehicle.
The NPRM is being published in the Federal Register and members of the public will have the opportunity to comment on the proposal for 90 days. NHTSA will also hold a public hearing on the proposed safety standard to solicit further public comment—the date and location of that hearing will be published in the coming weeks.