ing FMVSS No. 121 to improve the stopping distance of trucks. By 2011, most new trucks will be required to have the capability to reduce their stopping distance 30 percent more than had previously been required. (This is an important improvement, but the stopping distance is still much longer than that of light-duty vehicles.) The improved braking performance can be accomplished by the use of larger drum brakes or air disc brakes. In time, the NHTSA would be expected to conduct field tests to assess the effects of this new braking requirement.
It should be noted that the field performance of antilock braking systems (ABSs) required by FMVSS No.121 on all air-braked vehicles of 10,000 lb or greater manufactured after March 1, 1997, has shown mixed results. In a comprehensive study published by the NHTSA in July 2010 (Kirk, 2010), it was found that there was a statistically significant 6 percent reduction in the number of crashes where ABS is assumed to be influential, and a large reduction in jackknives and off-road overturns; yet it was found that there was not a statistically significant reduction in fatal crash involvement. Although improved braking was influential in reducing the number of accidents as noted above, it is possible that accidents that are so severe as to cause a fatality cannot be avoided simply by improved braking. In addition, drivers need to be trained not to push the ABS technology to its limits.
Over the past 5 years, truck manufacturers have been offering electronic stability control on several truck models, and ESC has become standard on some truck models. DOE (2010) provides a detailed explanation of how ESC works. Because the application of stability control systems is fairly recent, there are insufficient real-world data to assess its effectiveness. However, studies have shown that the systems do offer potential for accident and fatality reduction. In Woodrooffe et al. (2009), crash scenarios were selected from national databases and examined to assess the potential benefit of stability systems on 5-axle tractor semitrailers. Assuming that all 5-axle tractor semitrailers were equipped with ESC systems, the expected annual safety benefit related to combined rollover and directional (yaw) instability is a reduction of 4,659 crashes, 126 fatalities, and 5,909 injuries.
Anticipatory automatic braking and speed control systems may also be used for accident prevention. However, these systems were not included in the materials prepared by or presented by the DOE or DOT, and therefore were not evaluated by the committee.
Advancements in collision warning systems for heavy-duty trucks have continued over the past several years. The 21CTP supports this area, because it may have potential for significant benefit in improving highway safety. Warning systems currently available include the following:
• Lane departure warning (LDW),
• Forward collision warning (FCW),
• Side object detection, and
• Rear object detection.
These systems use radar, video detection, ultrasonic, and other sensor systems combined with sensor input analysis algorithms to determine if a crash situation is developing, and then they warn the driver (DOE, 2010). Some systems not only warn the driver but also take control of the vehicle by de-throttling or braking.
In the NRC Phase 1 report (NRC, 2008, Chapter 7), it was reported that, based on field operational tests (FOTs) that had been completed at that time, LDW systems could potentially provide a reduction in accidents for single-vehicle roadway departure of a little more than 20 percent. In a more recent study, estimates were made of the cost-benefit potential of LDW systems (Houser et al., 2009). General Estimates System (GES) data were used to estimate outcomes from different lane departure crashes.10 Then, using information from the aforementioned field operational test, efficacy rates were determined in order to estimate the types of crashes that could be prevented using LDW systems. Assuming that the systems had been in place from 2001 to 2005, and recognizing that certain types of accidents could not have been prevented by LDW (e.g., loss of steering control from brake lock-up), it was estimated that the mean average annual preventable fatalities could be 147 and preventable injuries could be 2,642.
The DOT has taken an approach of integrating forward collision, rear-end impact, road departure, and lane changing warning systems into what it calls Integrated Vehicle-Based Safety Systems (IVBSS). This program also involves the University of Michigan Transportation Research Institute, Battelle, Eaton, PACCAR, Conway, Navistar, Takata, and the Michigan Department of Transportation. A field operational test was recently completed—it was a 10-month test involving 10 trucks and 20 drivers. Some key findings of the FOT were encouraging (DOT, 2010a):
• Drivers stated that the system made them more aware of the traffic environment;
• Most of the drivers would recommend the purchase of such a system, would prefer to drive a truck with such a system, and thought that such systems would increase driving safety; and
• Seven drivers said that the system potentially prevented them from having a crash.
In an independent evaluation of the FOT results, the John A. Volpe National Transportation Center estimated that the integrated system would be 11 percent effective in preventing accidents of the type targeted by IVBSS, and therefore could prevent, annually, 13,000 crashes involving trucks.11