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BACKGROUND
RESEARCH PROGRAM OBJECTIVES
RESEARCH PROGRAM STATUS
RESEARCH PROGRAM TASK DETAILS

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By Task:
Task 1: Update Statistical Studies to Determine the Effects of ABS on Single- and Multi- Vehicle Accidents
Task 2: Conduct a Representative National Survey of Driver Experiences and Expectations Regarding Conventional Brakes versus ABS
Task 3: Study NASS Hardcopy Crash Files to Determine Typical Single-Vehicle Crashes for ABS-Equipped Vehicles
Task 4: Test Track Study of the Performance of Production ABS Over a Broad Range of Real World Situations
Task 5: Conduct Tests to Assess the Prevalence of Driver "Oversteering" in Panic Braking Situations
Task 6: Test the Effects of ABS When Performing a Two-Wheel Road Recovery Maneuver
Task 7: Study Driver Risk Compensation in Relation to ABS-Equipped Vehicles
Task 8: Assess the Overall Effect of Passenger Car ABS on Drivers’ Ability to Avoid Crashes
Task 9: Status Briefings

BACKGROUND

Antilock brake systems (ABS) have been introduced on many passenger car and light truck make/models in recent years. Brake experts anticipated that the introduction of ABS on passenger vehicles would reduce the number and severity of accidents. A number of statistical analyses of accident databases have been performed during the last three years. These analyses suggest that the introduction of ABS does not seem to have reduced the number of automobile accidents where they were expected to be effective. Kahane stated that involvements in multi-vehicle crashes on wet roads were significantly reduced by 24 percent, and nonfatal crashes by 14 percent (with ABS). However, these reductions were offset by a statistically significant increase in the frequency of single-vehicle, run-off-road crashes (rollovers or impacts with fixed objects), as compared to cars without ABS. Fatal run-off-road crashes were up by 28 percent and nonfatal crashes by 19 percent. It is unknown to what extent, if any, this increase is due to ABS or other causes. It is also unknown to what extent, if any, this increase is due to drivers incorrect usage of ABS or incorrect responses by drivers to their ABS.

In comparison, some benefits were observed for light vehicles other than automobiles (pickup trucks, sport utility vehicles, and vans), equipped with two-wheel ABS instead of the four-wheel ABS used on most automobiles. Two-wheel ABS has been effective in reducing the risk of nonfatal run-off-road crashes for almost every type of light truck. Nonfatal rollovers were reduced by 30 to 40 percent. Side impacts with fixed objects were reduced by 15 to 30 percent. Frontal impacts with fixed objects were reduced by 5 to 20 percent.

In general, ABS appears to be a very promising safety device when evaluated on a test track. Under many pavement conditions, ABS allows the driver to stop a vehicle more rapidly while maintaining steering control, even during extreme panic braking. Therefore, NHTSA wishes to, as rapidly as possible, determine why the real world performance for existing, production ABS is not producing the anticipated effectiveness that has been suggested under test track conditions.

Many possible reasons as to why ABS appears not to be effective in vehicles under certain conditions have been hypothesized. These reasons include:

1. The apparent increase in single-vehicle accidents involving ABS-equipped vehicles may be a statistical fluke. While the accident data studies have found, with a very high probability of significance, that single-vehicle accidents have increased for ABS-equipped vehicles, the make/models studied have, for technical reasons, been "sportier" or "more luxurious" than the common family sedan. Therefore, the possibility exists that the apparent increase in single-vehicle accidents observed for these types of vehicles may be due to some characteristic(s) of those vehicle types and may not necessarily be present for some of the more common models.

2. The apparent increase in single-vehicle accidents involving ABS-equipped vehicles may be due to one or more flaws in the hardware and/or algorithm of previous ABS models, i.e., an engineering problem. Newer, better performing ABS hardware and software may have already solved the problem. The ABS accident data studies that show the apparent increase in single-vehicle accidents for ABS-equipped vehicles involved systems that are now at least four years old. A new generation of light vehicle ABS hardware and software has since been introduced; this new generation of ABS may have improved performance which will enable drivers to avoid single-vehicle, as well as multi-vehicle accidents.

3. The apparent increase in single-vehicle accidents involving ABS-equipped vehicles may be due to the fact that ABS hardware and software may not perform adequately in all driving situations. Situations (such as braking in a hard turn, grass, split mu, washboard roads, etc.) may arise in which the activation of the ABS in a panic stop produces a reduction in vehicle braking capability compared with a non-ABS-equipped vehicle. This loss of braking capability could result in an increase in single-vehicle accidents.

4. The apparent increase in single-vehicle accidents involving ABS-equipped vehicles may be due to a driver-related problem. In situations of extreme, panic braking, drivers may not be aware of the steering control that four-wheel ABS provides. Without four-wheel ABS, aggressive braking may lock the front wheels of the vehicle, eliminating directional control capability, rendering the driver’s steering behavior irrelevant. With four-wheel ABS, the vehicle does not lose directional control capability during hard braking due to front wheel lockup, therefore allowing drivers to maintain steering control. It is possible that instead of making controlled steering movements, drivers perform extreme steering movements in panic situations. This could result in drivers avoiding multi-vehicle accidents by driving off the road, resulting in single-vehicle accidents instead.

5. The apparent increase in single-vehicle accidents involving ABS-equipped vehicles may be due to changes in driver behavior due (i.e., risk compensation) to their perceptions of the benefits provided by ABS. Drivers of ABS-equipped vehicles may operate their vehicles at higher speeds due to the perception that ABS increases the vehicle’s handling and braking performance. This increased vehicle speed could result in more run-off-road, single-vehicle accidents, particularly on curved roads.

  LIGHT VEHICLE ABS RESEARCH PROGRAM OBJECTIVES

The overall objective of the research program was to determine why ABS does not appear to be effective in reducing all accidents. Accident studies show increased involvement of ABS-equipped vehicles in single-vehicle accidents and less involvement in multi-vehicle accidents. Identifying the cause(s) of the apparent lack of benefits provided by ABS were accomplished through the following four specific goals:

1. Determine whether the apparent shift for ABS-equipped vehicles involving a decrease in multi-vehicle accidents and a corresponding increase in single-vehicle accidents continues to be seen in vehicles equipped with newer generation ABS.

2. Determine the reasons why ABS-equipped vehicles appear to be more involved in single-vehicle crashes than in multi-vehicle crashes.

3. Determine whether the shift from multi-vehicle accidents to single-vehicle accidents will also occur for other types of light vehicles (pickup trucks, sport utility vehicles, and vans).

4. Develop consensus between/within NHTSA, the auto industry, and other stakeholders as to the research findings and conclusions.

The research program is summarized in the following publication

 LIGHT VEHICLE ABS RESEARCH PROGRAM STATUS

The agency has completed and published 7 final reports and 3 professional society papers covering 5 of the 7 research tasks included in the study All research tasks (those that involve testing or experimentation) for the program have been completed. The final report on Task 6 (off-road recovery testing) will be published by the end of May 2003. The final report for Task 3 (crash report study) will be published by December 2003. The two non-research tasks (Tasks 8 and 9) involved compiling the findings of the research tasks and disseminating them. These two tasks have also been completed. We are not planning to publish a unified document detailing the results of the entire research program. Briefings highlighting results for all tasks have been given to industry members of the Motor Vehicle Safety Research Advisory Committee. Results have also been presented at meetings of the Society of Automitive Engineers and the Human Factors and Ergonomics Society. The research program did not identify any glaring vehicle performance or driver behavior issues that appeared to contribute substantially to the unexpectedly low crash reduction benefits. However, analysis of more recent crash statistics subsequent to completion of this research program indicates that ABS crash reduction benefits are improving when compared to the benefits assessed in the mid-1990s. The completed publications on the research conducted under this program are available through the agency’s website or the agency’s Vehicle Research and Test Center.

LIGHT VEHICLE ANTILOCK BRAKE SYSTEMS RESEARCH PROGRAM TASK DETAILS:

  ABS Effectiveness Based on Crash Data Analysis Studies

  • TASK 1: Update Statistical Studies to Determine the Effects of ABS on Single- and Multi- Vehicle Accidents

This task involved the analysis of accident data to determine the effects of equipping vehicles with ABS on single- and multi-vehicle accidents. This study differed from previous ABS accident data studies in several ways. First, high volume make/models, such as the Ford Taurus and Honda Accord, were be studied as compared to the "sportier" and "more luxurious" models used in previous studies. Earlier studies only used make/models that changed from standard conventional brakes one model year to ABS as standard for the next model year. This study obtained assistance from the automobile manufacturers to fully decode Vehicle Identification Numbers (VIN) numbers, thereby determining the presence or absence of ABS on vehicles.

Also, in previous studies, accident data from one year for a make/model was compared with accident data for another year without taking into account generational differences that may have existed in different make/models. Since data from make/models with optional ABS are being used, this study may be able to compare accident data for the same model year. Finally, this study focused on make/models equipped with newer generation ABS designs.

Status: Task completed.
Published Results:

  • TASK 3: Study NASS Hardcopy Crash Files to Determine Typical Single-Vehicle Crashes for ABS-Equipped Vehicles

For this task, National Accident Sampling System (NASS) hardcopy files were examined to further investigate the circumstances surrounding crashes involving ABS- and non-ABS-equipped vehicles. This analysis attempted to determine whether crashes of vehicles were due to brake performance or whether they were due to some other operational factor, such as road departure as a result of falling asleep while driving. Crashes that are determined to be brake performance related were used to assist in the development of test plans for other tasks.

Status: Task completed. Final report in progress.
 
Drivers' Experiences and Expectations About ABS

  • TASK 2: Conduct a Representative National Survey of Driver Experiences and Expectations Regarding Conventional Brakes versus ABS


  • For this task, a questionnaire concerning driver experiences and expectations regarding ABS- versus non-ABS-equipped vehicles were designed and nationally distributed. The survey data were analyzed to determine differences in driver’s expectations of brake performance between ABS- and non-ABS-equipped vehicles. From these findings, the need for an educational campaign for specific demographic groups were assessed.

Status: Task completed.
Published Results:

ABS Hardware Performance  

  • TASK 4: Test Track Study of the Performance of Production ABS Over a Broad Range of Real World Situations

This task endeavored to measure the braking performance of an assortment of ABS-equipped vehicles over a broad range of driving conditions. The goal was to attempt to find situations and/or conditions in which many vehicles equipped with ABS do not perform as expected due to shortcomings in either the ABS algorithm, hardware, or software. This effort involved in-depth examinations of one system from each major ABS manufacturer resulting in a total of approximately 8 systems being examined.

The ABS were evaluated across a broad and comprehensive range of driving maneuvers with respect to vehicle speeds, vehicle loading conditions, road surfaces (adhesion and texture), steer angles, split mu’s, transition surfaces, etc. This involved testing many maneuvers for each ABS design. The focus was to find situations in which there are gross differences in vehicle performance due to the presence/absence of ABS. For example, ABS-equipped vehicles are known to have poorer performance on gravel and snow. The goal of this task is to find if there are other situations where the performance of all ABS-equipped vehicles is grossly worse in comparison to non-ABS-equipped vehicles and in comparison to the population.

Status: Task completed.
Published Results:

  • TASK 6: Test the Effects of ABS When Performing a Two-Wheel Road Recovery Maneuver

The task addressed the problem of driver recovery of vehicle control after having allowed two wheels to stray from the road surface onto a surface having a different frictional coefficient (e.g., gravel). This vehicle control recovery scenario is a common one which frequently occurs at curves wherein drivers enter a curved portion of roadway at a speed which is too high for them to maintain adequate control resulting in their inability to maintain the vehicle in the lane. The scenario in which two wheels leave the roadway also may occur when a driver falls asleep at the wheel or during an evasive maneuver. ABS could prevent optimal braking performance on gravel or other non-paved surfaces. Thus ABS may complicate the drivers ability to return their vehicle fully onto the roadway. This task assessed issues relating to the effects of ABS on drivers’ ability to recover vehicle control and lane position in this type of situation.

Status: Task completed. Final report in progress.

ABS Human Factors Studies  

  • TASK 5: Conduct Tests to Assess the Prevalence of Driver "Oversteering" in Panic Braking Situations


  • This task involves the examination of driver behavior in panic, crash-imminent situations involving passenger car conventional and ABS. Accident data suggest that drivers may be using ABS inappropriately or otherwise exhibiting incorrect behaviors with ABS which lead to increased accidents. For example, drivers may use the same strategies, in terms of pedal application, in panic braking situations involving ABS-equipped vehicles as they do in non-ABS-equipped vehicles. Aspects of driver behavior to be focused on in an effort to address this issue also include steering and other types of crash avoidance reactions.

    A previous study of driver behavior in an intersection incursion scenario for non-ABS-equipped vehicles which was conducted on the Iowa Driving Simulator found that drivers tend to first apply the brakes aggressively, locking all four wheels, and then steer. The observed steering amplitudes were generally very large. In the case of conventional brakes, large amplitude steering has no effect on the vehicle’s direction of travel since vehicles with all four wheels locked travel in basically a straight line. A hypothesized reason for the increase in single-vehicle crashes and decrease in multi-vehicle off-road crashes with ABS is that driver’s use the same braking and steering strategy for ABS-equipped vehicles. Since with ABS (four-wheel ABS, in particular) all of the vehicle’s wheels may not lock, the vehicle’s steering remains fully effective. Large amplitudes of steering motion (such as are frequently seen for vehicle not equipped with ABS) have the potential to cause the vehicle to run off of the road. This may be a plausible explanation of why four-wheel ABS-equipped vehicles tend to be involved in more rollover incidents or off-road crashes with fixed objects.

    The main goal of this task is to determine whether drivers exhibit instinctive crash avoidance responses (i.e., braking, steering, "over-steering", etc.) in an effort to avoid hitting another vehicle. A second goal of this task is to determine, if drivers do, in fact, exhibit an instinctive behavior pattern involving heavy braking and steering, and whether drivers exhibit this behavior with vehicles equipped with both conventional and antilock brakes. This task involved both simulator and test track studies (using DASCAR) which examine driver behavior in a variety of different conflict situations (e.g., intersection incursion, brake-in-curve leading to run-off-road, and other obstacle avoidance).

    Task 5.1: Iowa Driving Simulator Study


    NHTSA conducted research examining driver crash avoidance behavior and the effects of ABS on drivers’ ability to avoid a collision in a crash-imminent situation. The study was conducted on the Iowa Driving Simulator and examined the effects of ABS versus conventional brakes, speed limit, ABS instruction, and time-to-intersection (TTI) on driver behavior and crash avoidance performance. This study found that drivers do tend to brake and steer in realistic crash avoidance situations and that excessive steering can occur. However, a significant number of road departures did not result from this behavior. Drivers in the ABS group showed significantly increased stability and control relative to conventional brakes.

    Status:
    Task completed.  
    Published Results:  
    Task 5.2: Dry Test Track Study


    NHTSA conducted research examining driver crash avoidance behavior and the effects of ABS on drivers' ability to avoid a collision in a crash-imminent situation. The study described here was conducted on a test track under dry and wet pavement conditions to examine the effects of ABS versus conventional brakes, ABS brake pedal feedback level, and ABS instruction on driver behavior and crash avoidance performance. This study found that drivers do tend to brake and steer in realistic crash avoidance situations and that excessive steering can occur. However, a significant number of road departures did not result from this behavior for either pavement condition. ABS was found to reduce crashes significantly on wet pavement as compared to conventional brakes.

    Status:Task completed.
    Published Results:

  • TASK 7: Study Driver Risk Compensation in Relation to ABS-Equipped Vehicles


  • This task endeavored to assess passenger car ABS-related driver risk compensation. This task examined the behavior of drivers to determine whether they tend to drive more aggressively (e.g., drive faster, brake harder, make more abrupt maneuvers) with ABS-equipped vehicles. Aggressive driving behavior is likely to contribute to increased incidence of rollover accidents. Aspects of driver behavior to be focused on in an effort to address this issue include steering and braking. Observations were also made to determine whether drivers tend to operate ABS-equipped vehicles at higher rates of speed than non-ABS-equipped vehicles. Studies were conducted to collect data on driver behavior during normal driving maneuvers and activities such as speed maintenance and braking aggressivity.

    Task 7.1: License Plate Study


    An experiment was conducted to unobtrusively determine whether drivers of vehicles equipped with ABS have a tendency to drive faster than drivers of conventionally braked vehicles. Several locations on public roadways around Ohio were selected as data collection sites. At these sites, the speed and license plate information of passing vehicles were unobtrusively measured and recorded using a laser speed gun, video camera, and laptop computer. Data were collected at each site for specified periods during daylight hours (balanced for AM/PM) in both wet and dry road conditions. Using the license plate number, the VIN number was obtained and then decoded to determine whether each vehicle had ABS. Average speed data for specified conditions and locations were compared for vehicles with ABS versus those without.The results of this study showed that type of brake system (ABS or conventional) had no significant effect on driving speed under the conditions examined. This finding of no significant speed effects was true for all sites, pavement conditions, and model years of vehicles observed. A consistent, but insignificant, trend was seen in mean speed by brake system for each site where higher speeds were observed for drivers of ABS-equipped vehicles. Significant results that were found included higher speeds for drivers of newer model cars, higher speeds for dry pavement, and that speed as a function of location (site). Overall, based on observed vehicle speed results, evidence of passenger car ABS-related driver behavioral adaptation was not observed using the methods employed in this study. Based on these and other related results from NHTSA’s Light Vehicle ABS Research Program, the authors believe that behavioral adaptation due to ABS is not occurring during “real world” driving. Thus, the results of this study suggest that the apparent increase in single-vehicle crashes involving ABS-equipped vehicles cannot be attributed to behavioral adaptation.

    Status: Task completed.
    Published Results:
    Task 7.2: On-Road MicroDAS Study

    An experiment was conducted to determine, in a real-world environment, whether people drive differently in vehicles equipped with antilock brake systems than they do in vehicles equipped with conventional brake systems. Six instrumented vehicles were provided to qualified subjects for 2-month periods (1 month with ABS and 1 month without) in order to gather naturalistic driving data with a minimum of experimental artifacts. A secondary goal of this study was to investigate whether any observable behavioral changes assumed to be a result of behavioral adaptation might vary as a function of driver age.The results of this study showed that the type of brake system (ABS or conventional) had no significant effect on any of the vehicle performance metrics examined in this study. These metrics included four types of speed evaluations, lateral and longitudinal accelerations, steering rate, mean and mean minimum time headway, and lane change and passing activity. In addition, no significant interaction effects were found when examining these metrics. However, significant effects due to driver age were found. These age effects were as expected but served as a valuable check on the validity of the methodology used. This NHTSA effort to assess behavioral adaptation under real-world conditions has produced data contrary to earlier test track studies. Drivers of ABS-equipped vehicles did not drive faster, exert higher brake pedal forces, or accelerate faster than their conventionally braked counterparts. This study does validate other recent NHTSA studies that found lack of observable differences in speed and brake pedal force. These results continue to suggest that the inclusion of ABS has no impact on the manner in which a vehicle is driven, especially under normal operating conditions.

    Status: Task completed.
    Published Results:

Information Dissemination and Consensus Building

  • TASK 8: Assess the Overall Effect of Passenger Car ABS on Drivers’ Ability to Avoid Crashes


  • This task will involve the examination of ABS performance and the ability of drivers to accurately make use of ABS to avoid crashes. ABS performance data and ABS-related driver behavior data were compiled to form conclusions regarding the underlying cause(s) of the shift in crash statistics with ABS. Forming this conclusion will facilitate the development of a general consensus among the ABS research community and ABS manufacturers as to why past studies of ABS effectiveness in real world operation have found mixed results. Research results were shared with ABS stakeholders and meetings were held to discuss research and test program results and determine appropriate future activities to increase the effectiveness of ABS for passenger vehicles.

    Status: Task completed.  

  • TASK 9: Status Briefings


  • NHTSA will conduct status briefings as necessary for the purpose of sharing knowledge and new-found facts regarding ABS performance and driver interaction with ABS with interested parties within NHTSA and the public at large. Summaries of current research efforts and results-to-date were presented for discussion. Briefings are tentatively scheduled for January of each of the fiscal years covered by the plan. Additional status briefings were conducted to keep stakeholders abreast of task progress and acquire their input as NHTSA sees fit.

    Status: Task completed.