May 2015

  1. What are crash avoidance technologies?

    The term "crash avoidance" can encompass a wide variety of vehicle features designed to help the driver operate the vehicle safely. Vehicles increasingly offer advanced technologies that assist the driver with warnings or automatic braking to avoid or mitigate a crash. These advanced technologies vary in their function and how they operate. In general, they monitor driver input and the environment around the vehicle and warn the driver when they detect the potential for a collision. In some cases, they increase braking power or adjust steering response to make the driver’s input more effective. They also may automatically brake or steer the vehicle if the driver does not take action to avoid the collision. 

  2. What kinds of crash avoidance technologies are currently available for passenger vehicles?

    There are many vehicle features that could be considered crash avoidance technology. This list summarizes some of the most common or promising systems. This is not a comprehensive list of technologies, as more are being developed and introduced each year. The descriptions are general and may not capture every variation of a given technology.

    Front crash prevention systems use various types of sensors, such as cameras, radar, or light detection and ranging (LIDAR) to detect when the vehicle is getting too close to one in front of it. Most systems issue a warning and precharge the brakes to maximize their effect if the driver responds by braking. Many systems brake the vehicle autonomously if the driver doesn't respond. In some cases, automatic braking is activated without a preliminary warning. An autobrake system may not always be able to prevent a crash but may reduce vehicle speed, mitigating the severity of the crash.

    Some front crash prevention systems can recognize pedestrians. Pedestrian detection systems use advanced algorithms coupled with some combination of sensors and cameras to spot people who are in or about to enter the vehicle's path.

    In addition to providing driver warnings and/or braking automatically, some front crash prevention systems perform other functions to potentially reduce the severity of a collision and the likelihood of injury. These functions include pretensioning safety belts, closing windows or adjusting seat positions or head restraints.

    Some vehicles also are equipped with night vision assist technologies. Night vision assist uses infrared imaging to produce an enhanced view of the road ahead. Some systems may provide an audible or visual alert if there is a pedestrian or animal ahead.

    Adaptive cruise control is related to front crash prevention, but it is typically marketed as a convenience, rather than a safety feature. As with regular cruise control, the driver sets the desired speed. The difference is that the forward-mounted sensors track the distance to a lead vehicle, and the engine and brakes are used to maintain a safe gap if traffic slows. As traffic speeds up again, the vehicle accelerates to maintain the preset cruise speed. Some systems allow drivers to adjust the following distance. If the vehicle slows below a certain speed as it approaches another vehicle, some systems are designed to disengage and require the driver to resume control, while others can bring the vehicle to a complete stop.

    Lane departure warning and lane-keeping support systems use cameras to track the vehicle's position within the lane, alerting the driver if the vehicle is in danger of inadvertently straying across lane markings. Some systems use haptic warnings, such as steering wheel or seat vibration, while others use audible and/or visual warnings. Some systems cause the vehicle to actively resist moving out of the lane or help direct the vehicle back into the lane through light braking or minor steering adjustments.

    Blind spot detection uses sensors to monitor the side of the vehicle for vehicles approaching blind spots. In many systems, a visual alert appears on or near the side mirrors if a vehicle is detected. An audible alert may activate if the driver signals a turn and there is a vehicle in the blind spot. Some systems also may activate the brake or steering controls to keep the vehicle in its lane.

    Park assist and backover prevention systems help drivers park and back up. Rear object detection systems use cameras and sensors to help the driver look for objects behind the vehicle when backing up. Rearview cameras display what is behind the vehicle. Systems that use radar or ultrasonic sensors, along with some camera systems, warn the driver if there are objects in the way when the vehicle is in reverse. Some systems automatically apply the brakes to keep the vehicle from backing into or over an object. A cross-traffic alert system detects approaching vehicles that may cross the path of a backing vehicle, warns the driver, and may automatically brake to prevent a collision. Some parking assist systems also are capable of automatically parallel parking the vehicle.

    • See the backover crashes Q&A for more information on the use of cameras to prevent backover crashes.

    Curve-adaptive headlights help drivers see better on dark, curved roads. The headlights pivot in the direction of travel based on steering wheel movement and sometimes the vehicle’s speed to illuminate the road ahead.

    Fatigue warning systems use sophisticated algorithms that monitor driver steering and other behaviors, such as the driver's eye blink rate and blink duration. A system will alert the driver if it detects inattention or drowsiness.

    Electronic stability control utilizes sensors and a microcomputer to monitor how well a vehicle responds to a driver's steering input. The system selectively applies the brakes and modulates the engine power to keep the vehicle traveling along the path indicated by the steering wheel position.

    Antilock brakes prevent wheels from locking up and skidding during hard braking by monitoring the speed of each wheel and automatically pulsing the brake pressure on any wheels where skidding is detected.

  3. Do crash avoidance features reduce crashes?

    Front crash prevention is reducing crashes, analyses of insurance claims data show. The jury is still out on many other advanced crash avoidance technologies, which aren't yet widespread enough for researchers to analyze their effectiveness.

    Volvo's City Safety, designed to help a driver avoid rear-ending another vehicle in slow-moving traffic, is proving to be effective. The Highway Loss Data Institute (HLDI) compared insurance claims for 2011-12 Volvo S60 sedans and 2010-12 Volvo XC60 SUVs equipped with City Safety with claims for other 2011-12 midsize luxury cars and 2009-12 midsize luxury SUVs, respectively. The XC60s had 15 percent fewer claims under property damage liability coverage, which pays for damage to vehicles that an at-fault driver hits. Highway Loss Data Institute. 2012. Volvo City Safety loss experience - an update. Loss Bulletin Vol. 29, No. 23. Arlington, VA. The S60s showed a similar reduction, with 16 percent fewer property damage liability claims in vehicles with the technology. There also were fewer claims under bodily injury liability coverage, which pays for injuries to people in other vehicles and collision insurance, which covers damage to the insured vehicle. Highway Loss Data Institute. 2012. Volvo City Safety loss experience - an update. Loss Bulletin Vol. 29, No. 23. Arlington, VA.

    In separate analyses of front crash prevention systems that function at higher speeds, HLDI found 14 percent fewer insurance claims under property damage liability coverage for Acura and Mercedes-Benz vehicles with forward collision warning with automatic braking than for the same vehicles that weren’t equipped with the technology. Highway Loss Data Institute. 2012. Mercedes-Benz collision avoidance features: initial results. HLDI Bulletin 29(7). Highway Loss Data Institute. 2011. Acura collision avoidance features: initial results. HLDI Bulletin 28(21). Mercedes-Benz offers a version of forward collision warning that does not include autobrake, which also showed lower crash rates but not to the same extent as the version that includes it. Systems without autobrake probably have more modest benefits because they rely on drivers to respond appropriately to warnings and can’t directly avoid crashes. However, a combined forward collision and lane departure warning system without autobrake on the Honda Accord and Honda Crosstour reduced property damage liability insurance claims by 10 percent. Highway Loss Data Institute. 2014. Honda Accord collision avoidance features: an update. HLDI Bulletin 31(16).

    Curve-adaptive headlights were also found to be effective in HLDI’s analysis, even beyond what researchers expected. HLDI looked at curve-adaptive headlights offered by Mazda, Mercedes and Volvo and found property damage liability claims fell as much as 10 percent with adaptive headlights. Highway Loss Data Institute. 2012. Mercedes-Benz collision avoidance features: initial results. HLDI Bulletin 29(7). Highway Loss Data Institute. 2012. Volvo collision avoidance features: initial results. HLDI Bulletin 29(5). Highway Loss Data Institute. 2011. Mazda collision avoidance features: initial results. HLDI Bulletin 28(13). That was surprising, since only about 7 percent of police-reported crashes occur between 9 p.m. and 6 a.m. and involve more than one vehicle. An even smaller percentage are multiple-vehicle, nighttime crashes occurring on a curve, where curve-adaptive headlights would be expected to have an effect. It's possible that other differences between the adaptive headlights and conventional ones besides steerability — for example, brightness or beam pattern — may have played a role in reducing crashes with other vehicles. In an experimental study with volunteer drivers with one vehicle's system, adaptive high-intensity discharge (HID) headlights allowed drivers to spot a hard-to-see objet on a dark, curvy road about one-third second earlier than they would have with conventional fixed headlights. Reagan, I.J.; Brumbelow, M.L.; and Frischmann, T. 2015. On-road experiment to assess drivers' detection of roadside targets as a function of headlight system, target placement, and target reflectance. Accident Analysis and Prevention 76:74-82. The study also showed that even fixed HID lights had a small advantage over halogen ones.

    There is more evidence available for systems that have been around longer, such as ESC.

    ESC is effective in reducing crashes. Institute researchers have found it reduces fatal single-vehicle crash risk by 49 percent and fatal multiple-vehicle crash risk by 20 percent for cars and SUVs. Farmer, C.M. 2010. Effects of electronic stability control on fatal crash risk. Arlington, VA: Insurance Institute for Highway Safety.

    On the other hand, antilock brakes are an example of a technology that hasn't panned out as expected. For reasons not completely understood, antilock brakes haven't had a significant effect on passenger vehicle crashes.

  4. Even if a crash avoidance system doesn't prevent the crash, can it still be beneficial?

    Yes. Front crash prevention systems can be beneficial even if they do not avoid the crash altogether but still reduce speeds, mitigating the severity of the crash. To show why reducing speed is important, IIHS conducted two demonstration crash tests at different speeds. In each test, a 2013 Mercedes-Benz C-Class ran into the back of a stationary 2012 Chevrolet Malibu. The tests illustrate what happens in a 25 mph crash when the striking vehicle doesn't have autobrake, compared with what happens when autobrake reduces the speed by 13 mph, the amount the C-Class's autobrake system reduced the car's speed in IIHS track testing. Total damage in the higher speed crash test was about $28,000. The Malibu was a complete loss. Lowering the speed to 12 mph trimmed the damage to $5,700. A similar speed reduction in a higher speed crash could significantly reduce injury risk, as well as vehicle damage. Insurance Institute for Highway Safety. 2013. Crash tests show how autobrake can mitigate crash severity, damage costs. Status Report 48(7):5.

  5. What resources are available for consumers who want to purchase a vehicle with crash avoidance features?

    Advanced crash avoidance features started out as options on a few luxury vehicles and have steadily spread to more of the fleet, including many nonluxury models. Information by make and model on the availability of forward collision warning, autobrake, lane departure warning, lane departure prevention, adaptive headlights and blind spot detection can be found here.

    An Institute test program rates the performance of front crash prevention systems to help consumers compare them and to encourage automakers to speed adoption of the technology. The Institute rates models with optional or standard front crash prevention systems as superior, advanced or basic, depending on whether they offer autobrake and, if so, how effective it is in tests at 12 and 25 mph. Since research from insurance claims indicates that systems with autobrake reduce crashes to a greater extent than similar systems with forward collision warning only, Insurance Institute for Highway Safety. 2012. They’re working: insurance claims data show which new technologies are preventing crashes. Status Report 47(5):1-7. vehicles must effectively brake automatically to get the top ratings. Vehicles rated superior have autobrake and can avoid a crash or substantially reduce speeds in both tests. For an advanced rating, a vehicle must have autobrake and avoid a crash or reduce speed by at least 5 mph in one of the tests. To earn a basic rating, a vehicle must have a forward collision warning system that meets performance criteria specified by the National Highway Traffic Safety Administration (NHTSA). For the NHTSA endorsement, a system must issue a warning before a specified time in a number of test track scenarios. Information on IIHS ratings can be found here.

    In addition to forward collision warning, NHTSA has recognized the potential importance of lane departure warning systems and rear-view video systems for backover prevention by incorporating them into its New Car Assessment Program. Vehicles are credited with having one of these systems if their system can pass specified track tests.

    As of the 2012 model year, the government requires ESC on new passenger vehicles. Information on which models from previous years are equipped with ESC can be found here.

  6. If the new technologies work as intended, how many crashes could they potentially prevent or mitigate?

    If all passenger vehicles were equipped with forward collision warning, lane departure warning, blind spot detection and adaptive headlights, about 1 in 3 fatal crashes and 1 in 5 injury crashes could potentially be prevented or mitigated. Jermakian, J.S. 2011. Crash avoidance potential of four passenger vehicle technologies. Accident Analysis Prevention 43(3):732-40. Those numbers are from an Institute analysis of 2004-2008 crash data and represent the best-case scenario, presuming the systems perform as advertised and drivers respond to them correctly. They reflect known limitations of crash avoidance systems available at the time of the study. However, they don't take into account potential reductions in effectiveness due to driver interactions with the systems or increased effectiveness due to enhanced system capabilities. Of all four features, current forward collision warning systems have the potential to prevent or mitigate the most crashes, and lane departure warning could come into play in the most fatal crashes.

    Another Institute study looked at crash avoidance technologies in large trucks. Based on an analysis of 2004-2008 crashes, it found that blind spot detection, forward collision warning, lane departure warning and ESC together could prevent or mitigate as many as 28 percent of large truck crashes a year, including 1 out of 5 fatal ones. Jermakian, J.S. 2012. Crash avoidance potential of four large truck technologies. Accident Analysis and Prevention 49:338-46. Of the four technologies, blind spot detection is applicable to the largest number of crashes. ESC showed the most potential for fatal crashes, possibly preventing or mitigating 15 percent of fatal large truck crashes each year.

    As crash avoidance technologies become increasingly common, more data will be available to determine how effective these systems really are in preventing or mitigating crashes.

  7. How do drivers respond to new crash avoidance features?

    Appropriate driver responses and acceptance of crash avoidance technologies are critical to their success. If drivers find the systems annoying or not useful, they may disable them. Similarly, if drivers experience warnings but don't understand them, don't trust them, are overwhelmed by them, or don't take an appropriate corrective action, then the systems will be ineffective.

    Early research using simulators has shown collision warning systems can redirect the driver’s attention to the road and improve reaction time, but little is known about how drivers respond in real-world driving. Lee, J.D.; McGehee, D.V.; Brown, T.L.; and Reyes, M.L. 2002. Collision warning timing, driver distraction, and driver response to imminent rear-end collisions in a high-fidelity driving simulator human factors. Human Factors  44(2): 314-34.

    Institute surveys of owners of vehicles with crash avoidance technologies found that, despite some annoyance about false alerts, for example, the majority of drivers left the systems turned on most of the time, felt the systems made them safer drivers and would want them in their next vehicle. Braitman, K.A.; McCartt, A.T.; Zuby, D.S.; and Singer, J. 2010. Volvo and Infiniti drivers' experiences with select crash avoidance technologies. Traffic Injury Prevention 11(3):270-8. Eichelberger, A.H. and McCartt, A.T. 2014. Volvo drivers' experiences with advanced crash avoidance and related technologies. Traffic Injury Prevention 15(2):187-95. Cicchino, J.B. and McCartt, A.T. 2015. Experiences of model year 2011 Dodge and Jeep owners with collision avoidance and related technologies. Traffic Injury Prevention 16(3):298-303. Eichelberger, A.H. and McCartt, A.T. 2014. Toyota drivers' experiences with dynamic radar cruise control, the pre-collision system, and lane-keeping assist. Arlington, VA: Insurance Institute for Highway Safety. The surveys included owners of both luxury and non-luxury vehicles that were equipped with some combination of front crash prevention, adaptive cruise control, lane departure warning and prevention, blind spot detection, fatigue warning, rear cross-traffic alert, and rear park assist.

    One concern is that drivers might rely on the systems too much and feel freer to look away from the road or take other risks. In the Institute's surveys, many owners reported safer driving habits with the systems (e.g., following less closely with adaptive cruise control, using turn signals more often with lane departure warning). Fewer owners reported potentially unsafe behavior, such as waiting for an alert before braking or allowing the vehicle to brake for them at least some of the time. Depending on the vehicle, between one-fifth and one-half of owners with front crash prevention reported the system had helped prevent a crash, Eichelberger, A.H. and McCartt, A.T. 2014. Volvo drivers' experiences with advanced crash avoidance and related technologies. Traffic Injury Prevention 15(2):187-95. Cicchino, J.B. and McCartt, A.T. 2015. Experiences of model year 2011 Dodge and Jeep owners with collision avoidance and related technologies. Traffic Injury Prevention 16(3):298-303. Eichelberger, A.H. and McCartt, A.T. 2014. Toyota drivers' experiences with dynamic radar cruise control, the pre-collision system, and lane-keeping assist. Arlington, VA: Insurance Institute for Highway Safety. and that proportion was even higher for some blind spot detection and cross-traffic alert systems. Eichelberger, A.H. and McCartt, A.T. 2014. Toyota drivers' experiences with dynamic radar cruise control, the pre-collision system, and lane-keeping assist. Arlington, VA: Insurance Institute for Highway Safety.

  8. What are some of the limitations of crash avoidance technologies?

    Many crash avoidance technologies rely on drivers to take action. The effectiveness of these systems depends on whether drivers accept the technologies, understand the information from the system and respond appropriately. If drivers find the systems annoying, overwhelming or unhelpful, they may disable them. Plus, interpreting warnings from multiple systems may be confusing or even distracting for some drivers.

    In addition to driver challenges, the technology itself can have limitations. For example, lane departure warning systems rely on the ability of the sensors to register lane markings, which may be problematic on roads that aren't well marked or are covered with snow. In a field test of a prototype road departure warning system, the system was available 76 percent of the time on freeways compared with 36 percent of the time on non-freeways. Wilson, B.H.; Stearns, M.D.; Koopmann, J; and Yang, C.Y. 2007. Evaluation of a road-departure crash warning system. Report no. DOT-HS-810-854. Washington, DC: National Highway Traffic Safety Administration. Sensors such as cameras, radar and LIDAR also may be influenced by environmental factors such as lighting or precipitation. In the same study, the lane departure warning system was available 56 percent of the time during dry, daytime conditions and only 4 percent of the time during wet, nighttime conditions.  Most current systems struggle to detect the outside environment in low light or inclement weather. Sayer, J.R.; Bogard, S.E.; Funkhouser, D.; LeBlanc, D.J.; Bao, S.; Blankespoor, A.D.; Buonarosa, M.L.; and Winkler, C.B. 2010. Integrated vehicle-based safety systems heavy-truck field operational test key findings report. Report no. DOT HS-811-362. Washington, DC: National Highway Traffic Safety Administration.

    Some systems only work at certain speeds. Other systems don't operate until turned on by the driver. Systems that rely on GPS to monitor the car's location, such as curve speed warning, are limited by the accuracy and availability of the digital maps.

  9. What other kinds of crash avoidance technologies can we expect in the future?

    The landscape of in-vehicle technologies is rapidly changing as new features continue to be introduced. Advances also are being made in intelligent transportation systems that allow vehicles to communicate with one another or with the roadway infrastructure.

    Vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communications are prototype safety systems in which vehicles and roadway infrastructure communicate over a wireless network. Drivers can then use this information to help avoid crashes.

    With V2V communication, vehicles transmit information regarding their actions to other vehicles. For example, in a long chain of vehicles, if the lead vehicle suddenly brakes, this information will be transmitted to every other vehicle in the chain so that the other drivers are alerted. It also could be possible for the trailing vehicles to automatically begin braking when the lead vehicle's signal is received.

    With V2I communication, cars receive and transmit information to roadway infrastructure. For example, highway systems could monitor vehicle location within a lane. If the vehicle is detected drifting out of a lane, the system could alert the vehicle. In urban environments, traffic signals can alert vehicles of an impending light change so drivers can prepare to stop.

    Beginning in 2012, a year-long pilot study in Ann Arbor, Mich., tested the functionality and reliability of these connected vehicle technologies. More than 70 lane-miles of urban, suburban and rural roads and nearly 3,000 vehicles, including passenger cars, commercial trucks and transit buses, were equipped with the V2V and V2I technology, enabling wireless communication between vehicles and infrastructure. Research and Innovative Technology Administration. 2012. Connected vehicle research. Available at Accessed: June 20, 2012. The results of the pilot study indicate connected vehicle technologies are technically feasible and would reduce property-damage and injury crashes. However, some barriers remain before wide adoption of the technology is possible, including issues related to privacy, security, and technical aspects and performance requirements of the systems. Harding, J.; Powell, G.; Yoon, R.; Fikentscher, J.; Doyle, C.; Sade, D.; Lukuc, M.; Simons, J.; and Wang, J. 2014. Vehicle-to-vehicle communications: readiness of V2V technology for application. Report no. DOT HS-812-014. Washington, DC: National Highway Traffic Safety Administration. The National Highway Traffic Safety Administration recently announced it will be taking steps to enable this technology in vehicles and recently solicited feedback from the public ahead of issuing a proposed rule.

  10. What is an autonomous vehicle and will it soon be driving me?

    An autonomous vehicle is equipped with technology that is capable of sensing the environment around it and driving itself without active physical control or monitoring by a human driver.  Vehicles with crash avoidance systems such as front crash prevention may have some automated functions but are not considered fully autonomous unless the vehicle can perform all safety-critical driving functions and monitor roadway conditions for an entire trip without any driver input. National Highway Traffic Safety Administration. 2014. Preliminary statement of policy concerning automated vehicles. Washington, DC: U.S. Department of Transportation. Available: Accessed: February 28, 2014.

    Autonomous vehicles already exist. The best-known example is Google’s driverless car, which was conceived in 2005. Many major auto manufacturers, including Audi, BMW, Ford, Nissan and Toyota, are also working on prototypes, some of which are now being tested on-road.

    Regulating the use of driverless cars on U.S. roads will be challenging. In 2011, Nevada became the first state to enact a law that permits testing. As of March 2015, only three other states (California, Florida and Michigan) and the District of Columbia permit testing on public roads. In these jurisdictions autonomous vehicle testing requires a human operator to be in the vehicle in case a manual intervention is required.

March 2015

  1. What is electronic stability control (ESC)?

    ESC is a vehicle control system comprised of sensors and a microcomputer that continuously monitors how well a vehicle responds to a driver's steering input, selectively applies the vehicle brakes, and modulates engine power to keep the vehicle traveling along the path indicated by the steering wheel position. This technology helps prevent the sideways skidding and loss of control that can lead to rollovers. It can help drivers maintain control during emergency maneuvers when their vehicles otherwise might spin out, or reduce vehicle speed to prevent running off the outside of a curve. The systems have been marketed under various names, including dynamic stability control, vehicle stability control and dynamic stability and traction control, among others.

  2. How does ESC help drivers maintain control?

    A driver loses control when the vehicle goes in a direction different from the one the steering wheel position indicates. This typically occurs when a driver tries to turn very hard or turn on a slippery road. Then the vehicle may understeer or oversteer. When it oversteers it turns more than the driver intended because the rear end is spinning or sliding out. When a vehicle understeers it turns less than the driver intended and continues in a forward direction because the front wheels have insufficient traction. ESC can prevent under- and oversteer by selectively braking wheels to produce a counteracting force which helps correct the vehicle's direction of travel. In some cases engine throttle also is reduced.

    How ESC works
  3. How effective is ESC in preventing crashes?

    In Institute studies, ESC has been found to reduce fatal single-vehicle crash risk by 49 percent and fatal multiple-vehicle crash risk by 20 percent for cars and SUVs. Many single-vehicle crashes involve rolling over, and ESC effectiveness in preventing rollovers is even more dramatic. It reduces the risk of fatal single-vehicle rollovers by 75 percent for SUVs and by 72 percent for cars. Farmer, C.M. 2010. Effects of electronic stability control on fatal crash risk. Arlington, VA: Insurance Institute for Highway Safety. Federal studies also show large benefits. The National Highway Traffic Safety Administration (NHTSA) estimates the installation of ESC reduces single-vehicle crashes of cars by 32 percent and single-vehicle crashes of SUVs by 57 percent. NHTSA estimates that ESC has the potential to prevent 72 percent of the car rollovers and 64 percent of the SUV rollovers that would otherwise occur in single-vehicle crashes. Sivinski, R. 2011. Crash prevention effectiveness of light-vehicle electronic stability control: an update of the 2007 NHTSA evaluation. Report no. DOT HS-811-486. Washington, DC: U.S. Department of Transportation.  

    ESC also has great potential to prevent rollover crashes of large trucks. NHTSA estimates that ESC on large trucks could prevent 40 to 56 percent of rollovers and 14 percent of loss-of-control crashes. Wang, J.S. 2011. Effectiveness of stability control systems for truck tractors. Report no. DOT HS-811-437. Washington, DC: Department of Transportation.

  4. Does ESC activate in typical everyday driving?

    For most drivers ESC isn't likely to activate frequently. It won't prevent most of the fender-bender crashes that occur so often in stop-and-go traffic, for example. It's designed to help a driver in the relatively rare event of loss of control at high speed or on a slippery road.

  5. Does the government require ESC?

    As of the 2012 model year, the federal government requires ESC in all cars, SUVs, pickups and minivans. Office of the Federal Register. 2007. National Highway Traffic Safety Administration – Final rule. Docket no. NHTSA-2007-27662; 49 CFR Parts 571 and 585 – Federal Motor Vehicle Safety Standards, Electronic stability control systems, Controls and displays. Federal Register, vol. 72, no. 66, pp. 17236-322. Washington, DC: National Archives and Records Administration. A similar requirement for truck tractors was finalized in 2015. Office of the Federal Register. 2015. National Highway Traffic Safety Administration – Final rule. Docket no. NHTSA-2015-0056; 49 CFR Part 571 – Federal Motor Vehicle Safety Standards, Electronic stability control systems for heavy vehicles. Federal Register, vol. 80, no. 120, pp. 36049-36110. Washington, DC: National Archives and Records Administration. Most new truck tractors will be required to have ESC as of Aug. 1, 2017. The remaining types have until 2019.

  6. How long has ESC been available?

    ESC was introduced in 1995 as optional equipment on luxury cars. By the 2001 model year it was standard on a number of high-selling vehicles and available as an option on many more. Since then, automakers have been increasingly equipping vehicles, especially SUVs, with ESC.

    NHTSA phased in its ESC rule, requiring the technology on 55 percent of model year 2009 vehicles and increasing the percentage each year until model year 2012, when manufacturers had to equip all their passenger vehicles with ESC. Office of the Federal Register. 2007. National Highway Traffic Safety Administration – Final rule. Docket no. NHTSA-2007-27662; 49 CFR Parts 571 and 585 – Federal Motor Vehicle Safety Standards, Electronic stability control systems, Controls and displays. Federal Register, vol. 72, no. 66, pp. 17236-322. Washington, DC: National Archives and Records Administration. Prior to 2012, the Institute required vehicles to have ESC in order to qualify for a TOP SAFETY PICK award.

  7. Can ESC help reduce insurance losses?

    Yes. Losses under collision coverage are about 15 percent lower for vehicles with ESC than for predecessor models without it, according to an analysis by HLDI. Highway Loss Data Institute. 2006. Electronic stability control. HLDI Bulletin 23(1). Arlington, VA. ESC doesn't have much effect on liability claims filed when an at-fault driver damages someone else's car or property or the frequency of personal injury claims filed to cover medical expenses.

April 2015

  1. What are antilock brakes?

    Antilock brakes are designed to help drivers avoid crashes. Without antilocks, hard braking can cause wheels to lock, sending a vehicle into a skid. Wheel lockup can result in longer stopping distances, loss of steering control and, when road friction is uneven, loss of stability if the vehicle begins to spin.

    The main benefit of an antilock braking system (ABS) is that it can reduce these problems on wet and slippery roads. ABS works with a vehicle's normal service brakes to decrease stopping distance and increase the control and stability of the vehicle during hard braking.

    The principle behind ABS is that a skidding wheel provides less stopping force and control than a wheel that is rotating. Antilocks prevent wheels from skidding by monitoring the speed of each wheel and automatically pulsing the brake pressure on any wheels where skidding is detected. ABS doesn’t make much difference in stopping distances on dry roads, although it can enhance vehicle stability and allow the driver to maintain steering control during an emergency stop, when conventional brakes might allow wheel lockup and skidding.

  2. How does ABS work?

    ABS differs among vehicles, but there are some basic similarities. Each system has sensors that monitor the rotational speeds of selected wheels when brakes are applied. When one of these wheels approaches lockup, a control unit reduces brake pressure to that wheel or set of wheels just enough to allow rotation again. This typically happens many times per second, resulting in improved control and, on many wet and slippery surfaces, shorter stopping distances.

    Most passenger vehicles have four-wheel systems with wheel-speed sensors on each wheel. In one type of system, ABS reduces brake pressure to both rear wheels whenever one approaches lockup. Brake pressure to the front wheels of four-wheel systems is controlled independently to maximize stopping power, which is concentrated in the front. In four-wheel independent systems, each wheel is controlled individually, so when any one approaches lockup, brake pressure is reduced to that wheel.

    Some pickups and cargo vans have rear-wheel-only antilock systems to address different braking needs when vehicles are loaded versus unloaded. ABS monitors the rotational speeds of rear wheels only and releases pressure to both when either is about to lock.

    Tractor-trailers have separate antilock systems for the tractors and the trailers. Ideally, both the tractor and trailer of a combination rig should have antilock brakes, but putting antilocks on either component should produce improvement compared with conventional brakes. With antilocks on the tractor only, a driver can maintain better steering control even if trailer wheels lock and the trailer swings. If only the trailer has ABS, trailer swing can be reduced even if steering control is lost.

    ABS is particularly important on motorcycles because locking a wheel during hard braking, especially the front wheel, often results in a serious fall. Motorcycle ABS systems operate on both front and rear wheels. They typically involve separate controls for each wheel, although ABS may be included in systems that link the operation of both brakes.

  3. Why doesn't ABS reduce stopping distances as much on dry roads as wet ones?

    Adequate braking is easy to achieve on dry roads with or without antilock brakes. Even if wheels lock, the coefficient of friction between tires and road surface still is relatively high, so a vehicle stops relatively quickly. It is even possible on some surfaces to stop sooner without antilocks than with them, although such instances are rare. They occur, for example, when loosely packed snow or gravel creates a "dam" effect in front of locked wheels, shortening the stopping distance more than antilocks could.

  4. Has ABS on passenger vehicles reduced crashes?

    Although antilocks perform well on the test track, it’s unclear whether they have made significant reductions in real-world crashes. A 1994 Highway Loss Data Institute (HLDI) study Highway Loss Data Institute. 1994. Collision and property damage liability losses of passenger cars with and without antilock brakes. Insurance special report A-41. Arlington, VA. and a subsequent 1995 study Highway Loss Data Institute. 1995. Three years' on-the-road experience with antilock brakes: an update. Insurance special report A-47. Arlington, VA. compared insurance claims for groups of otherwise identical cars with and without antilocks, finding no differences in the frequency or cost of crashes for which insurance claims for vehicle damage are filed. Because ABS should make the most difference on wet and slippery roads, researchers also studied the insurance claims experience in 29 states during winter months. Even here they found no difference in claim frequency for vehicles with and without antilock brakes. A 1997 Institute study Farmer, C.M.; Lund, A.K.; Trempel, R.E.; and Braver, E.R. 1997. Fatal crashes of passenger vehicles before and after adding antilock braking systems. Accident Analysis and Prevention 29(6):745-57. and a 2001 update Farmer, C.M. 2001. New evidence concerning fatal crashes of passenger vehicles before and after adding antilock braking systems. Accident Analysis and Prevention 33(3):361-9. reported no difference in the overall fatal crash involvement of cars with and without antilocks.

    According to one federal report, "the overall, net effect of antilock brakes" on both police-reported crashes and fatal crashes "was close to zero." Kahane, C.J. 1994. Preliminary evaluation of the effectiveness of antilock brake systems for passenger cars. Report no. DOT HS-808-206. Washington, DC: National Highway Traffic Safety Administration. A more recent federal report concluded that ABS reduces overall crash involvement risk by 6 percent for cars and 8 percent for pickups and SUVs but has no effect on fatal crash risk. Kahane, C.J. and Dang, J.N. 2009. The long-term effect of ABS for passenger cars and LTVs. Report no. DOT HS-811-182. Washington, DC: National Highway Traffic Safety Administration. Leonard Evans, a researcher with General Motors, reported that antilock-equipped cars were less likely to rear-end other vehicles but more likely to have other vehicles rear-end them. Evans, L. and Gerrish, P. 1996. Antilock brakes and risk of front and rear impact in two-vehicle crashes. Accident Analysis and Prevention 28(3):315-23. The net result was little effect on overall crash risk. In a study conducted for auto manufacturers, Failure Analysis Associates reported a net beneficial effect of antilocks on nonfatal crashes but no effect on fatal crashes. Padmanaban, J. and Lau, E. 1996. Accident experience of passenger vehicles with four-wheel antilock braking systems. Proceedings of the 40th Annual Conference of the Association for the Advancement of Automotive Medicine, 111-25. Des Plaines, IL: Association for the Advancement of Automotive Medicine.

  5. Why hasn't passenger vehicle ABS reduced crashes as expected?

    No one knows for sure why ABS test performance has not translated into a substantial reduction in real-world crashes. A possible reason is that the average motorist rarely experiences total loss of vehicle control, which antilocks are designed to prevent. There also is evidence that many drivers in the early days of antilock brakes did not know how to use them effectively. A 1994 Institute survey of drivers with antilock-equipped cars found that more than 50 percent in North Carolina and 40 percent in Wisconsin incorrectly thought they should pump the brakes. Williams, A.F. and Wells, J.K. 1994. Driver experience with antilock brake systems. Accident Analysis and Prevention 26(6):807-11.

  6. Is motorcycle ABS effective at reducing crashes?

    Yes. Results from recent studies by IIHS and HLDI compared crash rates for motorcycles equipped with optional ABS against the same models without the option. The rate of fatal crashes per 10,000 registered vehicle years was 31 percent lower for motorcycles equipped with optional ABS than for those same motorcycles without ABS. Teoh, E.R. 2013. Effects of antilock braking systems on motorcycle fatal crash rates: an update. Traffic Injury Prevention, in press. In crashes of all severities, the frequency at which insurance collision claims were filed was 20 percent lower for the ABS models. Highway Loss Data Institute. 2013. Evaluation of motorcycle antilock braking systems alone and in conjunction with combined control braking systems. HLDI Bulletin 30(10). Based on these findings, IIHS and HLDI have petitioned the National Highway Traffic Safety Administration to require manufacturers to equip all new motorcycles with this technology.

  7. How long have antilock brakes been around? Are they widely available?

    The idea of antilock brakes has been around for years. They first were used on airplanes in the 1950s. A rear-wheel system was developed for the 1969 Ford Thunderbird, and the 1971 Chrysler Imperial had four-wheel antilocks.

    Modern antilocks were first introduced on 1985 models. By the 1987 model year, they were standard or optional on about 30 domestic and foreign car models. Availability soared to 90 models the next year.

    ABS is a component of electronic stability control (ESC). Thus, the federal requirement for ESC has made antilock brakes standard equipment on all passenger vehicles as of the 2012 model year.

  8. Is ABS required on big truck rigs?

    In March 1995, the National Highway Traffic Safety Administration issued a rule requiring antilock brakes for heavy trucks, tractors, trailers and buses. All new truck tractors were required to have antilocks after March 1, 1997, and they were mandatory on new air-braked trailers and single-unit trucks and buses after March 1, 1998. New single-unit trucks and buses with hydraulic brakes had to be equipped with antilocks after March 1, 1999. This was not the first antilock standard for U.S. trucks. A federal brake standard took effect in 1975, but its antilock and stopping distance requirements were suspended after litigation in 1978.

    ABS is important for big trucks because of the poor braking capabilities of these vehicles compared with passenger cars. On dry roads, stopping distances for big trucks are much longer than those of passenger cars — 47 percent longer in Institute tests. Insurance Institute for Highway Safety. 1990. Special issue: Antilock brakes for trucks. Status Report 25(5):1-7. On wet and slippery roads, the stopping distance disparity is even worse. Tractor-trailer combinations also have the potential for loss of control and jackknifing, especially on slippery roads. (Jackknifing occurs when the rear wheels of a tractor lock up, allowing the tractor to skid and spin so that it folds into the trailer. This also can happen when trailer wheels lock and cause the trailer to swing around the tractor.) Antilock brakes not only reduce stopping distances on wet and slippery roads, but also help drivers maintain control.

    The standard for tractors requires antilock control on the front axle and at least one rear axle. On at least one of the tractor axles, each wheel must be independently controlled by an antilock modulator. This ensures that a wheel provides shorter stopping distances and optimal braking force on all surfaces, especially on roads where one side is slipperier than the other. For semi-trailers, at least one axle must have antilocks. Full trailers must have antilocks for at least one front and one rear axle.

    A 2010 report by the National Highway Traffic Safety Administration concluded that ABS on tractors reduced crash involvement by 3 percent. Allen, K. 2010. The effectiveness of ABS in heavy truck tractors and trailers. Report no. DOT HS-811-339. Washington, DC: National Highway Traffic Safety Administration. However, there was no significant effect on fatal crashes.

March 2015

  1. What are daytime running lights, and what safety benefits do they provide?

    Daytime running lights (DRLs) are headlights that are lit whenever a vehicle is running. A low-cost method to reduce crashes, they are especially effective in preventing daytime head-on and front-corner collisions by making it easier to see vehicles, particularly as they approach from far away.

  2. Where are DRLs required?

    Laws in Canada and many European countries require vehicles to operate with lights on during the daytime. Canada requires vehicles made after Dec. 1, 1989, to be equipped with DRLs. The European Union requires DRLs for new cars and small vans under a law that took effect in February 2011. New trucks and buses in the EU must have DRLs starting in August 2012.

    No U.S. state mandates DRLs, but some require drivers to operate vehicles with lights on in bad weather.

  3. Are DRLs available on vehicles in the United States?

    First offered on a handful of 1995 domestic and foreign model passenger cars, pickups and SUVs, daytime running lights have become a more common feature. They are standard on all General Motors, Honda, Subaru, Volkswagen and Volvo models. Other manufacturers also offer daytime running lights on certain models. GM offers retrofit kits for vehicles that do not already have DRLs. The kits can be used on non-GM models, too.

  4. How effective are DRLs?

    Nearly all published reports indicate DRLs reduce multiple-vehicle daytime crashes. A 1985 Institute study determined that commercial fleet passenger vehicles modified to operate with DRLs were involved in 7 percent fewer daytime multiple-vehicle crashes than similar vehicles without DRLs. Stein, H. 1985. Fleet experience with daytime running lights in the United States. SAE Technical Paper Series 851239. Warrendale, PA: Society of Automotive Engineers. Multiple-vehicle daytime crashes account for about half of all police-reported crashes in the United States. A 2002 Institute study reported a 3 percent decline in daytime multiple-vehicle crash risk in nine U.S. states concurrent with the introduction of DRLs. Farmer, C.M. and Williams, A.F. 2002. Effects of daytime running lights on multiple-vehicle daylight crashes in the United States. Accident Analysis and Prevention 34(2):197-203.

    Federal researchers, using data collected nationwide from 1995 to 2001, concluded that there was a 5 percent decline in daytime, two-vehicle, opposite-direction crashes. Tessmer, J.M. 2004. An assessment of the crash-reducing effectiveness of passenger vehicle daytime running lamps (DRLs). Report no. DOT HS-809-760. Washington, DC: National Highway Traffic Safety Administration. However, a 2008 federal study concluded that DRLs reduce crash involvements of pickups, SUVs, and vans, but have no significant effect on crashes of passenger cars. Wang, J.S. 2008. The effectiveness of daytime running lights for passenger vehicles. Report no. DOT HS-811-029. Washington, DC: National Highway Traffic Safety Administration.

  5. Will DRLs shorten headlamp bulb life or lower fuel economy?

    Running vehicle lights in the daytime does not significantly shorten bulb life. Systems like those on GM cars that use high beams are designed to operate at half their normal power during daylight hours, thereby conserving energy and reducing the effect on a vehicle's fuel economy. The National Highway Traffic Safety Administration (NHTSA) estimates that only a fraction of a mile per gallon will be lost, depending on the type of system used. 

  6. Are motorcycles required to have DRLs?

    Federal law does not require motorcycles to have DRLs, but some states require motorcyclists to ride with their headlights on at all hours. Since 1979 most manufacturers have equipped their cycles with automatic-on headlamps.