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AAA study finds automatic emergency braking and lane keeping assistance performance can be impeded by rain

A new study from AAA finds that moderate to heavy rain affects a vehicle safety system’s ability to “see”, which may result in performance issues. During closed course testing, AAA simulated rainfall and found that test vehicles equipped with automatic emergency braking (AEB) traveling at 35 mph collided with a stopped vehicle one third (33%) of the time. Lane keeping assistance test vehicles departed their lane 69% of the time.

Depending on the type of sensor, external influences such as weather and sensor cleanliness can have various effects on operation. Specifically, radar sensors are minimally affected by rain, snow, and fog relative to other sensor types and function equally well in lighting conditions ranging from complete darkness to blinding sun. Additionally, radar sensors tend to be less affected by bugs and dirt because they are frequently placed behind plastic bumper covers. In cases where radar sensors are exposed, emitted radio energy can penetrate these particles with minimal attenuation.

However, systems such as lane keeping assistance or lane departure warning systems require integration of additional sensors because radar is not effective at discerning object detail and cannot detect variations on a flat surface, such as lane markings. Image sensors (cameras) are currently utilized for object classification and to track lane markings. Depending on the type of electromagnetic radiation (visible, near-infrared, medium wave infrared, etc.) detected by the camera, the sensitivity to weather and dirt varies. Cameras that detect energy in the visible spectrum are most affected by weather, lighting conditions, and bugs/dirt relative to cameras specific to the infrared spectrum.

Automatic emergency braking systems utilize front-facing radar and/or camera(s) to obtain kinematic data pertaining to surrounding vehicles and objects. Lane keeping assistance systems currently rely on one or more cameras to track the position of lane markers. Lane keeping assistance and automatic emergency braking systems effect sustained lateral and temporary longitudinal motion control, respectively.

—“Effect Of Environmental Factors On ADAS Sensor Performance”

Advanced driver assistance systems (ADAS) are typically evaluated in ideal operating conditions. However, AAA believes testing standards must incorporate real-world conditions that drivers normally encounter.

Vehicle safety systems rely on sensors and cameras to see road markings, other cars, pedestrians and roadway obstacles. So naturally, they are more vulnerable to environmental factors like rain. The reality is people aren’t always driving around in perfect, sunny weather so we must expand testing and take into consideration things people actually contend with in their day-to-day driving.

—Greg Brannon, AAA’s director of automotive engineering and industry relations

AAA selected crossover vehicles for testing because of their continuing popularity in the United States. In 2020, sales of crossovers and utility vehicles accounted for 50% of the new vehicle market share. Additionally, the following criteria were utilized for vehicle selection:

  • Inclusion of domestic and import OEMs including European and Asian manufacturers

  • Variety of manufacturers (only one vehicle per manufacturer will be tested)

Based on those requirements, AAA selected the following vehicles for testing:

  • 2020 Buick Enclave Avenir with Automatic Emergency Braking and Lane Keep Assist

  • 2020 Hyundai Santa Fe with Forward Collision Avoidance Assist and Lane Keeping Assist

  • 2020 Toyota RAV4 with Pre-Collision System and Lane Tracing Assist

  • 2020 Volkswagen Tiguan with Front Assist and Lane Assist

Rain has the biggest effect on vehicle safety systems. AAA, in collaboration with the Automobile Club of Southern California’s Automotive Research Center (ARC), simulated rain and other environmental conditions (bugs and dirt) to measure impact on the performance of ADAS-like automatic emergency braking and lane keeping assistance. Generally, both systems struggled with simulated moderate to heavy rain, with results showing:

  • Automatic emergency braking engaged while approaching a stopped vehicle in the lane ahead

    • In aggregate, testing conducted at 25 mph resulted in a collision for 17% of test runs

    • In aggregate, testing conducted at 35 mph resulted in a collision for 33% of test runs

  • Lane keeping assistance engaged to maintain the vehicle’s lane position

    • In aggregate, veered outside of the lane markers 69% of the time

During testing with a simulated dirty windshield (stamped with a concentration of bugs, dirt and water), minor differences were noted, however, performance was not negatively impacted. While AAA’s testing found that overall system performance was not affected, ADAS cameras can still be influenced by a dirty windshield. It is important drivers keep their windshields clean for their own visibility and to ensure their ADAS system camera is not obstructed.

Also, some systems may provide an alert or deactivate in extreme situations, however, the conditions AAA tested under provided no such alert or warning.

To simulate rainfall, AAA engineers designed a system using a reservoir to hold water, a high-pressure pump for a consistent flow of water and a precision injector nozzle to spray the windshield. This system was secured in the cargo area of the test vehicle and was connected to a nozzle positioned above the windshield so that the spray pattern covered the entire windshield. Water sprayed by this system did not reach the pavement or interact with the test vehicle’s tires.

Previous AAA testing of vehicle safety systems in both closed-course and real-world settings show that performance is greatly impacted by driving scenarios, road conditions and vehicle design, finding issues such as the following:

  • Struggling to stay within in a marked lane in moderate traffic, on curved roadways and on streets with busy intersections.

  • Failing to stop for pedestrians in common scenarios such as crossing in front of a vehicle, a child darting out between two parked vehicles, or walking at night.

  • Colliding with a simulated disabled vehicle and instances of coming too close to other vehicles or guardrails.

AAA’s research continues to show that vehicle safety system performance varies widely, reinforcing that they are not a replacement for a fully engaged driver.

AAA recognizes these systems have the ability to lessen the chance of a crash and improve the overall safety of driving. Fine-tuning their performance and providing drivers with a more consistent experience will go a long way in unlocking their true potential.

—Greg Brannon

Based on the findings, the report makes three main summary recommendations for drivers:

  1. Drivers should understand the limitations of ADAS and remain engaged behind the wheel as these systems are an aide, not a replacement for an engaged driver.

  2. Drivers must use extra caution and avoid system overreliance, particularly in adverse weather conditions.

  3. To ensure optimal system performance, keep areas around cameras and radar sensors clean. The owner’s manual will describe the location of sensors and recommended cleaning procedures.



Guess what. Heavy rain affects humans ability to "see" as well. Automated driving COULD still be better than a human under those circumstances.

Bob Niland

This is definitely a "compared to what?" situation. Is the net outcome more or less dangerous than hand driving?

I'm really surprised that they didn't appear to have "control" runs with the same vehicles driven with the same impairments and with the automation disabled.

If a net degradation of safety, I'd also guess that looking forward, the suites need some threshold of sensor impairment below which the automation refuses to engage.

Mike Friederichs

Wonder why they didn't test Tesla Autopilot? It actually works in the rain, and properly requires that the driver take control when visibility is reduced. Unlike these failed attempts at ADAS that veered outside of the lane markers 69% of the time and should be investigated and recalled by NHTSA


It is curious that the vehicles ran into the back of the stopped vehicle - the radar should have picked it up, and radar should not be upset by rain.

Also, people can see road lines in the rain, unless it is absolutely pelting down.

It must be that the glass covering the cameras got covered in rain to an extent that the image was so distorted that the system gave up.
This is a solvable problem.

I suppose the key thing is that they always test in the wet as well as the dry.
Then it starts to get tricky - what about low sun, what about snow, fog etc.

One thing that is incredible is the ability of humans to drive when they can barely make the scene out (you narrow your eyes and maybe shade them with your hand; you can estimate the position of a road line even if you can barely see it.
Never underestimate the human visual system.


Rain has the biggest effect
Don't use it in the rain


I have one of the tested vehicles and can confirm the extreme rain down pours that Florida see a lot can temporarily disable these systems. Even radar cannot reliably see through the really heavy rain we get.

I also service one of the brands in question. Many customers come in with some complaint of the system and often I find faults "system disabled to to limited visibility" stored.

In reality the problem is two fold -
1) we continue driving in bad weather when we really should have pulled over and not continue driving.
2) once drivers get used to a new DAS they depend on it to take over and save them from their bad driving, instead of driving safely and DAS helping only as the backup safety it was designed for. (Tesla drivers - I'm looking at you sleeping and watching movies behind the wheel)


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