Driver-assistance technology can fail in rain, leading to crashes in AAA study

In testing, emergency braking failed to stop a crash 33% of the time at 35 mph

Human drivers don't necessarily see the road ahead as well when it rains, and it turns out that driver-assistance technology doesn't either. The systems used to help your car automatically brake and stay within its lane is significantly impaired by rain, according to a study by AAA released Thursday.

American Automobile Association researchers found that automatic emergency braking, in several instances during testing conducted in simulated moderate-to-heavy rainfall, failed to detect stopped vehicles ahead, resulting in crashes. Lane-keeping technology also faired badly. 

AAA cautioned drivers, who should always be vigilant of these systems even in ideal conditions, not to rely on them in the rain.

“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,” said Greg Brannon, AAA’s director of automotive engineering and industry relations. “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.”

Advanced driver-assistance systems, or ADAS, are common in newer vehicles. They do not perform autonomous driving, but they can automate some limited driving tasks such as adaptive cruise control and staying centered in one's lane. Auto emergency braking has been shown to significantly reduce rear-end crashes in tests by insurance groups.

For its tests, AAA employed a 2020 Buick Enclave Avenir, a 2020 Hyundai Santa Fe, a 2020 Toyota RAV4 and a 2020 Volkswagen Tiguan.

No test car crashed into a stopped vehicle under dry, ideal conditions. But then researchers turned on the simulated rainfall — and 17% of the test runs resulted in crashes at speeds of 25 mph (40 km/h). At 35 mph, the instances of crashes increased to 33%.

You can extrapolate from there as to the dangers at highway speeds.

Researchers simulated rainfall in the vehicles' field of vision by using a device involving a spray nozzle that obscured the sensors in the windshield, as shown in the photo above. That way, they were able to keep the roadway dry. AAA noted that wet roads in real driving conditions could result in even higher crash rates.

As for lane-keeping technology, vehicles crossed lane markers 37% of the time during ideal conditions in the AAA test — and that rate jumped to 69% once rain was added.

This is not the first AAA study to note shortcomings in driver-assistance systems. On the bright side, this study noted that merely having a dirty or bug-spattered windshield had little effect on the systems' sensors.

AAA emphasizes that while these systems have potential, they are no match for an attentive driver. For driving in rain, AAA offers these tips:

  • Keep windshield clean and ensure that wipers are not streaking the windshield.
  • Slow down and avoid hard braking and sharp turning. If possible, follow in the tracks of other vehicles.
  • Increase following distance to 5-6 seconds behind the vehicle ahead.
  • Do not use cruise control in order to stay alert and to respond quickly if the car’s tires lose traction with the road.
  • If the car begins to hydroplane, ease off the accelerator to gradually decrease speed until the tires regain traction, and continue to look and steer where you want to go. Don’t jam on the brakes—this can cause further traction loss.

Reuters was used in this report.

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