Vehicle safety systems struggle to "see" in bad weather

New research 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 traveling at 35 mph collided with a stopped vehicle 33% of the time. Lane keeping assistance didn’t fare any better, with test vehicles departing their lane 69% of the time. Vehicle safety systems, also known as advanced driver assistance systems or ADAS, are typically evaluated in ideal operating conditions. However, AAA believes testing standards must incorporate real-world conditions that drivers normally encounter.

How AAA tested braking & lane keeping systems

AAA worked with the Automobile Club of Southern California's Automotive Research Center to test how 2 ADAS options (automatic emergency braking and lane keeping assistance) performed in 2 separate scenarios (simulated rain and a simulated dirty windshield).

AAA engineers designed a system using a reservoir to hold water, a high-pressure pump for a consistent flow, and a precision injector nozzle to spray the windshield. The nozzle was positioned so that the spray pattern covered the entire windshield. Water sprayed by this system did not reach the pavement or the test vehicle’s tires.

A car windshield fitted with a water spraying nozzle for safety testing

A water nozzle sprayed the windshield to simulate rainfall.

To simulate a dirty windshield, engineers created a mixture of bugs, dirt, and water, and used a custom stamping tool to apply a consistent pattern of the mixture to the windshield around system cameras.

A mixture of bugs, dirt, and water was applied with a stamping tool to simulate a dirty windshield.

The results: Rain has the biggest effect

Generally, both ADAS systems struggled with simulated moderate to heavy rain:

Automatic emergency braking with simulated moderate to heavy rain

Testing conducted at 25 mph resulted in a collision for 17% of test runs
Testing conducted at 35 mph resulted in a collision for 33% of test runs

Lane keeping assistance with simulated moderate to heavy rain

Vehicles veered outside of the lane markers 69% of the time

In contrast, testing with a simulated dirty windshield produced only minor differences without impacting either system's overall performance. While this testing didn't find an effect, ADAS cameras can still be influenced by a dirty windshield, so it's important for drivers keep their windshields clean for their own visibility and to ensure their ADAS system camera is not obstructed.

A test vehicle touching a foam target car used for testing.

In the automatic emergency braking portion, the test vehicles were driven toward a foam target simulating a stationary car in the lane ahead.

What drivers can do to stay safe

1. Get familiar with how ADAS work

AAA urges drivers to take time to read the vehicle owner’s manual to learn when, where, and how to use features like automatic emergency braking and lane keeping assistance. The manual or online materials provided by the automaker are the best resource for understanding the ins and outs of an individual vehicle’s safety system.

2. Keep the windshield clean

Vehicles with ADAS cameras located behind the windshield, like those used in this research, “see” the same things a driver does. If the windshield wipers are worn and streak across the window, this could interfere with the system’s performance. Drivers should regularly inspect their wipers and replace them at the first sign of streaking, chattering, or if the blades appear hard and brittle. A good rule of thumb is to replace wipers each year, especially in hot, sunny climates or if a vehicle sits outside all day.

3. Know how to drive safely in wet conditions

Rain presents challenges to drivers regardless of whether their cars have advanced safety systems. AAA recommends using extra caution in slick conditions:

Keep the windshield clean and ensure that wipers are not streaking.
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.
Don't use cruise control. Stay alert and 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.