Car Safety Systems Don't Always 'See' in Bad Weather, AAA Says

driving in the rain — If you can't see to drive in bad weather, how can your car's advanced driver assistance systems do it? Amer Ghazzal/Getty Images

In the past five years or so, advanced driver assistance systems (ADAS) have exploded onto the market. Even entry-level car models sometimes come with long lists of high-tech safety equipment, like automatic emergency braking and lane keeping assistance.

These systems are not autopilot — they cannot do the driving for you in any circumstances — but they do help a lot with visibility and reaction times. However, if anything is blocking the systems' cameras, like mud or snow, they can't do their job.

Most new cars today use a combination of radar sensors, which are hidden behind plastic in the bumpers, and optical cameras mounted behind the windshield. Radar is not too affected by weather or lighting conditions, and since these sensors are behind plastic, bugs don't really bother them either.

But radar can't see things like lane markers or make out details. That's where the cameras come in. Cameras are better for classifying objects, but they have a harder time "seeing" in bad weather or bad lighting.

Putting Vehicle Safety Systems to the Test

To test camera performance of the automatic emergency braking and lane keeping assistance systems in simulated moderate to heavy rainfall, AAA worked with the Automobile Club of Southern California's Automotive Research Center. AAA released its findings Oct. 14.

The four common SUVs they tested, like most new cars today, used a combination of radar sensors in the bumpers and optical cameras mounted behind the windshield. Since rain doesn't really affect radar, though, the water only needed to be sprayed over the windshields to test the cameras. It's worth noting too that the track was dry, so the tires had their ideal grip.

Researchers tested the automatic emergency braking systems of vehicles at two low, neighborhood-appropriate speeds and found that at 25 mph, 17 percent of the test runs ended in collisions. When they increased the speed a bit to 35 mph, 33 percent of the test runs ended in collisions.

So the radar sensors still did their job as well as they could on their own, but without the "eyes" of the camera, it wasn't ideal.

The lane keeping assist feature struggled far more. Test vehicles veered outside their lanes 69 percent of the time. As we know, radar can't see lane markers, and a camera trying to peer through heavy rain can't really see them either.

Testers also simulated windshields by stamping the glass with "a concentrated solution of bugs and dirt," according to the full report we reviewed. Ew. Interestingly, there were no negative impacts for the cameras with this smattering of sludge on the glass. But really, if you can't see very well out of your filthy windshield, neither can your fancy ADAS. At least get the bugs off.

AAA has done previous research showing that these systems are not perfect. Curved lanes and high traffic can affect a car's ability to track a marked lane, and ADAS doesn't always see pedestrians walking at night.

What all of this research means is your car's ADAS can indeed assist you, but it cannot yet replace you. Human brains are still the best onboard computers.