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What Is a Blind Spot Monitor?

Blind Spot Warning

What Is a Blind Spot Monitor?

A blind spot monitor uses sensors to detect vehicles to the side and rear of a car that the driver may not be able to see. Some advanced systems can intervene automatically to keep the driver from changing lanes when another vehicle is in an adjacent lane. The standard industry term for blind spot monitoring is blind spot warning (BSW). The system is often bundled with another advanced driver assistance technology, rear cross-traffic alert.

How Does a Blind Spot Monitor Work?

Blind spot warning systems use small cameras or radar sensors to monitor the areas alongside and to the rear of the vehicle on both the left and right sides. The system usually works at speeds greater than 20-35 mph. Most BSW units will only detect other vehicles that are directly alongside, while some systems can sense vehicles that are a few car lengths back for an added measure of safety.

If another vehicle is detected, drivers will get a warning alert chime and/or a warning light will illuminate, sometimes on the mirror or the windshield pillar closest to the nearby vehicle. The system will typically give more insistent alerts if drivers switch on their turn signal. More sophisticated systems go a step further and intervene with subtle autonomous steering or brake input to keep drivers from changing lanes, avoiding a perilous situation. The driver is always able to overcome this with more forceful steering.

Blind Spot Monitor in Side View Mirror

Blind Spot Monitor in Side View Mirror

Research Shows Blind Spot Warning Prevents Crashes

BSW systems can keep motorists from running into another car that’s moving in the same direction in an adjoining lane. According to the Insurance Institute for Highway Safety (IIHS), blind spot warning systems can reduce the occurrence of lane-change crashes by 14 percent and cut the number of such collisions with injuries by 23 percent.

Known Drawbacks of BSW

Depending on the vehicle, some BSW systems may not operate at lower speeds, which limits its usefulness in slower traffic.

On some models, the warning light is a small symbol embedded in the side-view mirror that may be difficult for a driver to see at a quick glance, especially in bright sunlight. Others use larger warning lights mounted on the windshield pillar or on the side-mirror mounts that are easier to notice.

The audible or tactile alerts, or autonomous intervention included with some systems, only activate if the turn signals are used, which is something many drivers neglect to do.

Finally, the system may not work at all if the embedded sensors are covered in snow or mud.

Does the Car You’re Considering Have BSW?

Be sure to check the Research section here on Carfax.com for reviews of current- and past-model-year vehicles. Click on the Safety tab to determine availability of blind spot warning and other driver assist systems, along with crash-test ratings and other pertinent data.

If you’re thinking about shopping for a used car with BSW, you can use our Used Car Listings. Every vehicle comes with a free Carfax Vehicle History Report for peace of mind.

Research Other Advanced Driver Assistance Systems (ADAS)

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