Auto Tech

How blind-spot monitoring works

They're no replacement for an old-fashioned glance over your shoulder, but blind-spot monitoring and rear cross traffic alert systems can help watch your back.


If you don't properly adjust your car's mirrors or if your vehicle of choice has thick window pillars, you may find that there's a blind spot just over your shoulder where cars behind you can hide. A blind-spot monitoring system can help watch your back so that you don't find yourself trading paint when changing lanes.

Blind-spot monitoring systems are among the most common modern driver aid technologies, being found on the options list of many mid-tier or budget models. The tech is sometimes shortened to BSM or called blind-spot warning (BSW). I tend to call it a blind-spot information system (BLIS), as it was by Volvo, which originated the tech back in 2007 for use on its and Ford's vehicles.


The new Volvo XC60 features BLIS with Steering Assist and can actively guide the SUV back into its lane if it detects that you're about to collide with another vehicle.

Volvo Cars

What is it? and why?

This technology uses ultrasonic sensors located on the car's flanks to detect when a car is in your blind-spots. Most BLIS implementations will warn by illuminating a notification light on or near the side mirror on the appropriate side of the vehicle.

If you activate your turn signal while BLIS is triggered, you'll usually also get an audible beep or tone to let you know to look twice before changing lanes.

Most basic systems will let you know if a car is currently in your blind-spot, but more advanced versions will let you know when a vehicle is about to be there. Lane Change Assist, as Hyundai calls it, extends the range of the side sensors as much as three to five car lengths back and monitors the speed of oncoming vehicles. With this information, the system can sort of predict the future, alerting you to an upcoming car in your blind-spot before it's even there.

2016 Mazda CX-9 blind spot monitor

Blind spot information systems are increasingly a "must-have" feature for large trucks and SUVs with long tail ends and poor rear visibility.


What are the limitations?

Typically, blind-spot systems only work at speeds above 20 to 35-ish mph. This prevents false positives on city streets but tends to only make the tech useful at highway speeds. And as I mentioned, the audible alerts work only if you actually use your turn signal, which some people don't always use.

So, keep in mind that the technology is no replacement for an old-fashioned glance over your shoulder before changing lanes. Always look before you leap.

Rear Cross Traffic Alert

You'll often see blind-spot systems packaged with so-called rear cross-traffic alert (RCTA) systems because they use the same ultrasonic sensors. RCTA monitors the blind-spots of your car for traffic, but from the sides at low speeds when reversing rather than at highway speeds.

When backing up, sometimes your view is blocked by other vehicles or obstructions, making the blind-spot over your shoulder even larger. RCTA can let you know a car is coming down the aisle in the parking lot when you're reversing out of a spot or when backing out of your driveway onto the street.

Here also, the warning is most often an audible tone, but General Motors has made very interesting use of haptic feedback with its Safety Seat Alert, which vibrates the seat to grab your attention.

Who does it best?

BLIS and rear cross-traffic alerts are increasingly becoming "must-have" technology on larger SUVs and cars with poor rearward visibility and the next evolutions of the tech are already here.

Volvo is at the forefront of blind-spot monitoring tech -- it figures, they invented it -- with its BLIS with Steering Assist. Featured on the new 2018 Volvo XC60, BLIS with Steering Assist can actively guide the SUV out of harm's way and back into its lane if it detects that you're about to collide with another vehicle when changing lanes.

Ford and Toyota/Lexus, for example, have taken the next logical step of adding automatic braking assist technology to their most advanced RCTA systems. So, if you're backing out of a parking spot and another vehicle is approaching, the system will sound an alert and then, if you ignore the warning, automatically brake before you move into a possible collision path.

Be sure to check out the rest of our guide to advanced driver aid technology to learn more about how modern and future cars help make driving safer and more convenient.