Early on in Drivers’ Ed, you were taught to look over your shoulder before changing lanes because side view mirrors don’t see everything. What you may miss in a quick glance is what blind spot detection picks up. This driver assistance technology senses cars coming up in your blind spot behind or alongside you, and if your turn signal is on, it alerts you not to change lanes. You’re warned by a flashing light on the side view mirror and then a beep or steering wheel vibration. If you’re not planning to change lanes (there is no turn signal on), the warning light glows steadily but doesn’t flash and there’s no audible alert.
Blind spot detection is a key technology among driver aids that provide 360 degrees of electronic coverage around your car, whether you are at speed or moving slowly. This circle of safety also includes adaptive cruise control, lane departure warning, rear and front parking sonar, the rear traffic alert, and parking cameras (ranging from rear-only through four cameras providing a birds-eye view of the car as you snake into and out of tight spaces). Some driver’s aids make you safer, especially late on a long drive, and some earn back their cost when you don’t crumple a fender, where the insurance deductible costs more than the device.
Blind spot detection vs. lane departure warning vs. parking sonar
Three technologies protect the side and back of your car: blind spot detection, lane departure warning, and rear parking sonar. Here’s how they differ:
Blind spot detection (BSD) was developed by Volvo a decade ago. BSD tracks traffic just behind you as well as what’s coming alongside. The alert stays active until the car in the adjacent lane is in front of you, or at least directly alongside and you’d have to be blind not to see it. It doesn’t care if you are in your lane or have drifted a bit into the next and are at risk of sideswiping another car. BSD uses ultrasonic or radar sensors on the side and rear of the car.
The name comes from the blind spot to the side just behind the car where you may not see a car because the mirror doesn’t cover and if you turn your heard, it could be obscured by the B-pillar (the one between the front and back seat on four-door cars). The visual alert is a yellow (usually) indicator in the side mirror glass, inside edge of the mirror housing, or on the A pillar inside the car. It lights when it senses a car in the blind spot and flashes if the turn signal is flashing. You’ll also get an audible alert (beeping) or an induced vibration or light shake of the steering wheel if the turn signal is flashing.
The system is called blind spot detection, blind spot monitoring, and blind spot information system, depending on the maker, but it’s all the same thing. The first two are almost equal in frequency of use. BLIS is used by Ford, Lincoln and Volvo. Audi calls it Side Assist, General Motors side blind zone alert, and Infiniti’s name is blind spot warning. You’ll also see active blind spot monitoring.
The opposite, passive blind spot monitoring, means — wait for it — looking in the side mirror. Just as the names vary, so does the performance. Some only detect cars once they’ve overlapped your rear bumper (that is, already alongside), while others detect cars 3-5 car lengths back.
Blind spot detection also works (as BSD) to spot cars and trucks; even motorcycles and bicycles have enough mass to be sensed. Although: How often are you overtaken by a bicycle?
Lane departure warning (see our tech LDW backgrounder) tracks whether you’re centered in your lane or not. It uses a forward-facing camera usually in the rear view mirror mount so it’s typically an optical system. If you drift across the lane, you get a visual alert in the dashboard and an audible alert or a steering wheel or seat bottom vibration. If your turn signal is on, you won’t get an alert; the car assumes you intend to cross over lanes. That’s the opposite of BSD, which gives a full alert with the turn signal blinking, because the car fears you’re about to move into the path of another car.
Parking sonar uses ultrasonic (usually) or electromagnetic sensors in the back and sometimes front of the car to judge how close you are to objects nearby. Sometimes the sensors are part of the blind spot detection and cross traffic alert (see below) systems. They work like sonar in a U-boat movie: the sensor sounds a ping when it first senses an obstacle and the pings get more frequent, becoming a solid tone at about a foot away. You can have parking sonar (also park distance control, park assist, or Parktronic) without blind spot detection.
What else can blind spot detection do?
If you have blind spot detection, the sensors may also provide cross-traffic alert. When you start to back out of a head-in parking spot with cars on either side, the sensors will notice traffic on either side heading in your direction and sound an alert, usually louder and more insistent that backup sonar. It sees the traffic coming almost as soon as you start backing out, while you’re still trying to peer through the window glass of cars next to you.
The BSD side sensors also help find parallel parking spaces and then park the car. This is an automated parking system — tap a button on the console and drive slowly (say, 20 mph). The sensors see cars parked on the roadside and then gaps between the cars; if it sees a gap 3-5 feet longer than your car, that’s enough for the car to guide itself back into the space. The driver just stops the car, puts it in reverse, and applies the throttle and the brakes. The driver also has to watch for driveways and fire hydrants that the system misses. Ford has been aggressive in marketing both automated parking (active park assist) and cross-traffic alert and bringing it down from full-size cars to the low-cost Ford Focus.
The sensors at the back and side of the car may also be used for safe backing, called variously parking sonar, backup assist, reverse parking sensor. Not all rear sensors for parking are used for blind spot detection. If your car has rear parking assist, it may not have blind spot detection.
Annoying? Then turn it off
Blind spot detection is one of the technologies that car purists detest. Cranky editors at Car and Driver got out there early and announced BSD as one of Five Annoying Safety Technologies “sure to raise your blood pressure on the morning commute.” Six years later it remains near the top of the Google hit list when you search for “blind spot detection.” The editors may not have noticed the technologies they despised had off switches. A Consumer Reports blogger was likewise annoyed by “my own little commuting light show” under the could-see-it-coming headline “Ignorance is BLIS.” As for the complaint about annoying blinking lights, on most cars that’s only when you have your directional on; when it’s off, you got a non-blinking light that is no more distracting than others on the dash or instrument panel.
Some cars leave an indicator lamp lit in the instrument panel when you disable BSD and some critics say one more light is annoying. But consider the opposite: For some safety aids, especially stability control but also BSD and LDW, you ought to know that it’s off. Maybe you hit the button inadvertently since it’s typically below and to the left of the steering wheel where you can’t see them while driving. Maybe your car carries over the setting rather resets to each time you cycle the ignition and you ought to know if the previous driver in a family car had BSD off. If you want to tailor blind spot detection so it lights but never flashes, or so there’s no it’s-off light when it’s off, forget it. You can’t tailor your car to your tastes; leave that to the automakers.
Using blind spot detection you will get occasional false alerts, most often when you pass a solid object like a tunnel or a guardrail that’s close to the car (or you’re close to it). My experience has been that false alerts are few, you can recognize most of them when you see the Lincoln Tunnel way five feet off, and you can recognize the rest by turning your head, really turning it, to look over your shoulder.
Next page: Alternatives for blind spot detection…
Honda developed on offbeat blind spot detection system that requires driver judgment. On the Honda Accord, Lane Watch is a rear facing camera in the base of the passenger side mirror. Turn on your right directional and the image appears on the center stack display with three horizontal lines overlaid on the image. If it’s between the middle and closest line, don’t change lanes. If it’s between the middle and rear line, use your judgment, or watch to see if the car is accelerating.
We thought the system quirky (there’s no blind spot detection for the driver side), but not so quirky as to keep the Accord from being one of our ExtremeTech top tech cars.
Ford for several years has offered wide-angle side mirrors on both sides. The passenger mirror is convex, as is typical on virtually all cars now. The outside edge of the driver’s mirror (photo above) is curved (convex) and that gives you a significantly wider field of view. It’s also better than the $2.99 adhesive-tape-on convex blind spot mirrors that take up a small corner of your outside mirror and then after six months tarnish so you can’t see anything.
For everyone else, do this: crank the side mirrors farther out. At the least, you should not see your rear fenders in the mirror (they are not going to hit you). How far? Move yourself 6-12 inches to the right and crank the passenger mirror out until you can just see the edge of your car now. Then move yourself 6-12 inches left of center and crank the driver side mirror out until you just see the left edge of the car. By moving your head just a little, you’ll have a wider and safer field of view.
Should you buy? Yes, if you can afford the package
Safety groups almost all believe you should have blind spot detection. Between blind spot detection and lane departure warning, if you’ve only got money for one, you may find more safety group preference for BSD. On its own, blind spot detection runs $250-$500, but more often you’ll find it in a package. On the excellent new Chevrolet Impala, the Advanced Safety Package runs $940. On other GM cars, lane departure warning and forward collision warning run $295 as a package, so the other two interlocking components, side blind zone alert and cross traffic alert, would be $645.
Blind spot monitoring, like lane departure warning, is more of a highway than urban safety aid. It benefits drivers on long trips, especially once they are several hours in. It’s also a benefit to older drives whose head-turning ability is limited. (Florida ranks high in T-bone crashes where drivers don’t see cars coming at right angles, to say nothing of cars angled behind them.) That applies to younger drivers who used to be jocks and perhaps suffered shoulder or neck injuries that limit mobility.
