How Automatic Braking Systems Work

In pretty much every case, automatic braking is part of a suite of safety system technology that works together to save your sorry butt. The system in a Subaru is called EyeSight; in a Volvo, it's called City Safety. Other manufacturers, including Cadillac and Mercedes-Benz, have something similar.

"The system works very simply from an engineering perspective," said Adam Kopstein, a safety and compliance manager at Volvo. "Making the car stop is easy; we've got the sensors and ABS. The trick is making sure it only does it when it's supposed to." Kopstein is a very funny guy, because after calling this an easy and simple system, he explained in detail how that trick is accomplished.

Every manufacturer uses a different setup, so we'll use Subaru and Volvo's systems as examples of what's out there.

Subaru's EyeSight, not surprisingly, uses two black and white cameras that work like your eyes to triangulate the speed and distance of the vehicle in front of you. They're mounted at the top of the windshield, and they scan every 0.1 seconds, looking for contrast with the background and vertical surfaces. The software is programmed to recognize several types of images, like the rear ends of vehicles, motorcycles, bicycles and pedestrians.

Volvo uses lidar (which is not related to the lion-tiger mashup we all know as a liger) in its City Safety system. Lidar is laser radar, and it sends out a signal that pings off objects in front of it to determine distance and speed. Since lidar works best at short range, Volvo also has a camera mounted in the windshield and radar in the bumper that work together at high speeds as part of its collision warning system with full braking capability. The radar can see several hundred yards in front of the car, but it can't tell what it's seeing. The camera picks up on what the radar is seeing and can identify the object as a problem or something to ignore.

So far, this is just how your car knows that there's a Humvee in front of you, and that the Humvee has slammed on its brakes. While your car's computer does some panicky calculations, let's move to the next page.

So now your car has determined that you're about to hit the Humvee. It can also sense that you're not doing a thing about it. You're not steering away from its massive bumper, and you're not jamming on the brake. It's time for your car to take things into its own hands. Circuits. Whatever.

At under 20 miles per hour (32.2 kilometers per hour) or so, most systems can avoid the crash completely, although the goal is merely to minimize the impact and therefore the injury. "It's an additional safety layer, but we're not trying to take the responsibility of driving away from the driver," Sullivan said. "If the driver is distracted, EyeSight will warn them and help them out if they panic or spazz." That's the Macaulay Culkin face moment that we were talking about earlier.

EyeSight will warn you when you're about a second away from the Humvee's bumper, then apply the brakes lightly to help you out. Most people don't press the brakes hard enough when they're trying to avoid a crash, oddly enough. But if you're still not doing anything, the Subaru will apply full braking force to the tune of about 1 g.

The Volvo system is actually two systems layered one on top of the other: City Safety for slower speeds, and the collision warning system for higher speeds. Since in our example you're crawling along on your way to work, City Safety will come into play. If the lidar (rawr!) thinks you're too close to the car in front and you're not doing anything about it, you get no warning. It starts braking for you, and then lights up a red LED in the windshield that mimics a brake light to get your attention. The idea is that maybe then you'll react and press the brake on your own; but if you don't, Volvo's got it.

If you're going a bit quicker, Volvo's second system will pick up where City Safety leaves off. Above 30 miles per hour (48.3 kilometers per hour) or so, the system will give you a warning if you're following too closely. It will also precharge the brakes so they're ready to slow you down as soon as you heed the warning -- or take over if you don't.

These systems work best when the difference between the speed of your car and the car you might hit is less than 20 miles per hour (32.2 kilometers per hour). If the speed differential is greater than that, the rest is up to you. "If you're flying down the road, EyeSight isn't going to save you from yourself," Sullivan said.

Both Subaru and Volvo -- and really any manufacturer you talk to, as long as it's not Audi with its self-driving car -- make it clear that they are not trying to take driving away from the driver. "If any system begins activation but the driver decides to steer away, you will not be in a fight with the vehicle," Kopstein said. "The car will release the brakes so you can maneuver out of the situation." Sullivan put it even more plainly: "We want you to drive the car."

So, if no one is trying to take away the driver's responsibility, why bother to have the car do the braking for you at all? Why not just beep or blink or, in the case of Cadillac, vibrate the seat to let you know you need to pay attention and drive the dang car?

Research says that a third of all reported collisions happen when a front bumper meets a rear bumper. And in half of those accidents, the driver of the rear car doesn't brake at all. So, apparently, we could use a little help in the braking department.

The IIHS determined that cars with automatic braking systems had between 14 and 27 percent fewer insurance claims for damage to cars, depending on the system. And that was for 2010 cars, which is practically the dark ages for these systems.

Besides, automatic braking doesn't operate alone. It's part of more comprehensive systems that include throttle management, lane departure warnings, adaptive cruise control and other safety systems. And the technology is only getting better, with better cameras and sensors. As the technology moves forward (EyeSight and City Safety are both already on their third generation), it'll get cheaper, and as it gets cheaper, it'll be in more cars, which benefits everyone. Even the woman in the muumuu driving the Humvee.