How Automatic Braking Systems Work

Danger, Will Robinson! Danger!

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.