[bird]

Because lasers.

Using laser sensors, the system effectively keeps a 360-degree vigil and will automatically and gently brake before a vehicle hits any stationary object.If you drive full throttle at a wall, Active Assist will stop mere inches away, without the driver touching the brakes.

While collision-avoidance systems are not new—many cruise-controls automatically slow down when you close too fast on the car in front—BMW’s sensors keep track of walls, columns, and other stationary objects, constantly redrawing a map of the surrounding area.

http://www.bloomberg.com/news/articles/2015-04-06/bmw-s-new-sensors-do-the-braking-for-you