Front crash prevention systems alert the driver when the vehicle is getting too close to one in front of it. Most systems precharge the brakes to maximize their effect if the driver responds to the warning. Some systems are able to brake the vehicle autonomously if the driver doesn't respond. These systems use various types of sensors, such as cameras, radar or light detection and ranging (LIDAR) to detect vehicles in front of the vehicle. A forward collision warning system with autonomous braking may not always be able to prevent a crash but may reduce vehicle speed, mitigating the severity of the crash.

More information at http://www.iihs.org/research/qanda/crash_avoidance.aspx