it can receive the radar frequency and automatically do brake, but it will be TOTALLY illegal for automobile companies to add it to their products it can be a small hack to the car, but a small hack to the radar can detect the receiver.
i have this project idea of making a radar ( or laser ) based braking system in cars.
the transmitter transmits these waves which when detects any obstacle within a critical distance alarms the driver and applies automatic brakes to a safer speed or halt.
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you can go for a proto-type,and we may find its application in other feild,say in controlling a remote machine,anyhow i will look into my resources i will suggest you u shud study the rovers and remote controlled cars tat will help u in a vague idea,then u just have change the frequency of the signal & you will reach your objective!