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I am looking for a sensor that could be mounted on the front of a car, to tell the distance to the car ahead, to detect safe driving distance.

I was thinking of the possibility of a laser perhaps, but I am unsure of where to get a sensor like this, or if I am going to have to build my own.

What are some feasible and inexpensive ways/products to sense this distance?

Please note, I do not intend to break the rules by asking for specific products, I am just asking what should I search for.

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Pretty sure they use radar for adaptive cruise control in cars. Check this sensor out:

enter image description here

I've heard them called "headway sensors" or "radar headway sensors". There are other technologies like acoustic but I'm not sure if they'll have the range you're looking for. Laser could work but it's aim is pretty narrow, dust dirt and fog can get in the way etc.

Good Luck!

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    \$\begingroup\$ This is the best answer because Volvo (for instance) also uses radar as headway sensor, but it has a SWEET BONUS: it is considered a 'safety feature' of the car.. BUT since the radar will interfere with the cop's speed-gun, this 'feature' MUST be automatically turned off when cops are doing speed-checks.. But then your 'safety-feature' (that you 'depend' on) is momentary disabled and in most country's it is then mandatory (instead of prohibited) to give an auditable signal when the sensor is turned off and when it is turned on again. IN OTHER WORDS: LEGAL RADAR-DETECTION :-) fck the cops \$\endgroup\$ – zduk Oct 28 '12 at 22:48
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Search for "lidar". This is the technology that autonomous vehicles (e.g., DARPA Grand Challenge) use to detect obstacles around them.

For the narrower goal of detecting cars specifically — and just distance, not position — you might be able to take advantage of the fact that the red tail reflectors on automobiles are really good retroreflectors, and send a strong signal back towards the source it came from.

A fanned-out laser diode sending a code-modulated beam, along with a sensitive photodetector with a suitable code correlator, should be able to get good results without the expense of a scanning lidar.

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