I want to make a safety sensor to let me know when a car is following to close for a give speed. I can get my speed from gps, but I cant find and ranging radar. are there any on the market?
Look for TOF sensors (Time of Flight)
Not sure if the limitations in an open environment,it you should find something.
Drill 2 holes in the back of the car and mount an ultrasonic distance sensor?
Have you considered COTS (Commercial Off The Shelf) radar units? There are many available. Here is a link to a company that specializes in this area: "Continental Automotive - Short Range Radar",
That link says: "SRR520 is a high performance 77GHz short range radar sensor for various premium backward & forward looking applications. Its small package can be integrated easy and robust behind all kinds of painted bumpers.".
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What exactly are going going to do with this information once you have it?
Any TOF sensor is going to be limited to vehicles directly behind you. So, they will be fooled as soon as you go around any type of curve at all. Also, they will change depending on the conditions outside. Whether it's sunny or cloudy will make a big difference in your readings. As well as the color and reflectance of the vehicle behind you.
It says that it is good for short range like for blind spot detection. Once someone is that close behind you, I highly doubt you'd need a sensor to tell that they're too close. I doubt it would work for something multiple car lengths away.
Hi,
Forget Ultrasonics, any major air movement that causes a change in air pressure will act as an acoustic mirror and give false readings.
Found this out with Halloween prop that triggered when people approached, it was mounted outside ad the wind would trigger it.
Tom.. 
I'm not saying an ultrasonic sensor would be the best choice for the OP, but color me skeptical about a breeze triggering an ultrasonic sensor, unless the wind is causing something in the sensor's FOV to move. Or the code didn't trap the ubiquitous occasional glitch that ultrasonic sensors are known for. Or a goblin did it. 
Ultrasonic sensors are used on UAVs for terrain following and collision avoidance, so "wind" (relative wind and rapid air movement and pressure changes from rotor wash) in that application isn't a factor (which seems more demanding than wind in a normal residential outdoor setting).
LIDAR.
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Quick glance in the rear view mirror works for me.
Come to think of it, has done for millions for quite some time.
The trouble with knick knacks (such as what you are proposing) in a motor vehicle, is they draw ones attention away from the most important part, concentrating on driving the vehicle.
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The OP would also have to make sure HIS radar did not interfere with the radar in the car behind him and cause a rear-end collision.
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