Over the years I've read of a number of projects that use vibration (haptics) to warn people of people rapidly approaching from the rear. They might use ultrasonic sensors. Note this was not on a motor vehicle, it was on foot.

People who only have one eye dont have depth perception so they have to estimate distances based on size. They might be able to use something like that. The eyes are better for objects in front.

In the rear I think this kind of thing makes a lot of sense but it requires something with more precision like microwaves or lidar. Which could be fairly expensive. The cost on all electronics is typically falling fairly fast (Moore's Law) so what isn't cost effective today may well be cost effective "tomorrow". (18 months, is the period for value to double in CPU value for dollar) Cost of high precision GPS and Networking bandwidth is also falling so it might also be possible to do mapping-related things with haptics. I think its quite valuable safety wise not to have to look at a GPS, for example, in navigation. (navigation by feeling)

For Electromechanical parts the rate is much slower.

If you include a GPS in this thing, think ahead and use one that can do raw data and 10 or 20 Hz. Because then the possibilities for accurate positioning improve greatly with the right software and antenna and a correction signal available.

Specific raw-data capable modules from ublox and skytraq (S1315f) are popular in apps like that. Check out http://gpspp.saku ra.ne.jp/indexe.html for more on that.