You'll need an accelerometer at the front and probably another at the back of your "vehicle" to sense impact. A search of industrial accelerometers puts the cost well out of reach, so i checked instructables.com for a DIY version. Low and behold, high on the search list is a project using the Arduino and, what i hope would be, an inexpensive gyroscope (accelerometer) chip. It includes a downloadable sketch file too.
Here's the url for the project:http://www.instructables.com/id/ArduinoGyroscopeProcessing/
and the gyroscope (accelerometer) datasheet:http://www.alldatasheet.com/datasheet-pdf/pdf/272733/EPSONTOYOCOM/XV-8100CB.html
Looks like fun. Good luck
EDIT: Found this site which describes accelerometer chips in some detail:http://www.sparkfun.com/tutorials/167