Detecting sudden changes in velocity?

I'm working on a racing game where the vehicle is normally colliding with pretty much anything/everything in the level (ground, walls, jumps, etc). So I can't use normal collision detection methods to check if the vehicle has actually "crashed" and not safely glanced off of something.

I think the solution is to check for sudden changes in velocity, but I'm not sure how to do it. I'd like to check if the velocity falls below a specific minimum within a specific amount of time. Any ideas?

You could compare the magnitude of the velocity of the previous frame with the magnitude of the velocity of the current frame (or rather, the sqrMagnitude, which is faster):

private var oldVelocity : float;
var crashThreshold = 10.0;

function Start () {
    oldVelocity = rigidbody.velocity.sqrMagnitude;
    crashThreshold *= crashThreshold;   // So it works with sqrMagnitude
}

function FixedUpdate () {
    if (oldVelocity - rigidbody.velocity.sqrMagnitude > crashThreshold) {
        // Crash
    }
    oldVelocity = rigidbody.velocity.sqrMagnitude;
}