Think about it this way.

If it was you working in your Royal Mail van and some pikey smashed into you so hard it broke your back, would you want someone who had seen them driving like wankers to come forward as a witness to the state of their driving???
Or would you be happy for it to possibly just be put down to an unfortunate accident?
If you were potentially parylised, would it be of any consolation to know it wasnt your fault?

If your friend/son/partner/ whatever had been killed in a crash-would you not want to know why?

You havent done anything wrong so whats going to incriminate you?