The most violent or traumatic method I can imagine would be parking on a steep hill- then placing the car in reverse and using a stick to hold the brake pedal in place. Using rope or chain to tie my neck/head to one fixed object, and my arms to another- with my legs chained to the vehicle. Then pulling on a rope attached to that stick to dislodge it from the break pedal. High strength piano wire would probably be better, as it would cause things to sever instead of being pulled apart.
I think it'd be pretty difficult to accomplish, though- with a high probability of being found and stopped.
Attaching high strength metal wire to a long chain, putting the wire around the neck, attaching the chain to a fixed object, routing that chain out the drivers seat window, and then stomping on the gas pedal would be quite violent as well, and likely result in decapitation.
I remember seeing an image on the internet a long time ago of someone who sawed themselves in half with a band saw.