P.S. I'm guessing you mean retainer rather than retension? I don't think that should apply in this sitation anyway, since that's usually used in a business context or for big balling ass muthafuckas or criminals who need legal services on a somewhat regular basis. In a personal injury context, you've either signed a contingent agreement or you haven't.

Also, Spanks comments would apply more to your damages rather than your bases for suit. You'd likely be going with general negligence unless you could show the driver was stoned or had a poor driving record the company knew of or something like that, then you'd probably also go allege gross negligence and/or malicious/wanton conduct that would allow you to claim punitive damages. Maybe a little negligent entrustment or something like that - it would depend on your jurisdiction.

Hope it goes well for you.