It seems to me that rate of car accidents would reflect only minor influence from the properties of the car: anti-lock brakes, bigger mirrors(?), etc. Things that make people think of a safe car - seat belts, air bags, etc. - serve only to prevent physical harm, not accidents. To examine the effect of volvos, you could explore accident rates for drivers on their previous cars vs. their current, comparing the transition values.
It seems to me that driver fatality is a better example than accident frequency. Driver fatality is something a Volvo would actually mitigate, and which would correlate with cautious driving.