A disaster of the commons is that if people are in smaller vehicles and getting a car accident there is less likely to be injuries or fatalities. These vehicles also require significantly less fuel per mile. However, for any given individual, physics shows that being in the larger and heavier vehicle increases your likelihood of survival and reduces your odds of injury.
To address this tragedy of the Commons, the best approach would be to have an exponentially rising tax rate per pound of vehicle. Then people would have a strong incentive to choose smaller vehicles rather than larger. You also could create a law that puts more fault in any car accident on someone in the larger vehicle. These two policies would combine to mean that only the safest drivers would be hired to drive big semis and any private individual driving large vehicle would be much more careful on the roads. These policies would both reduce fatalities and decrease emissions.