I'm not sure which tragedy of the commons you're referring to. Do you mean that people shouldn't replace an old car with a new car just because the new one is safer, because of e.g. the impact on the environment?
the fact that people are caught in the safety arms race (which really just means buying a larger, heavier car). a rational decision individually, but trashes up the city and makes it less safe for everybody else, especially pedestrians and cyclists.
