This is a good point to bring up but kind of lands in a different place than the GP's argument. Cars aren't boosted because a rational analysis has determined

This is a good point to bring up but kind of lands in a different place than the GP's argument. Cars aren't boosted because a rational analysis has determined that the negative externalities are worth it; rather, cars are boosted despite the evidence that the measures you mention are cost-effective ways to save lives. The continued subsidy of cars is proof that emotion trumps reason.