I don't know, how much goes into transportation in the US? I mean, aside from the vehicles, including researching new tech for safety, advertising those features, law enforcement of safe driving laws, DMV money, roadwork, and whatever else goes into it. It seems like a lot to me.

Another thing to think about is that maybe we're hitting the safety cap in automobiles. Maybe the cost of making things more safe nowadays is just way too high. It could be that the cost of going after some terrorists is even higher, but we might not have been so sure at the time.

Also, the problem with ignoring people who set out to murder you is that it shows an unwillingness to do so. It might prompt other would be killers to attack. A car, on the other hand, isn't going to notice all the accidents happening, and choose to crash because of them.