I think they will eventually have it dialed in for highway use, but maybe never 100% in the city. It just goes to show how much information our brains take in, filter and process from one second to the next while driving.
Maybe something about the aliens knowing what is best for us, and that depopulation is the only way they think humans can survive.
I saw a couple people say rabies and I would have to agree with that one. Mostly because untreated rabies is nearly 100% deadly so any side effect of the shot for it are most likely less severe then that.