Your wife had every reason to be angry at the driver - who among us would not have felt the same way?

>But I also can’t believe other cars just drove by and watched this man physically threatening a young woman.

The moral of the story is that if you choose to engage with a reckless driver and potentially escalate the situation, assume that no one will be coming to your assistance if it turns violent. You can chalk it up to bystander apathy, cowardice, minding one's own business, or whatever you want. Based on news reports that I've read, I think it is a healthy self-preservation interest at work. Sometimes people try to get in the middle of a conflict between a couple, with an eye toward protecting the woman in the couple. The male partner becomes enraged at the interloper and a fight breaks out. And then, later, instead of being appreciative toward the good Samaritan, the woman tells the police that he is the bad guy and that the couple was just having a spirited disagreement. I'm not saying that happens all the time, but it happens enough that you can foresee it as a possible outcome that would make you wish you had not gotten involved.