You're missing the point. There likely isn't a problem at all. His perception that drivers all of a sudden are greater in number and more aggressive since he moved in is probably completely inaccurate.

It's definitely the case that most drivers gain a bit of speed going down a hill, but it's been that way forever and it's not something you're going to change. So if one of your main concerns is the speed of traffic outside, don't buy a house at the bottom of a freaking hill.