> For example a well understood engineering concept is that narrow roads produce slower speeds and wider and more lanes produce faster speeds. Instead of putting up a sign that says "Maximum 30mph", which "requires" police to enforce, you could make the road so narrow that drivers only feel safe driving at 30mph.

We have an older bridge connecting Louisville, KY and Jeffersonville, IN. The speed limit is 35, the lanes are very narrow, and yet people still regularly go 50. I was hit while stopped at the end of that bridge in 2021 by a distracted driver, likely on her phone, who was going so fast, she wrecked into me and the 2 cars in front of me and her car did a 180 and ended up facing the opposite direction in opposing traffic.

IMO a key safety measure would be "if you are in an accident while on your phone, you lose your license for 12 months times the percent at fault.