One of the issues is that posted speeds often don’t match the speed the road allows people to comfortably do. To bring speeds down, the things that are generally most effective are street narrowing, speed tables and humps (to bring to speeds safe in neighborhoods), and then obstacles like protected bike lanes or pedestrian refuge islands.

When a street is designed to allow someone to comfortably do 45 because it’s maybe 2 lanes in one direction instead of 1, but posted as a 25, people will speed on it.