See, the thing is, those roads that were 45mph have more than enough room for both cars and bike/ped infrastructure. The issue is that, generally, the lanes for higher speed roads are wider. Narrow the lanes and people feel they have to go slower to stay in the lines (which is true for a lot of drivers, bless their hearts). It's an unconscious response to narrowing roadways that can be used to actually make people slow down, rather than just telling them not to. And when you narrow the lanes, that leaves more room in the same space for stuff like bike lanes and sidewalks.