My city converted a lot of 4-lane roads to 2 traffic lanes, a centre-turn lane, and 2 wide bike lanes. It's great, cheap, and it really doesn't affect car traffic at all.
Anecdotally, where I live has been doing something similar, and it really depends on the specific road as to whether or not it's effective. One of the roads near me had this 'road diet' done to it, and the net result was that drivers actually ended up speeding more often on that road because they ended up driving faster to avoid driving next to cars in the narrowed lanes. Since the road diet occurred, I've basically not seen any cyclists using that road anymore because the car traffic has gotten more dangerous.
These are all on curving roads through residential neighbourhoods, which might have something to do with it. Roads that really did not need 4 lanes to begin with. Some areas near schools also have enforced 40 km/hr speed limits.
