This was tried on a fairly large scale in Milton Keynes, a new planned city in the UK founded in 1967. The traffic grid is high-speed (and roundabout-heavy), an

This was tried on a fairly large scale in Milton Keynes, a new planned city in the UK founded in 1967. The traffic grid is high-speed (and roundabout-heavy), and there was an extensive network of grade-separated pedestrian and cycle lanes. The results are... mixed. Some people love the place, but the grade separated cycle paths are not popular. Well worth looking at, if this is an area you're interested in.