"The main way to encourage cyclists on the roads is to reduce the threat from motor vehicles. The road should either be narrow enough that cars can’t pass cyclists, or wide enough that they can pass easily – the middle ground leads to collisions. Some cities are now experimenting with other options: Västra Hamngatan in central Gothenburg has recently introduced a street with smooth tarmac in the middle and cobbles on the sides. This encourages cyclists to take a central position in the road, preventing drivers from attempting dangerous passes, and also slows the cars down.
Park Belle-Vue in Leuven is planning something similar – a track with smooth bike lanes on either side and a raised cobbled section in the middle, meaning any drivers on the street will have one wheel on each section, encouraging them to drive slower and take more care."
Quote from: https://www.theguardian.com/cities/2017/jun/17/10-best-cycling-innovations-dockless-bikes-solid-tyres