More and more urban areas are having 20mph roads but but the humps and chicanes are very expensive to install. Cyclists don't like using these roads as the humps interfere with the rhythm of cycling. It causes less noise pollution from vehicles accelerating and braking.
Allowing 20 MPH zones to be put in place without having to have speed bumps, chicanes and other physical hindrances to speed. 20 MPH signs would suffice. Speed cameras could be used if accidents happen with vehicles travelling faster than 20MPH.
These would be much cheaper to put in place. They would be more attractive to cyclists. There would be lees objections from locals to having a 20MPH zone.