If you drive a vehicle in the areas affected then you will see new road signs and markings showing that a 20mph limit is in place. You should not drive faster than 20mph in these areas.
20mph limits will mainly be on residential streets, and in some selected locations on main roads.
New speed limits + community action + targeted enforcement = a safer city for everyone
The vast majority of main roads will keep their existing 30mph or 40mph speed limits, so once drivers leave residential areas and join the main roads there should be no impact on journey times.