Will 20 mph limits increase fuel consumption and carbon emissions?

NO!

This is a common myth. Driving style is the main factor which affects fuel consumption in built-up areas, not the numbers on the speed limit signs. The Department for Transport states driving more slowly at a steady pace will save fuel and reduce pollution, unless an unnecessarily low gear is used.

Accelerating only to 20 mph on a residential road, instead of up to 30 mph uses less fuel. Even when cruising at a steady speed What Car found that the slower you go with the vehicle running smoothly, the less fuel you will use.

Even the AA admit: “Targeted 20 mph speed limits in residential areas are popular and improve safety. Along shorter roads with junctions and roundabouts, limiting acceleration to up to 20 mph reduces fuel consumption.”

Short car journeys around town are the most inefficient and polluting use of cars. The engine does not have time to warm-up and the catalyst does not work properly when cold. Slower speeds on residential streets will make the roads safer and encourage more people to leave their car at home when they don’t need it and walk or cycle instead, meaning a 100% reduction in fuel consumption!