There is clear evidence that both the number and severity of road accidents increase markedly with speed

Transport Minister Sarah Boyack

The pilot schemes were shown to cut traffic speeds in more than half of the locations, reduced the number of accidents, and helped local residents feel safer, said Ms Boyack.

"There is clear evidence that both the number and severity of road accidents increase markedly with speed," she said.

"Slower speeds mean fewer and less severe injuries."

Compulsory 20mph speed limits use traffic calming measures to force drivers to slow down.

But the "advisory" limits rely on local traffic paying heed to them.

Researchers involved in the pilot project found the advisory limits led to an overall reduction in average speeds from 23.4 to 22.2 mph, and traffic speeds fell in 60% of the locations.

There was a fall in the accident rate, and serious or fatal accidents were reduced from 20% to 14% of the accident total.