The majority of vehicles break the speed limit in new 20mph zones in residential areas, a Government commissioned study has found.
Just 47% of motorists comply with the limit on roads near houses, rising to 65% in city centres.
Although a “substantial proportion” of people are speeding in those areas, there has been an increase in drivers travelling at less than 24mph, which suggests “faster drivers have slowed down”.
The study also found there is “insufficient evidence” that the lower speed limit has led to a significant reduction in crashes and casualties in residential areas.
Journey times are estimated to have increased by 3% in residential areas and 5% in city centres, adding less than a minute to a five-mile trip.
The Government commissioned consultancy firm Atkins to examine vehicle speeds in 12 areas where the limit was reduced from 30mph to 20mph.
There has been a substantial growth in the number of 20mph limits following the Department for Transport’s decision in 2013 to encourage local authorities to consider the measure in a bid to reduce casualties and boost walking and cycling.
AA president Edmund King said speed limits must reflect the nature of roads so drivers can “easily understand why the limit is set”.
He went on: “We need more variable speed limits linked to time of day. For example, in the USA most drivers slow down outside schools with flashing yellow lights but not at 3am when there are no children around.
“The research suggests blanket 20mph zones dilute the speed limit’s effectiveness and compliance.”
Joshua Harris, director of campaigns at road safety charity Brake, said: “Breaking the speed limit is breaking the law and those who do so should be punished.
“We must make a success of 20mph limits but to do so we need more enforcement which is delivered consistently across the country.”