The blanket 20mph speed limit imposed on traffic in Portsmouth, one of two cities chosen in a government pilot scheme, is neither effective nor enforceable, residents said yesterday.

People questioned by The Times in Queen’s Road, where demonstrators once blocked the street after a spate of fatal accidents, pointed to an electronic speed detector erected by the council as an example of how the scheme was – or was not – working.

Its lights, designed to flicker into action whenever a vehicle breaches 20mph, remain dead, but not because drivers are paying attention to the limit. On the contrary: the sensor could not react to the cars haring past yesterday afternoon because a builder had inadvertently blocked it with a stack of metal containers.

The useless sign, which is meant to caution drivers as they approach the docks where HMS Victoryand the remnants of the Mary Rose are kept, is emblematic of residents’ attitude towards the new limits. Even those who supported the scheme did not believe that it would make a difference unless speed humps or police cameras were used to enforce it.

Michelle Geater, 33, a healthcare support worker living in the Paulsgrove area of the city, observed the scheme’s ineffectiveness first hand recently when a van travelling at 70mph in a 20mph zone smashed into the back of her parked car, propelling it through her the gates of her driveway.

“The 20mph sign didn’t make any difference,” she said. “He got in the van and put his foot to the floor. The gates were bolted and locked with a padlock, but they were ripped apart.”

A tree planted outside her front door was also taken away after a car uprooted it in a collision before driving off. “He left his bumper behind on the grass. The police asked me on the phone, ‘Did you get his numberplate?’ I said, ‘Yes. I’m holding it in my hand’.”

Signs will not be enough on their own, residents say, and the police do not have the manpower to enforce it.

Tom Nash, 54, a driving instructor, said that the blanket scheme would be less effective than restricting 20mph zones to specific areas such as schools. “They’re putting 20mph signs in places where you don’t need them. So people are just ignoring them.”

Alex Searsbrook, 64, a duty manager of the Royal Maritime Club in the Queen’s Road, said that those who did obey the lower limit were naturally careful drivers. “It works some of the time, but you stand here and you can still see them speeding – especially buses and taxis screaming down. If you really want to bring it down to 20mph, you have to put ramps in.”

He estimated that 30 per cent of drivers were ignoring the limit. “I think that people will get used to it once a few of them have been pulled over. They need a kick up the arse.”

A shopkeeper in the road – which links the historic dockyard to Unicorn Gate, where Navy court martials take place – said that he thought the new limits were pointless. The man, who declined to be named, said: “You need to educate people to cross the road properly. I talked to a policeman about the 20mph limit. He said it is impossible to enforce.”

Portsmouth City Council maintains that the scheme is a triumph. The 20mph zone was introduced to Queen’s Road after three fatalities in 2005, when a pair of pensioners were killed on their way to a bingo game and Darren King, a cyclist, was hit while riding his bicycle across a pedestrian crossing. Since the lower limit was imposed, accidents have fallen by almost two thirds, to eight.

The council said that average traffic speeds have dropped by 5mph on the road, although a study by the car magazine Auto Expresssuggested that drivers in 20mph zones elsewhere in the city have not been so compliant. A team armed with a speed gun recorded average speeds on three low-speed roads of 23mph, 27mph and 29mph.

Hull, another city at the forefront of the 20mph road safety strategy, recorded a halving of accidents in zones in trials as far back as 2002.

The statistics, put before Parliament in evidence to the Transport Select Committee that year, suggested that accidents resulting in death and serious injury dropped by 90 per cent, while accidents involving child pedestrians reduced by threequarters. The evidence became an important factor in the growth of 20mph zone usage.

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