Police tried to warn speeding motorists in Barton, Oxford, only to discover their speed indicator device was not working.

They took the equipment to Bayswater Road at the junction with Wayneflete Road yesterday and Monday in an attempt to catch drivers in the act. But on both days battery problems meant officers were unable to tell if drivers were breaking the law.

Police said it still had the desired impact and motorists stuck to the limit when they saw the officers.

The road has a 30mph limit coming down the hill from the Oxford Crematorium, but changes to 20mph once on the estate.

Sgt Jim Holmes, from the Barton neighbourhood team, said: “We realised the machine wasn’t working, but it had much the same effect, all the cars slowed down.

“You go from rural to urban straight away and as soon as you cross the bridge there is the junction to Wayneflete Road and a pedestrian crossing which a lot of people use to take their kids to school.”

Sgt Holmes said the device would be fixed very soon.

And he said that the police presence in the road was so effective that officers planned to make the operation part of their regular duties.