Nottingham is the first city in the world to install the digital cameras

They track vehicles' average speed and then send a photograph of the offending cars to a police computer.

They can photograph the number plate of the vehicle, electronically check who owns it, and send out the penalty notice.

Other forces in the cash for cameras scheme have reported similar successes to Northamptonshire.

South Wales police said the number of speeding tickets had doubled.

Cleveland and Thames Valley police also reported increases.

All the money generated from the fines go to local councils and police authorities rather than to the Treasury.

Reduce road casualties

There has been some criticism that the cameras could be used to generate revenue for cash-strapped local services.

But police say the aim is not simply to raise money but to reduce road casualties and improve the quality of life in local communities.

Jacqui Elliott of the cameras' manufacturer, Speed Check, said the aim was to reduce casualties by one third over the two-year scheme.

"That means over 78 serious accidents could be avoided, 12 of which could have been fatalities.

"Motorists may need to change their driving behaviour to drive within the speed limit for the whole stretch of road."

Speeding drivers going past conventional cameras often escape unpunished because there is no film in the camera to record their offence.

The first set of cameras are being installed on the main link road from the M1 motorway at Junction 26 into the city.