Roadside speed indicators designed to persuade drivers to slow down are being widely ignored, a study has found.

Hailed by motoring groups as the driver-friendly alternative to cameras, they have become increasingly popular with councils and police and are used on more than 1,000 roads. But the radar devices, which display each vehicle’s speed as it passes with either a smiling or frowning face, are said to have only a “novelty effect” on drivers. Motorists slow down for the first week then quickly resume their bad habits. After three weeks, speeding is just as bad as before – and in half the sites tested even more drivers broke the speed limit when the devices were removed than did so in the period before they were installed.

The study, by the Transport Research Laboratory (TRL), makes awkward reading for police forces and local authorities that have chosen to use the devices as a cheaper alternative to speed cameras and radar guns.

Forces often encounter fierce criticism from drivers when installing speed cameras but hardly ever receive complaints about devices that merely remind drivers of their speed and do not result in fines or penalty points. A speed indicator device costs about £3,000, whereas a Gatso camera costs more than £20,000.

TRL was commissioned by Transport for London to test the effectiveness of the devices. It chose ten sites on roads with 30mph limits in South London, Surrey and Kent and measured speeds before, during and after the devices were installed.

It found that the average speed of vehicles passing the sites fell by 1.9mph in the first week, but that the impact then began to wear off. During the second week, the reduction in average speed was only 1.6mph, and this continued to deteriorate in the third week.

The study also found that, even in the first week, drivers accelerated back over the limit within a few seconds of passing the device. Measurements taken 400m farther down the road found that some drivers broke the limit by an even wider margin. Overall, the study found that “any effect after 400m is likely to be negligible”.

Some authorities, such as Kingston upon Thames, acknowledge the limited impact of the devices and move them to new sites after three weeks.

Robert Gifford, the director of the Parliamentary Advisory Council for Transport Safety, said: “This report underlines that these devices have only a limited effect and are not enough to protect communities from dangerous drivers. They have a short-term effect but it would be a mistake to think that they alone can solve a speeding problem. Other measures, such as road humps and camera enforcement, are needed to bring down speed in the longer term.”

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