The government's policy of painting roadside speed cameras yellow is to be challenged in the High Court on Thursday.
Speed cameras were pained yellow last year
Road safety groups are to ask the courts to order ministers to use the less conspicuous grey cameras instead.
They claim brightly painted cameras are less effective and only encourage drivers to slow down temporarily before speeding up again.
Ministers introduced yellow cameras last year. It followed concerns that grey cameras actually increased the risks of accidents because drivers tended to brake suddenly after seeing them.
The change in policy also aimed to deflect criticism that the cameras were more about raising money through speeding fines than about road safety.
However, campaigners believe a return to grey cameras will improve road safety.
They say less conspicuous cameras mean drivers do not know where they are and are, they argue, more likely to stay within the speed limit.
Lawyers for the groups will argue that the government acted unlawfully in restricting the use of an important law enforcement mechanism when it declared that fixed speed cameras should all be yellow.
They will suggest that those responsible for the cameras, including police and local authorities, should be allowed to decide themselves whether or not they should be clearly visible.
We hope the High Court will reverse this decision
Paige Mitchell, Slower Speeds Initiative
The campaigners are also calling for a study to find out which cameras are most likely to improve road safety.
Transport 2000, one of the groups taking the action, said ministers had decided to paint cameras yellow because of media pressure.
Vicky Cann, its assistant director, said: "The government has admitted that it made the decision to paint speed cameras yellow, not because it had evidence that yellow cameras were more effective in slowing drivers down, but because a few journalists were running scare stories about cameras trapping 'innocent motorists."
Paige Mitchell, of the Slower Speeds Initiative which is also involved in the action, said she hoped judges would rule in their favour.
"We hope the High Court will reverse this decision and allow the police discretion in enforcing speed limits. Let's have trials to see whether yellow or inconspicuous cameras are more effective."
Jenny Jones, the London mayor's road safety ambassador, criticised the yellow cameras.
"These yellow boxes are the starting flag for speeding elsewhere and in London. As soon as a driver goes past one they know they can put their foot down on the accelerator and won't be caught."
That view was echoed by Ian Roberts, professor of public health at the London School of Hygiene and Tropical Medicine.
"Painting cameras yellow without any research into the effect this could have on safety is typical of a government more concerned about keeping the road lobby happy than on preventing the 3500 deaths that occur on our roads each year," he said.