Numbers of deaths and crashes have significantly reduced since mobile speed cameras were introduced on the M4 motorway in Wiltshire, figures show.
The mobile speed cameras were brought into operation in April 2005
Wiltshire and Swindon Safety Camera Partnership (WSSCP) began enforcing the 70mph limit in the county last April.
Figures suggest that since then, there has been a 57.89% cut in the number of people killed or seriously injured.
But campaign group Safe Speed, which opposes the cameras, said there was insufficient data to justify the claim.
Spokesman Paul Smith said the assumptions should not be based on a single year's figures, which did not take into account national trends and local conditions.
"Analysis and interpretation of road accident statistics requires considerable care and scrupulous honesty because road accidents are rare and exceptional events," he said.
"It is necessary to separate random effects from real changes."
David Frampton, manager of the WSSCP admitted there was no specific evidence to show that the cameras had caused the reduction.
But he said: "Nothing else has changed on the motorway since they were introduced. There are no extra police patrols, only the cameras, and we have fewer collisions, so we have to draw some conclusions.
"When we started enforcing on the M4, some people said we would make the road more dangerous, these figures prove that this is not the case. The signs and cameras encourage motorists to slow down."
He added the partnership would continue to enforce the speed limit on the motorway.