Electronic signs that allow drivers to compare fuel prices at motorway service stations have failed to reduce costs, a study has concluded.
In a trial aimed at promoting competition, Highways England installed signs on the M5 southbound, displaying the real-time price of petrol and diesel for the next three motorway service areas along the route, together with the distance to the services.
Five motorway service stations between Bristol (junction 18) and Exeter (junction 30) were included in the trial.
However, a report has found no evidence of any savings.
Highways England said it would cost £50m to roll out the scheme across the country and there is "no case" for doing this.
RAC fuel spokesman Simon Williams commented: "We had hoped this trial would help to promote an element of competition among motorway services' fuel operators by overtly displaying prices to drivers, but sadly all it's proved is that motorway fuel is extremely expensive wherever you buy it.
"This has been a very costly trial that has unfortunately confirmed what we already knew. Motorway service area operators take advantage of drivers having to make 'distressed' fuel purchases."
Filling up on the motorway is on average 16p a litre more expensive than buying it elsewhere, the motoring organisation said.
"We urge the government to investigate fuel pricing at motorway services to make it fairer for every motorist who needs to fill up at one," Williams added.
Highways England plans to decommission the signs, and is exploring options for their future use.
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