Electronic signs comparing fuel prices at motorway services fail to reduce costs for motorists, according to an official report.
Forecourt prices at five rest stops along the M5 between Bristol and Exeter have been displayed since March 2016 in a trial aimed at promoting competition.
But a report by government-owned Highways England found there is no evidence that the introduction of the signs has reduced prices.
It would cost £50 million to roll out the scheme across the country and there is "no case" for doing this, the study concluded.
RAC fuel spokesman Simon Williams said: "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.
"The RAC does not know of any reason why buying fuel on the motorway should be on average 16p a litre more expensive than buying it elsewhere.
"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."
Highways England said the signs will be decommissioned and it is exploring options for their future use.