Thousands of speeding convictions could be overturned because the wrong font was used on signs along parts of the M42 near Birmingham.
The Crown Prosecution Service told Warwickshire Police in November 2012 that the numbers on the variable speed limit signs were too tall and narrow and may not have complied with traffic regulations.
As a result police stopped using the signs between junctions 3a and 7, and 7 to 9, as a means of enforcement and dropped prosecutions it was going to go ahead with relating to affected stretches of the motorway.
The Highways Agency said it believed they were the right size and were clearly visible.
In a statement it described it as a “minor discrepancy”, saying: “The signs were considered to meet the appropriate standards when they were installed and became operational.
“Following queries that suggested that this may not be the case, the Highways Agency clarified the issue and a specific authorisation was signed in November 2012.”