Image caption Variable speed limit sign that did not meet the Department for Transport's design criteria

Thousands of motorway speeding convictions could be overturned because the font used to display the numbers on some variable speed limit signs may not have complied with traffic regulations.

The Crown Prosecution Service said the signs showed mph numbers taller and narrower than they should have been.

The signs are used to reduce congestion on busy stretches of motorway.

The CPS alerted Warwickshire Police last November to the irregularity of the signs on the M42 west of Coventry.

Some lawyers and traffic consultants now want any penalties that were handed out over the course of the six years the signs were in place to be quashed, arguing they are not legally enforceable.

The Highways Agency, which installed the signs, said it believed they were the right size and were clearly visible to motorists.

The signs were on two stretches of the M42, between junctions 3a and seven and junctions seven to nine.

Cases dropped

As a result, police took the decision to stop using the signs as a means of enforcement and dropped prosecutions they were intending to pursue on the stretches of road affected.

We felt it was quite clear what these numbers were and how visible they were to the road user Ginny Clarke, Highways Agency

However, by then thousands of motorists had already received fines and convictions built up since the first of the signs went into operation in 2006.

At least 11,000 fixed penalties were issued to motorists breaking the variable speed limit between junctions seven and nine of the M42 last year.

Richard Bentley, a traffic management consultant and former police officer, thinks it is only fair that previous speeding fines, points and driving bans be reconsidered.

"There should be a situation where cases are opened in the magistrates court to have the cases reheard and the convictions quashed," he said.

"If there are no traffic signs, the Act of Parliament prohibits the conviction - and these are definitely not traffic signs."

The regulations governing variable speed limit signs are set out in a government document called Traffic Signs Regulations and General Directions 2002.

Image caption Traffic consultant Richard Bentley says he believes cases should be reviewed

If a sign fails to comply with the document's specification then it needs separate approval from the Department for Transport before it can be used as a means of enforcement.

That did not happen in this instance.

'Fully illuminated'

The BBC has spoken to one solicitor who is challenging a conviction for a client caught speeding on the M42 last April.

"Unfortunately the client received a six-month disqualification from driving," said Hojol Uddin.

"But then the cases that came in a week later, they all got off, they were completely in the clear - no further action was taken - so I think it's not equal," he said.

Warwickshire Police admit there was an issue relating to the shape of the numerals in the signs but said the variable speed limit signs were fully illuminated and clearly showed what the correct speed limit should be.

But the force's camera enforcement unit manager Gary Hollis said, "Motorists who have already had their cases dealt with by the courts, conditional offers of fixed penalty or speed awareness courses are advised to take independent advice regarding how this affects their individual cases."

Remove uncertainty

The Highways Agency said it was first made aware of concerns about three years ago but believed they did conform to the regulations.

"When it was first brought to our attention in 2009, we felt it was quite clear what these numbers were and how visible they were to the road user," said Ginny Clarke, director of network services for the Highways Agency.

Despite that four months ago the Highway Agency applied to the Department for Transport seeking permission to use the signs in their current form.

In doing so it revealed that a number of similar signs had also been used on other variable speed zones including stretches on the M40, M6, M1, M20 and M25 although it is unclear what the issue was with these signs.

"We felt certain that it didn't make a difference, but because there was some uncertainty we wanted to remove any of the difficulties by having this additional authorisation," said Ms Clarke.

The Department for Transport has since granted authorisation for all the signs and Warwickshire Police says it started enforcing variable speed limits again on those sections of the M42 on 1 Jan 2013.

You hear the full report on 5 live Investigates on Sunday, 10 March, at 11:00 GMT on BBC 5 live.

Listen again via the 5 live website or by downloading the 5 live Investigates podcast.