The Association of British Drivers (ABD) claims highway authorities by law have to ensure signs indicate where speed limits start and finish and remind drivers of those limits with repeater signs.
But it suggests that even if local authorities manage to install the signs correctly they often fail to maintain them properly, meaning they can become damaged or defaced, are obscured by foliage or simply disappear altogether.
And, it adds, if speed limit signs are not maintained in accordance with the regulations then the limits to which they relate are unenforceable.
It says police officers should check that all the necessary signs are present and correct before they begin enforcement, but often fail to do so. The ABD believes many drivers are being prosecuted when they shouldn't be and that most simply pay up because they are unaware of the legal requirement involving signs.
It has added a new page to its website setting out these requirements and it can accessed at www.abd.org.uk.
ABD roads and traffic spokesman, Mark McArthur-Christie, said: 'The ABD is receiving increasing numbers of requests from drivers for advice after they have been charged with exceeding a speed limit they didn't know was there.
'When they have complained to the police, they have been told to pay up or go to court and risk a bigger fine and more points. Our website will help drivers to check if the signs are correct before they obtain legal advice.'