Drivers have been warned not to rely on sat-navs for the speed limit on residential roads in Wales when it is reduced to 20mph from 30mph on Sunday (17 September).
Some GPS firms say map updates may not be immediate when Wales becomes the first UK nation to adopt 20mph in built-up areas.
The law will change the speed limit on about 35 per cent of Welsh roads where lamp-posts are no more than 200 yards (183m) apart.
Because it will be the new national speed limit, 20mph signs will no longer be needed on those roads – except when the limit changes.
Police in Wales have said enforcing the lower limit will be a “last resort” as drivers adjust to the change.
The RAC said motorists should follow road signs for the speed limit.
“Until sat-nav systems have been fully updated, they shouldn’t rely on them to know what the speed limit is on any particular stretch of Welsh road,” said Simon Williams, of the RAC.
“It’s vitally important that drivers are fully aware of the arrival of the 20mph limit in Wales, and pay full attention to all road signage.”
Of the almost 22,000 miles of road in Wales, an estimated 7,700 miles will change from 30mph to 20mph from Sunday.
“Our priority is to help people understand why they should slow down and the benefits to their community,” said South Wales Police Assistant Chief Constable Mark Travis.
“Where we find people driving above 20mph, we will stop drivers and speak to them about the dangers and risks about driving above the speed limit.
“We won’t be taking enforcement action against those people who are actually engaging with us and trying to bring their speed down – so they won’t receive a fine.
“The people who will be fined will be those driving at the highest levels. Ultimately, if necessary, we will enforce – but that is always a last resort for us.”