Drivers have been warned not to rely on sat-navs to tell them speed limits on residential roads in Wales when it is reduced to 20mph from 30mph on Sunday.
Some GPS firms say map updates might not be immediate when Wales becomes the first UK nation to adopt a 20mph (32km/h) speed limit in built-up areas.
Police in Wales have said enforcing the lower limit will be a “last resort” as drivers adjust to driving slower.
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.