Speed limits painted on to the road are designed as reminders of the limit and should be placed in support of vertical signs displaying the speed limit bordered by a red circle. A speed limit is only valid where these signs are present and a valid Traffic Regulation Order is in place. However there are some exceptions, such as in national parks and Areas of Outstanding Natural Beauty where specific environmental protections mean that the placement of physical signs is limited and therefore signs painted on the road are used instead. Ultimately it is a matter of how both physical and painted signs are used and placed within a specific area, so while generally speaking painted signs do not by themselves constitute adequate and enforceable signage, this is not always the case.