The United Kingdom has a network of roads, of varied quality and capacity, totalling about 262,300 miles (422,100 km). Road distances are shown in miles or yards and UK speed limits are indicated in miles per hour (mph) or by the use of the national speed limit (NSL) symbol. Some vehicle categories have various lower maximum limits enforced by speed limiters. A unified numbering system is in place for Great Britain, whilst in Northern Ireland, there is no available explanation for the allocation of road numbers.