Roads in the United Kingdom form a network of varied quality and capacity. 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. Enforcement of UK road speed limits increasingly uses speed guns, automated in-vehicle systems and automated roadside traffic cameras. 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.