The Department for Transport have collected statistics on the estimated lengths of all roads maintained at public expense in Great Britain since the early 1900's.
These roadways fall into three main categories:
Motorways
A-roads
Minor roads
Motorways can be distinguished by large volumes of traffic moving at high speed, multiple lanes, a lack of pedestrians and cyclists, a hard shoulder, and blue signage; A-roads may retain the high speed and similar levels of traffic, but the signage is green and there's (typically) only two lanes; the minor roads are everything else - distributor roads which have far lower traffic densities and get you from local streets onto main roads.
Scroll down to see how Britain's road network has grown in size since the early 1900's.