From Roader's Digest: The SABRE Wiki
The Transport alphabet appears on road signs in the United Kingdom, and is used for all text, except route numbers on motorway signs. It is available in two weights: Transport Medium is the standard weight, and is used for pale text on a dark background, while Transport Heavy is bold, and is used for dark text on a pale background.
The Transport alphabet was created in the 1960s by designers Jock Kinneir and Margaret Calvert, as part of a wider-ranging remit to design motorway signage.
The Motorway alphabet is used instead of Transport for route numbers on motorway signs.