The M4 Airport Spur links the M4 to the central area of Heathrow Airport. It is unusual as it heads away from the M4 in two directions. When motorway numbers were being planned, the spur (being more than a mile long) was given its own number in several schemes (M44 and M30 being the main choices), however it was constructed and opened as a spur of the M4. Driver Location Signs give chainages prefixed with AS and then a figure in hectometres (tenths of kilometres), rather than the standard kilometres.tenths format.
The spur was the first motorway in the UK to gain a bus lane.