High Quality Content by WIKIPEDIA articles! The M4 bus lane was a controversial 3.5-mile (5.6 km) bus lane on the eastbound (London-bound) carriageway of the M4 motorway between Heathrow Airport and central London. It operated between junction 3 (A312) to the start of the elevated 2-lane section near Brentford. The lane, which had no intermediate junctions, was reserved for buses, coaches, motorbikes, emergency vehicles and licensed taxis but not mini-cabs.