The road begins in Gunnersbury at the Chiswick flyover (junction 1 of the M4), from which the South Circular Road (A205) heads south over Kew Bridge. The first section runs along Gunnersbury Avenue through Gunnersbury Park to Ealing Common, with a mix of single and dual carriageways, where it becomes Hanger Lane. The road crosses the Great Western Main Line west of Paddington to the Hanger Lane gyratory system, a large roundabout on top of the Western Avenue (the A40) with Hanger Lane tube station. This is one of the busiest junctions in London, incorporating 10,000 vehicles an hour.