The Security Service (MI5) was based at 35 Cromwell Road from 1929 to 1934.[3]
Cromwell Road was not always the main traffic route it is now, as when it was built, it ended at Earl's Court. The Cromwell Road extension, across the West London railway line and towards Hammersmith, was authorised as a bridge across the railway in 1884 but completed only in 1941. Thus, it was only after the Second World War that it became the main A4 route into London. The large traffic increase brought much demolition and road rearrangement beyond Earls Court Road in 1967 to 1972, but the main part of Cromwell Road has not had its basic building line changed.
The Cromwell Road Extension was a mid-late 1950s roadlaying program involving widening and demolition from Hammersmith Broadway to the Great West Road at Chiswick Roundabout as part of the A4 route for Heathrow and the West. The Hammersmith flyover(1961), avoiding Hammersmith Broadway, complemented these works which eventually fed the M4 Motorway (1965) . Some of the streets widened by demolition in Chiswick to form the route have retained their former names. [4][5]