The A4 (Great Western Road) runs 127 miles from London (Holborn Circus) to Avonmouth. It heads west out of London through Slough, Reading, Newbury, Marlborough, Bath and Bristol. Today it is paralleled by the M4.
Wikipedia: Link
By my reckoning, on the extreme left of this image Basher has captured the steps up to number 46, from which address Lord Lucan disappeared 40 years ago today.