The A40 is one of the two principal road routes from the West into London, the other being the A4.
Most of the A40 is known as the Western Avenue. It's generally a 6-lane dual carriageway (3 lanes each way) with grade-separation (no traffic lights) until Gypsy Corner in Acton. (There is also one set after this at Western Circus, then grade separation over the Westway flyover until [Lisson Grove]?.)
If you are driving into London (see Drive yourself) on the A40 then keep an eye on the speed limits since there are speed cameras at regular intervals.