The A408 through the village of Sipson is one of the few roads in the area of Heathrow Airport that still follow their pre-airport route (although West Drayton has been bypassed). Make the best of it. When the new runway is built then it is likely that this end of the A408 will disappear.
Heathrow - Uxbridge
We start on the A4 Bath Road just to the east of M4 Junction 4A, heading north past airport hotels, etc., before cutting left and crossing the M4 Heathrow Spur and passing through the village of Sipson. Not being overly familiar with the village, I did a Google search and, apart from the usual list of estate agents and sites offering dates in Sipson, the highest ranked pages said things like Sipson is a place in London, England - nearest places are Heathrow, Harmondsworth and Harlington. I couldn't think of anything more to say either, so we'll move on.
North of the village, we reach a roundabout with the A3044. Not a road number that jumped to my attention as a famous road, until I realised it went through Harmondsworth (past the Animal Hospital immortalised by Rolf Harris) and then forms the western boundary of Heathrow (not the Western Perimeter Road. But I digress. At the roundabout north of Sipson, we turn right (the original road continued ahead, now severed by the M4, and through West Drayton) and pass under the M4 before we reach a hamburger roundabout immediately north of Junction 4. There is a short link road to the M4 and beyond that the Heathrow Spur itself.
Turning left at the hamburger (the A408 link to the M4 forms the burger), the road is dual carriageway, and you might be forgiven for thinking that this route provides a good cut through to the A40/M40 corridor. If that is your objective then the A312 heading north from M4 junction 3 is likely to be a better bet. Nevertheless, the A408 begins with good promise. We bypass West Drayton, running parallel to and just to the west of the Heathrow Express railway which emerges from tunnel in time to curve to the east to meet the Great Western mainline, which we soon cross.
There's even a grade separated junction, leading to some industrial and business parks, then we cross Stockley Park Golf Course, and begin to curve round to the west. But the promising start fades away now, as we pass a couple of roundabouts, losing dual carriageway status in the process, and passing into suburbia. We pass along Falling Lane then turn right into High Street, passing some more business units before reverting to the usual mix of residential, schools and small shops etc. as we pass through Cowley. There's a mini roundabout with the B470 (the B road that runs parallel to the M4 a few miles west of here), then we continue north, terminating on a one-way system in the middle of Uxbridge.
My memories of the route are sketchy - it's a while since I've driven it, but I recall it being very busy and congested with school traffic (it was about 3:15pm). It's not a bad drive as it goes, passing through leafy suburban streets, but for people who need to make progress, the A312 would be a better bet.