The fastest road in England is also the highest (630m), dropping to around 300m in about two miles. It is dead straight most of the way except for a wiggle at the top. Visability is excellent though, and if you do it... please wait for a car to start the descent... then scream past it. Be sure to look at the driver's face, and grin!
It maintains a steady 17% gradient, and faces north westerly pretty much dead on. Under normal weather conditions (ie, south westerly), a typical speed is in the low 50's. If the wind is from the west/north or east, this speed can drop to the high 40's (thankfully, as the road is also in the windiest part of Europe, winds from these directions are rare). The ideal direction, then, is due south, and as strong as possible. This happens only a few times a year, so glue yourself to a forecast, just waiting for the right conditions.
I have achieved 65+mph on about three occasions and 61mph more frequently, 50-something can be achieved almost any time.
Where is this road... well...
Get a map of the north of England. Find the road linking Penrith to Alston to Durham, the A689.
Find a village called St Johns Chapel, which is about two-thirds of the way from Durham to Alston, after a place called Stanhope.
The fast road itself jons this road (A689) in the Wear Valley to a road beneath in the Tees Valley at a place called Langdon Beck. The Tees Valley road starts near Barnard Castle, runs through Middleton in Teesdale, and eventually reaches Alston (it is a B11276 or something like that).
So, I advise an approach from St John's Chapel, up the hill towards the Valley to the south. Once at the summit (about 20 minutes), turn around, wait for a car, and go for it.
You should approach 50mph by the first bend (about 300m away) if the wind is favourable. You will know if the wind is favourable... because you will hardly be able to stand at the summit. The second bend can be taken on the other side of the road at about 55+mph, then let the hill coupled with the tailwind do the rest.
Not many people know about this road.