oxon: speed limit changes

Just a warning to oxon members that the A417 between Hendred and Rowstock will be dropping from 50mph to 40mph in a weeks time, but the 30mph section will increase to 40mph making the section from Rowstock/Harwell roundabout up to the Hare ph at Hendred 40mph.

I expect this section will get rigerously inforced in the old 50mph section anyway, but the 30 to a 40 change is a good thing as this is what the limit use to be an a natural limit for that section.