I happened to drive that road today. If you are heading south on Rolling and go straight through the Alban Rd/Pohick Rd intersection, Rolling Rd turns into Pohick. The speed limit was 35 mph for the longest time BUT it is not any longer.
Just south of the intersection there is a 40 mph speed limit sign and just before the I-95 overpass there is a yellow warning sign indicating that the speed limit decreases to 35 mph on the other side of Lorton Station Road.
If you got a ticket after the speed limit changed, just go to court with pictures of the signs - which clearly show the road - and the judge will amend the speed. It is still speeding though.
If you look here
http://lortonvalleystar.com/ you will see that the "Lorton Valley Star" posted an article saying VDOT was increasing the speed limit. I'm not sure when the article was posted, but scroll down to the article titled "Speed Trap Limit Increasing."