Just in case, like me you sometimes need to go across the Forth road bridge, watch out for the new average speed cameras. Recently I went over to Dunfermline and couldn't understand the need for these cameras as there didn't appear to be any good reason and no explanation.
http://www.scotsman.com/edinburgh-evening-news/transport/average_cameras_to_enforce_40mph_limit_at_forth_bridge_1_1985744?commentssort=1

Could it be that the speed restrictions are there to create lots of income by fining drivers who exceed the limit so they can pay for the bridge?