Average speed cameras near Forth road bridge

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?