Operators are MEANT to visually check the speed before targetting a vehicle as the primary evidence is infact the suspicion of speeding, the laser system is used to provide confirmation of this. Doesn't stop many vans being used incorrectly and every passing vehicle being checked.
As to direction, vans can target vehicles travelling in both directions, regardless of whether they are sited at the side of the road or on a bridge over it. The van situated on the Rogiet bridge just before the toll booths for the 2nd Severn crossing is nominally there to cover the 50 limit approaching the booths but does a good business catching people heading towards Bristol on the other side of the motorway.