The crossing is worked manually by a crossing keeper, so there's no (or minimal!) risk from vehicles breaking down on or otherwise becoming unable to leave the crossing despite the inability to see the other side.
Entirely correct, of course, and if Bob hadn't gone quite so far as to suggest abolishing
all crossings, it'd all sound a lot more reasonable. In fact, he'd probably have a stronger argument as far as his members' job security goes if he'd talked about (as has happened in some cases) replacing AHBs with CCTV crossings. Full-barrier crossings (those which provide jobs for Bob's memberships!) must be about the safest type of crossing, so any case for replacing these with a bridge would more likely be based upon reducing traffic congestion (like Tallington).
Going back up the thread a bit to fsmr's post - yes, CCTV crossings aren't generally monitored once 'crossing clear' has been pressed, and indeed in most cases the picture disappears once that's happened (though quite a few Signalmen do put the picture back on periodically if they're not busy elsewhere in the box). However, a fracture strip is (usually? always?) fitted to the barriers to indicate back to the box if a barrier is significantly damaged.
Just to clarify too following a couple of earlier posts, there should be no
need for a vehicle trapped on a crossing (perhaps because they'd driven onto the crossing with no clear exit) to drive through a barrier to escape danger. If the crossing isn't checked by someone before a train is allowed to approach (i.e. AHBs), there'll be no exit barrier. In all other cases (excepting UWCs etc. where the user should open both gates themselves), a trapped vehicle shouldn't be in danger as the Signalman (or Crossing Keeper)
should check the crossing first so wouldn't
need to drive through the barrier. I can think of only a couple of exceptions - those strange RC or MCBR crossings (for example,
Ashwell Gatehouse) where, after the barriers have been raised for a road user, they lower themselves after a short time, and there's no requirement for the Signalman to check that the crossing is clear (although a basic CCTV installation is provided at Ashwell now). However, clearly signed emergency plungers are provided which do (I think) raise one barrier for a short time to allow a vehicle to escape, and also sound an alarm in the signal box.