MCB is Manually Controlled (full) Barriers. AHB is, as you say, Automatic Half Barriers - but, rather than as you describe, they only block the entrance side of the crossing, with no barrier at all on the exit side - which does give the opportunity for motorists to zig-zag around them, but more importantly gives an escape route for anyone trapped on the crossing - important because they're operated automatically by an approaching train, with no check that the crossing's clear, and no time to do much about it.