Replacing signals when a train is approaching

With AHBs there is usually a pair of strike in treadles at the extreme end of one track circuit, and either track occupied or one of the treadles down will initiate the sequence. Is there a block joint by your treadles?

" said:

Rather wide of the mark there, I'm afraid - an ABCL is rather a different beast, in operation, to an AHB, and only really suitable for relatively quiet roads

Which reminds me: what's the difference between an ABCL and an AOCL+B?

I think instead you have to install an ABCL (basically the same thing but with Obstacle Detection and ERTMS capable).

(with, IIRC, the barriers dropping as soon as the ambers change to reds) if the signal is SPADed.

Which reminds me: what's the difference between an ABCL and an AOCL+B?

Isn't it simply the case that an AOCL+B is an AOCL retrofitted with barriers, whereas an ABCL is one fitted with barriers from new?

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I think instead you have to install an ABCL (basically the same thing but with Obstacle Detection and ERTMS capable).

You're thinking of MCB-OD crossings which are indeed now being used where you would have otherwise used AHB's

MCB-OD's are full barrier crossings which are automatically initiated by approaching trains. They have obstacle detectors which will stop the lowering sequence if there is an obstacle detected and will automatically clear protecting signals when the barriers are proved down and the crossing is detected to be clear.

" said:

In the US and probably elsewhere they have traffic light preemption. This is an early warning from a level crossing (grade crossing) that a train is approaching and alters the traffic light sequence to release any trapped vehicles on the crossing. Not sure if that's done in the UK.

Bradford-on-Avon town council did a good one at Greenland Mills AHB. They narrowed the road in such a way that a car waiting on one side of the crossing would block the exit of a car coming the other way. It didn't last long.

It's called phased advanced. We have a hurry button on Crawley High Street CCTV on our panel 7 which when pressed causes the road traffic lights at the road junction immediately beyond the crossing to advance though the sequence to give the traffic coming over the crossing a green road traffic signal to clear the crossing. We also have a sealed key release to turn the road traffic lights off, only to be used in extreme circumstances of which we've never had to do that in the 13 years I've been at TBASC. I also believe Lancing has one for one of their CCTV crossings.

Getting back to N/S selectors we have one for Parsonage Road AHBs for trains calling at Littlehaven or not, set on "N" and the signal clears straight away set on "S" and there is a delayed clearance based on TC occupation.

"Rare as it is", the possibility of a track circuit failing to detect a train does exist hence the treadles. It's also not as rare occurrence as people may think, although blips are more common than complete failures, and the less the line is used the greater the risk.