"If a level crossing were to be closed to road traffic (including pedestrians) for an extended period of time, the gates or barriers would need to cover the full road width, or the road barricaded in some other way. And emergency exists, if they are any, would also need to be locked. The warning bell would also need to be disconnected." - Myrtone
What Standard says this? What is defined as "an extended period of time"? Why do they need to cover the full road width? If the crossing is active no traffic should be able to enter it, and all traffic that was on it at the time of the activation should have cleared.
If you are talking about a crossing being closed for a special event, (such as track work) then traffic controllers would have to be called to place signage and cones around the crossing on both sides. Once traffic control was in place, a signal technician could then deactivate the crossing, at which point, the boom gates should fall to the horizontal position (unless secured upright) with no lights or alarms sounding until the crossing was reactivated.
Sometimes booms get held down because of equipment failures, train breakdowns, and incidents on trains. We had one occasion where a train had to be held at Gosnells station (WA) because a passenger was unwell and needed an ambulance. The crossing would have been active for at least 20 mins before the ambulance arrived and took the person to hospital, and the train was able to clear the crossing.
If a crossing is near a residential area and noise from the alarm is a concern, then timers can be set up so the alarm doesn't sound at certain times of the night. I believe this is the case on a few boom gate crossings between Berry and Bomaderry on the NSW South coast as the crossings are next to farm houses, and the booms were only recently installed (well recent being within the last 10 years) replacing passive crossing protection methods (Stop, Look for Trains signage)
Crossings can also be set up so that the bell only rings from activation to boom down. (Pitt Street crossing on the Midland Line in WA is an example of this).
"Today the Boom appears to be made of a plastic (Carbon Fibre?) which would perhaps bend more than the previous wood booms would and not break." - gordon_s1942
Although a failure for every scenario would be difficult to simulate, having the booms break if they are struck is sometimes the best thing that can happen, especially in the electrified area, as a bent boom could rise up and come into contact with the overhead.
On all crossings on the PTA network in WA, they all still have wooden booms, and, when crossing equipment is being renewed (2018) they still are being installed with wooden booms. I have seen the boom gates you are talking about outside the PTA territory
"I would suggest you dont try to use a level crossing in NSW where there is a double line as you will find that after a train passes one way, another can be approaching from the opposite direction on the other line so the booms could begin to rise only to be brought back down by the other train." - gordon_s1942
Can't comment on NSW practice, but in WA they have "approach" circuits on crossings with more than one track so the booms stay down, if they can't clear before another train arrives.