Level Crossing Safety

Ah Yes! but the obstruction would not be able to place itself across the track when the train is close because the exit barriers would be blocking the entrance for the drive arounders but like johnb said, too much of a costly exercise. 20 seconds sounds rather late to lower the barriers for a 100mph train. I thought they had to lower three signal blocks in advance in case there was something wrong with the crossing to give the train sufficent time to stop. What would happen if there was a problem with the barriers.

On an AHB, the yellow light starts to show 25 seconds before a train travelling at maximum line spead would reach the crossing. There is no scope for stopping the train (other than by manually setting preceding signals to red if someone uses the emergency handset).

The idea is that there are enough failsafe systems (lights, sirens, barriers, etc) that if one of them should fail by chance when it's too late to stop the train, the others will still leave the crossing acceptably safe. And obviously if any of the safety systems do fail, this will be indicated to the signaller by the equipment and then steps can be taken (TSRs, proceeding with caution, etc) to reduce the risk.

While this doesn't achieve the same absolute levels of safety as a system where the crossing needs to be secure before the train is signalled onto it, it works in practice. As I understand it, there has never been an AHB accident caused by mechanical failure - every single one has been caused by driver error.

Correct.