AHBC crossing question.

[King Lazy]

One for anyone with knowledge of the technical operation of Automatic Half Barrier Level Crossings.

A level crossing I regularly use of this type and one where I often observe trains pass appears to have a ‘quirk’ which I’ve never noticed anywhere else.

When observing the crossing operation, after the passage of a train, the barriers raising fully and the lights all extinguishing. There is a pause (clearly long enough that the crossing has completed its sequence) followed by a very brief single flash of all the red ‘wig wag’ lights.

The crossing also has traffic enforcement cameras facing it. They are tall, yellow masts with a variety of cameras, what look like infra red sources and sensors as well as the “speed camera” warning signs.

I have no idea how the cameras work in conjunction with the crossing activation or how they detect misuse.

Would it be possible (even if the odds of timing it such are huge) to set off then be caught by the camera apparently crossing through a red light?

I am certain the crossing is just completing a post-operation self test or similar and this would not be classed as a traffic offence.

But it is genuinely possible to see the lights extinguish then set off only to receive a brief flash of all the red lights without the yellow warning light operating first.

 

[alxndr]

I don't know how crossing enforcement cameras work, so can't comment on that.

The behaviour of the crossing does sound rather strange though. There is not usually any self-test feature built into the flasher which would explain this (although with it being the railway there are always oddities). Can you recall if the barriers are fully raised when the wigwags flash again?

 

[TSG]

I don't know how crossing enforcement cameras work, so can't comment on that.

The behaviour of the crossing does sound rather strange though. There is not usually any self-test feature built into the flasher which would explain this (although with it being the railway there are always oddities). Can you recall if the barriers are fully raised when the wigwags flash again?

I seem to remember seeing this where one of the barriers wasn't well damped and it 'bounced' a little at the end of its movement. There are a few failure scenarios where the reds can come straight on. For that reason, I am pretty sure the enable for the enforcement camera is proved against a full sequence, including the yellow lamps, otherwise it wouldn't be enforceable.

 

[LYradial]

There is a video on YouTube (crowhurst bridge level crossing) shows this happening, the lights extinguish before the barriers are fully raised, then one very brief flash of both lights as the barriers fully settle.

 

[alxndr]

I seem to remember seeing this where one of the barriers wasn't well damped and it 'bounced' a little at the end of its movement. There are a few failure scenarios where the reds can come straight on.

That was what I was trying to ascertain with my question. It's been at least 3 years since I worked on AHBs, and none of ours had enforcement cameras, but as I can recall the red lights will come straight back on if the barriers aren't raised between 81-90° after 7.5 seconds.

 

[King Lazy]

Yes, the barriers fully raise. The lights are extinguished and then there is a short (but considerable, in context -if that makes sense) gap before one single short flash of the red lights.

And I too would suspect the enforcement cameras would activate with the full sequence.

Edit- I’ve just viewed a video of the crossing in question. It is as LYradial states, regarding Crowhurst Bridge. The lights extinguish as the barriers raise and as the barriers reach the full extent of their upward travel there is a single flash of the red lights.

Edit 2. I also checked Crowhurst Bridge and reading the comments a few people used the term ‘tail ring’.

Searching for that threw up another crossing in Ceredigion that exhibited the same behaviour. It’s is clearly not unusual and most likely as TSG mentions relating to the damping at the end of barrier travel.

 

[alxndr]

Yes, the barriers fully raise. The lights are extinguished and then there is a short (but considerable, in context -if that makes sense) gap before one single short flash of the red lights.

Edit- I’ve just viewed a video of the crossing in question. It is as LYradial states, regarding Crowhurst Bridge. The lights extinguish as the barriers raise and as the barriers reach the full extent of their upward travel there is a single flash of the red lights.

Searching for that threw up another crossing in Ceredigion that exhibited the same behaviour. It’s is clearly not unusual and most likely as TSG mentions relating to the damping at the end of barrier travel.

From viewing this video of Crowhurst Bridge level crossing it actually appears that the boom closest to the camera is very slightly slow to rise, hence the momentary flash.

2:31 - the barriers start to rise
2:35 - the lights initially extinguish
2:38 - the lights re-light (notably ~7 seconds from the barriers starting to rise)
2:39 - the lights extinguish

If watched on 1/4 speed you can just see that the barrier nearest to the camera hasn't fully finished raising, and it appears that it may be slightly below the crucial 81° measurement at this point, although it's hard to tell for certain due to the camera angle.

A relay which takes 7.5 seconds to drop holds the relay controlling the red road light flasher unit up for the 7.5 seconds to give the barriers time to raise. Ideally the barriers would raise within this time period and the lights never reilluminate. If any of hte barriers don't make it in time then they will reilluminate as they must be proved to be above 81-90° to be considered "up" by the circuitry and extinguish the lights. The 7.5 second relay effectively just buys them time to get to that "up" position.

 

[King Lazy]

Thanks for the detailed reply. I’ve certainly learnt something!

I intend to time it next time I see the crossing operate and I’m sure it will match the 7 seconds you mention. I’ll have a look at the barrier position too.

My family have noticed this as well and it’s nice to have an answer for them!

 

[sprite]

From chatting with a signalling engineer :

The cameras sense the current going to the lights using the hall effect. Passing a red light for any reason unless directed by a traffic controller is an offence.

So the lights and camera are connected.

 

[Islineclear3_1]

So woe betide the suspecting motorist who happens to pass the wig-wags at the exact time when they flash and the camera takes the picture. The automatic camera isn't going to know that this is the end of the crossing sequence rather than the beginning

 

[LYradial]

So woe betide the suspecting motorist who happens to pass the wig-wags at the exact time when they flash and the camera takes the picture. The automatic camera isn't going to know that this is the end of the crossing sequence rather than the beginning

Not necessarily, at the start and end of the sequence both lights are illuminated simultaneously, so the camera should be able to detect that the lights are not flashing alternately.

 

[Islineclear3_1]

Not necessarily, at the start and end of the sequence both lights are illuminated simultaneously, so the camera should be able to detect that the lights are not flashing alternately.

I guess that depends on the shutter speed of the camera... I assume it has a fixed aperture