Every legend a gantry can show is a numbered entry in a library, and the control room can only send a number from that list. Gantry 12 shows something once a month, and the log records a command with a number the library does not contain.
I am a control room operator on a regional motorway network. Eleven years.
How a sign gets on a gantry.
There is a library of legends, each with a number: speed limits, lane closures, a small set of warnings and a handful of national messages. The operator selects the legend by number and the system sends it to the gantry.
You cannot type free text. There is no free text field, and that is deliberate, because a signal over a live carriageway is a legal instruction and a mistyped one has killed people.
The gantry acknowledges the command and reports back what it is displaying, and both go in the log with an operator identity and a timestamp.
Roadside cameras look at the gantries, because a sign that does not match the command is a safety fault.
Gantry 12 on the northbound displays something about once a month between two and four in the morning.
The log records a set command with an operator identity of SYSTEM and a legend number that is not in the library.
The library numbers run in ranges, with gaps between them, and every number in use is in a document I have on my desk.
The number in these commands is a five digit one in a range nothing uses.
The gantry acknowledges it and reports that it is displaying it, and the camera confirms the gantry is lit.
What is on it.
The camera looking at gantry 12 is a low resolution one meant for confirming that a sign is present, not for reading it, and at night an illuminated matrix blooms.
You can see that it is lit. You can see roughly where the lit lamps are and that they form characters across the top row rather than the big red ring of a speed limit.
Nobody in the control room can read it off that camera and I have tried with the picture blown up on three screens.
What has been done about it.
The gantry has had its controller replaced twice, in 2019 and 2022.
The library has been reloaded onto it from the central system on both occasions.
The comms were replaced when the section went to a new network in 2021.
And in 2022 a maintenance crew went out under a closure at three in the morning to sit and watch it, at my request, which took me four months to get approved.
It did it that night at ten past three. The two of them were parked on the hard shoulder about two hundred metres back with a camera.
Their report says the gantry displayed a legend for approximately ninety seconds and that neither of them could read it. One of them wrote that the characters were of the correct height and pitch for the matrix and that they were not English.
He is a signals technician with eighteen years in and he has read ten thousand gantries, and that sentence is in a company report with his name on it.
The photographs they took show a lit gantry with a bloom across the top row.
What happens on the day.
It clears itself after about ninety seconds and the gantry reports blank, and the log records a clear command from SYSTEM.
Nothing else on the network does anything. The speed limits either side are unaffected, no lanes are closed, and the traffic underneath it, which at that hour is about four vehicles a minute, does not slow down.
Not one driver has ever reported it. That is in its own way the thing I find least comfortable, because a strange sign over a motorway at three in the morning is exactly the sort of thing people ring in about, and the incident line takes calls all night about far less.
What we do.
It is on the fault register as an intermittent unknown legend, with the log entries attached, and the standing instruction for operators is to record it and take no action.
I wrote that standing instruction. The reason for taking no action is sound: the sign is not instructing anybody to do anything dangerous, and setting a legend of our own over the top of it would put a real instruction on a carriageway to solve a cosmetic problem.
What I do.
My desk covers that section on the night shift and I have logged perhaps forty of them.
The one thing I changed is that I stopped putting the gantry 12 camera up on the wall.
The control room has a video wall and you can throw any camera onto it, and for a while I put that one up when the log flagged the command, because I wanted somebody else in the room to see it.
In 2023 I did that at about twenty past three with two colleagues in and one of them looked at it for a while and asked me whether the camera was on the right gantry, because the sign in the picture was the wrong way round.
It was on the right gantry. The bloom on that image is the far side of the matrix, which faces away from us, and a gantry can only be lit on the face that is signalling, and the face that was signalling was the one pointing up the carriageway at nothing coming.