The beam is set at the bridge's height and anything that breaks it is too tall. A second sensor measures every vehicle's height as it approaches. Twice a month the beam breaks for a vehicle the second sensor puts at four hundred and ninety millimetres of clearance.
I am a highways technician and over height detection is one of my systems.
How the installation works, because there are two independent measurements and that is the point.
At the approach to a low structure there is an infrared beam across the carriageway, set fifty millimetres below the signed height. A vehicle that breaks it is going to strike the bridge. Breaking it triggers a flashing sign telling the driver to stop, and it sends an alarm to the control room.
Separately, forty metres back, there is a profile sensor. It is a laser curtain that measures the height of every vehicle passing, continuously, and logs it, and that data exists so the highway authority can see how many over height vehicles use a route.
So every alarm has a measurement in front of it. The beam says too tall and the profiler says how tall.
The bridge on this approach is signed at four point four metres and the beam is at four point three five.
Twice a month, roughly, the beam breaks for a vehicle the profiler measured at three point eight six.
Three point eight six under a four point four bridge is half a metre of daylight.
The profiler is accurate to a few centimetres and it is calibrated with a target on a pole, which I do myself.
What happens on the road when it fires.
The sign lights and the driver stops, or does not, and either way the control room takes a call.
Nobody has ever hit that bridge. Not once in the eleven years the system has been in, and the bridge's own strike record, which the railway keeps because it is their bridge, is clean since 1998.
What has been checked.
The beam alignment. Checked to a target, re-set twice, and the transmitter and receiver both replaced in 2021.
Birds and debris. This is the honest common cause: a bird flying through the beam or a bin bag off a lorry gives you a false trigger, and it is why the system requires the beam to be broken for a minimum period before it alarms.
That minimum is a hundred and twenty milliseconds, which at motorway speeds is a solid object several metres long.
The correlation. Every alarm is matched against the profiler by time, and every one of these has a vehicle passing at that moment. It is not firing at an empty carriageway. There is a lorry there, and the lorry is three point eight six metres tall.
What the profiler shows on those passes, which I only looked at properly in 2023.
The profiler does not give you one number. It gives you a profile, a height against distance along the vehicle, sampled as it goes past, and the log keeps the whole thing.
On a normal lorry it is a shape you would recognise: the cab, a dip, then the trailer flat at its height for twelve or thirteen metres, then nothing.
On these passes the profile is a normal lorry, flat at three point eight six, and then at about eleven metres along the trailer it goes up.
Not a spike. A rise over about a metre and a half to four point four one, held for two metres, and back down.
Four point four one is above the beam and above the bridge.
What that means and what I have done with it.
Something on the back of those trailers stands half a metre proud of them for two metres of their length, and the beam sees it, and the bridge does not.
I raised that in 2023 with the structures team. Their reply is the only sensible one available: if a vehicle were carrying something at four point four one it would have struck a four point four metre bridge, and the bridge is unmarked, and therefore the profile is an artefact.
I have asked for the profiler to be replaced on that basis and I have been told, correctly, that a profiler which is right about every other vehicle on that road is not a profiler you replace.
What we do.
The system stays as it is. It alarms, the sign lights, the control room logs it as a probable false trigger, and the traffic goes under the bridge.
I have stopped calling them false triggers on my own paperwork. I call them unexplained beam breaks, which took an email to defend and which is what they are.
What I do.
The site visit is quarterly and it is a lay-by, a cabinet and a walk up the verge with a target on a pole.
The change is that I do the calibration facing the traffic now.
You set the target under the beam and walk it up until the receiver drops out, and you do that with your back to the carriageway because the target has to be square to the beam, and there is a barrier between you and the road and it is perfectly safe.
In June 2023 I was doing exactly that at about eleven in the morning with a lorry coming, which is background noise on that job, and the beam dropped out while my target was well below it and the sign lit up behind me.
I turned round in time to watch the lorry go under the bridge with nothing on the back of it at all: a flatbed trailer, empty, strapped, and the straps lying flat.