Road marking brightness is measured with a retroreflectometer in millicandelas and it only ever goes down, because the glass beads that do the work get worn out of the paint. Two hundred metres of ours reads three hundred and ten, and new paint reads two fifty.
I am a highways inspector and road markings are part of my round.
How the measurement works, because the direction of it matters more than the number.
A retroreflectometer is a box you stand on the line. It shines light at it at the angle a headlamp would, measures what comes back at the angle your eye would, and gives you a figure in millicandelas per square metre per lux.
New thermoplastic with beads in it reads between two hundred and two hundred and fifty. The specification for a motorway edge line is a hundred and fifty as a maintenance level, and below a hundred it gets programmed for replacement.
That figure only goes down. The retroreflection comes from glass beads dropped into the hot material, and traffic and grit wear them out and pull them out, and no process on a road puts them back.
The whole discipline of marking maintenance is watching a number fall and deciding when to act.
Two hundred metres of lane one edge line on the southbound reads three hundred and ten.
It was last laid in 2016 with the resurfacing. Everything either side of it reads between a hundred and thirty and a hundred and sixty, which is what nine years of traffic does.
Three hundred and ten is higher than that material has ever read, including on the day it went down.
I have the laying records. The site tests in 2016 for that whole section came in between two hundred and twenty and two hundred and forty.
What has been checked.
The instrument. Two of them, both calibrated against the reference tile that lives in the case, before and after every reading. I have taken four different meters over that stretch including one borrowed from another authority.
The surface. It has not been repainted. Marking work is programmed, procured and recorded, and there is no order against that chainage since 2016, and the marking contractor confirmed that in writing when I asked them whether anybody had done anything there off the books.
The material. You can see the difference between old and new thermoplastic with your eyes, at the joint, and there is no joint. The line runs continuously through those two hundred metres and out the other side, one pour, with the same edge and the same wear pattern as the rest of it.
What I did in 2023, which is the thing I go back to.
I took a core.
Not of the line, which you cannot really core, but a sample of the material lifted with a chisel in three places, with permission, under a closure: one in the bright section, one fifty metres before it, one fifty metres after.
They went to a laboratory that does exactly this.
The report says all three samples are the same product, consistent with the 2016 specification. It says the binder is aged to a comparable degree in all three, which is what you would expect of material laid on the same day and exposed for the same time.
And it says the bright sample has a bead retention of eighty one percent against nineteen and twenty two percent in the other two.
Eighty one percent bead retention in nine year old motorway marking is a number that does not exist. New material out of the machine is about eighty five.
The beads in that two hundred metres are still in it, unworn, under traffic that has taken the others out.
What happens about it.
Nothing has to happen and that is the whole reason this has never been a problem. A line that is too bright is not a defect. It passes, it goes in the return, and the asset team are pleased.
The section is programmed for replacement in 2027 with everything either side of it, and when the crew do it they will lay new material over the top and it will read two hundred and forty, and it will be less visible than what is there now.
I have made an argument for leaving those two hundred metres and I have lost it, on the sensible grounds that a road with two different ages of marking on it is a road nobody can plan.
What I do.
I read the line twice a year and write the figures on the sheet.
The one thing I have changed is that I read that section first, in daylight, at the start of the round rather than the end.
Retroreflection is a night property but you measure it whenever you like, because the instrument brings its own light.
I used to take that stretch last, which put me there at about four in the afternoon in winter, in the half dark, on foot on the hard shoulder with the meter.
In November 2022 I was doing it at ten past four with the traffic on and my back to it, and the meter sat on the line reads a number every time you press it, and I took nine readings along those two hundred metres and wrote each one down.
The last three came out at three ten, three ten and three ten, and the ninth reading was taken at a place where my own boots had been in the line's way for the previous eight, and the line under them was dry, and it had been raining since two.