Odometers in vehicles measure distances travelled in tenths of a kilometre or tenths of a mile, depending on the unit used in the target market. A tenth of a mile is 176 yards. However, there is nothing on British roads that is measured by tenths of a mile.
The Traffic Signs Regulations and General Directions (TSRGD) regulations do not allow decimal numbers of miles or tenths of a mile on road signs. The TSRGD only allow whole numbers of miles, the ¼, ⅓, ½, ⅔ and ¾ fractions of a mile and combinations of the two. And whole numbers of yards, of course.
The marker posts on major roads are 100 metres apart. And signs leading to roadworks are placed at multiples of 100 metres. That is one tenth of a kilometre, not one tenth of a mile. Motorway exit signs with white bars on a blue background are placed at 100-yard intervals.
This is another dimension of the measurement muddle in the UK. Odometer measurements in British vehicles do not relate to anything on British roads. This is a consequence of the limited progress the UK has made in using metric units on British road signs. Half a century after a UK transport minister postponed the metrication of British road signs indefinitely, they are still almost exclusively imperial.
Metric Views 
Motorway exit signs …
I’ve always thought it was said (according to the old H/way Code) that these are at 100- yard intervals, but the DfT Highways Manual actually specifies that they are placed at 100 metre intervals.
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Metricviewer:
Could you please provide a quotation from the DfT Highways Manual where it actually states that? I also seem to be believe that these markers are posted to metric distances. Can highways engineers today actually even measure in imperial? Do they have imperial tapes? Or do they have to do a conversion?
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The only place I recall seeing 1/3 and 2/3 mile signs is on M55 Junction 1 (aka Broughton Roundabout) together with the nearby approach onto the M6 Junction 32 (no doubt I’ve driven past others). There is a now-rare “Motorways merge 1/3 mile” sign on the southbound slip road.
This was built in the mid 70s and I wonder if it was to cater for future metrication being approximately 1 km and 500 m. The new and as yet unopened M55 Junction 2, which is about 4 km west of J1, has the more usual 1 mile and 1/2 mile signs.
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Location markers on motorways (and also on certain other roads) are at 100 metre intervals. These are designed primarily for use by the highway authority to identify particular locations on the highway. Anybody who has sharp eyesight can check them against their own odometer.
My experience is that countdown markers on the other hand appear to be at 100 yard intervals. They occur in 3’s – three stripes for the 300 yard warning, two stripes for the 200 yard warning and one stripe for the 100 yard warning.
The use of metres of yards can easily be verified by a passenger by comparing the spacing between three successive countdown markers and the three location markers closest to them. If the distances appear to be the same, then the countdown markers have the same spacing at the location markers (100 metre intervals), but if there is a significant change in spacing, then the countdown markers’ spacing is different to that of the location markers, so they are probably are at 100 yard intervals.
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Traditionally, these countdown markers have been referred to as “hundred-yard markers”. I cannot find any reference to them in the present or previous versions of the Highway Code. The publication Know Your Road Signs refers to the as “about 100 yards apart”. So, what does “about” mean? “Approximately”, maybe?
I heard somewhere that the separation of the markers is variable, according to the nature of the exit road and how quickly vehicles need to slow down. But it must be equidistant.
Like Martin, I have compared the spacing of the countdown markers with the location markers. Occasionally they can be as much as 100 metres apart, but usually they are less. I would guess that in many cases they are not even as much as 100 yards apart.
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Martin & Metricmac,
I’m sure it can be both 100 yards and 100 m. If the road is old and the lines were originally in yards, the paint fades then the repainting is just over the existing lines and the 100 yards is preserved.
If there is a new road built or an old road is repaved and the 100 yard lines are gone, new lines will have to be measured out and they are measured out in 100 m increments.
Thus some roads will be 100 yards and some will be 100 m. Make sense?
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@Daniel, Martin, and Metricmac
More of the joys of the metric muddle, eh?
Given the local election results, I hold out a glimmer of hope that a Labour government with push from the Lib Dems and the Greens will take over Parliament and Number 10 at the next general election and maybe will be more amenable to final finish the job of metrication in the UK.
Fingers crossed, lads and lasses! 🙂
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Countdown markers also occur on non-motorways. Here is one (3 stripes) close to Sandhurst: https://www.google.co.uk/maps/@51.3577599,-0.817469,3a,75y,184.36h,79.91t/data=!3m6!1e1!3m4!1spuP9VfrxLm28VKln9afcLg!2e0!7i16384!8i8192. It appears that this sign is about 150 metres from the speed limit indicated.
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Thanks, Martin. This illustrates the point. The markers will be approached at less than motorway speed and require a substantial reduction of speed, so it is appropriate to space them only 50 m apart.
However I would dispute the choice of signage by the responsible authority. By my interpretation of the sign regulations, advance notification of a speed limit or any hazard should be within a rectangular sign, with the distance ahead in a sub-panel below.
In this case, an observant and careful driver could see the speed limit roundel well ahead but not detect the countdown marker, or could expect the countdown marker to refer to something else, like a junction. The driver would then slow down earlier than necessary.
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I checked the southbound exist sliproad of M6 J32 and the countdown markers do seem to be at 100 yard / 91 cm intervals. The current form of this junction dates from well after metrication was repudiated, probably from the 90s when the motorway was changed to 4 lanes in each direction.
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