Recent Metric Views articles have discussed the poor awareness of the meaning of driver location signs amongst the general public, and argued that despite their inclusion in recent editions of the Highway Code, there is still a need for a new public information campaign about these signs.
However, could there be another reason why driver location signs are poorly understood? And is there a solution that would both increase public awareness and increase their use?
If we look at current road signs for a typical motorway junction, we can see that 3 incompatible ways of describing distances are used:
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| 3 incompatible ways of indicating distances | |||
- kilometres (unit name or symbol not shown) – on driver location signs, placed at 500-metre intervals along the side of each carriageway of a motorway, indicating the location relative to the nominal start of the motorway.
- miles (“m” used to indicate miles) – on signs in advance of a junction, indicating the distance from the sign to the exit.
- junction numbers – a sequential based reference system. Not strictly-speaking a measurement unit, but this is in effect a non-linear measurement of distance relative to the nominal start of the motorway.
Metrication of road signs will see the replacement of miles with metres (and kilometres), thus reducing the number of measurement units used. However, this won’t address the main issue of drivers not using, and potentially not understanding the meaning of driver location signs.
This is because drivers, and travel news reports on the radio, generally use junction numbers to indicate locations on motorways. e.g. If you listen to travel news while driving on the M6 in the Midlands you will be familiar with travel reports such as, “The M6 southbound has long delays between junctions 10 and 8.”
Describing motorway locations using junction numbers can be sufficient to give a rough idea of a location but this method is unsuitable for road engineers and emergency services.
For a travel news report to indicate the precise location of a trouble spot on a motorway, such as a car breakdown blocking a lane, they could use the kilometre-based reference system shown on driver location signs, but in practice they don’t. This is probably because of a reluctance to introduce a second system of referencing locations.
It seems that in the USA and Canada, junction numbers (or “exits”) are generally numbered according to their location, rather than sequentially as in the UK (perhaps one of our US readers could confirm the current situation there).
Exit sign and location marker post at Exit 40 on the I-495, USA
Numbering junctions using the same reference system as location marker posts has the advantage that only one reference system is needed for all purposes.
If carried out as part of the metrication programme for all road signs, the re-numbering of all UK motorway junctions in line with the nearest whole kilometre of the location reference system used on driver location signs, would create a single coherent location reference system for all purposes on motorways, using a single unit of measurement.
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| A single unit of measurement for all purposes | |||
The numbers on driver location signs would then be more obvious to drivers, and radio travel reports could be more precise when needed, without needing to resort to two systems.
Travel news such as,
“The M6 southbound has long delays between junctions 10 and 8.”
would become,
“The M6 southbound has long delays between K198 and K193.”
and
“Debris is blocking lane one on the M6 northbound between junctions 14 and 15.”
would become,
“Debris is blocking lane one on the M6 northbound at kilometre 244.”
Re-numbering motorway junctions is another benefit of metrication that we are missing out on due to our government’s continuing refusal to upgrade road signs to kilometres.
References
https://metrication.uk/transport/roads/driver-location-signs/#metrication
https://en.wikipedia.org/wiki/Exit_number
Metric Views 
How do you deal with the case where there are more than 1 exit within a kilometre, for example, 3 exits in quick succession? Do you use letters in this scenario?
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US reader here. Most US states number freeway exits in miles from origin of the roadway (to the south or west, whether the state line or beginning of the road within the state. If the road continues into another state, the numbers begin over at the state border. A handful of states have retained the sequential numbering system, but they are a small minority (I would have to look them up to name them). If there are multiple exits in a one mile stretch, a sequential letter is added to the exit number, A, B, etc. The only other exception I am aware of is I-19 in Arizona, exit numbers are in kilometers from the southern border of the state.
The exit numbers are rounded to the nearest mile. However, the freeways also have small mileage markers set accurately each 0.1 mile (I do not know if I-19 uses kilometers for that purpose).
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It is indeed true that US Interstates number their intersections according to the nearest mile from the origin. (I cannot speak for Canada.) if a number of intersections occur within the same mile, they are suffixed with ‘A’, ‘B’ or ‘C’.
It was not always so. I understand that initially their intersections were numbered like ours. However, apart from anything else, such a numbering system gives no indication of the distance between successive junctions.
A few years ago a group of us made a road trip from Johannesburg to the Limpopo (all set about with fever trees). I began to wonder if the same system applied to South African highways, only, using kilometres. It really didn’t take very long to confirm that it did.
I have long thought that it was a system that we should do well to adopt. It would make better sense, when inserting a new junction, such as J 10A at km 91 on M 20 between J 10 at km 90—the notorious Inland Border Facility at Sevington—and J 11 at km 101.
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On the Interstate highways exits are numbered sequentially.
Ezra aka punditgi
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Junctions on Spanish motorways are also numbered using location markers. Using kilometre-based numbering rather than mile-based numbering has the advantage that if two or more junctions are tightly clustered, one is less likely to have clashes of numbers as the kilometre markers are closer to each other than are mile markers. I recall having seen one instance in Spain where the two exits for a junction are separated from each other by a few hundred metres. One was labelled “329a” and the other “329b”. In contrast, Junction 21 and Junction 21A on the M6 are two separate junctions with Junction 21A having two separate exits, each of which are about one kilometre apart. One of the exits leads to Liverpool and the other to Manchester. If one is looking at junction numbers only when leaving the motorway, one can easily end up going to Liverpool rather than Manchester. In my view, we should be numbering exits, not junctions and using qualifiers “a” and “b” where two distinct exits are less than one kilometre apart, such as on the M8 through Central Glasgow.
A further refinement would be to number service areas as “Snnn” where “nnn” is the nearest whole-numbered location marker.
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One logistical problem associated with renumbering junctions using a kilometre base is that during the change-over process, would J45 on the M1 refer to the exit for Leeds (current system) or Milton Keynes (new system)? The way around that would be to phase change-overs over a two-year period – by mid-year in Year 1, everything from kilometre marker 100 would be renumbered and from mid-year onwards in Year 2 everything less than 100 kilometres would be renumbered. If necessary, the Year 2 renumbering could be for junctions between km 10 and km 100 and the process repeated a year later for the remaining junctions (km 0 to km 10).
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Apparently today, 2024-06-17 is the 200-th anniversary of imperial units. Let’s hope they are long dead when the 300 year anniversary rolls around.
https://www.dailymail.co.uk/debate/article-13523441/Why-great-British-pint-one-thing-modernisers-havent-dared-try-abolish-200-years-officially-came-being.html
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@Daniel (and editor of the Daily Mail)
The litre was defined in 1795 and the imperial gallon in 1824. One would therefore hope that the definition of the gallon was superior to that of the litre.
The mass of one litre of water is one kilogram while the mass of one gallon of water in 10 lbs. The definition of the litre is comparable to the definition of the gallon (and hence of the imperial pint).
The volume of the litre is equal to the volume of a cube with sides of 10 centimetres (1000 cubic centimetres). One gallon is equal 277.419 cubic inches. The litre’s definition is much cleaner.
In short, the definition of the gallon is inferior to the definition of the litre.
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