Motorway DLS starting above 0 (i.e. unusual datum points)

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wrinkly
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

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Vierwielen
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

Post by Vierwielen »

The A3 is another interesting case. The bridges have numbers on them that appear to use Central London (London Bridge?) as a reference point. Travelling out of London through Surrey, the marker posts appear to use the Surrey/"London" border as a reference point while travelling northwards, the Hampshire section appears to use the M27 as a reference point. The Hindhead Tunnel section is a continuation of the Surrey numbering system.
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

Post by RichardA35 »

Vierwielen wrote:The A3 is another interesting case. The bridges have numbers on them that appear to use Central London (London Bridge?) as a reference point. Travelling out of London through Surrey, the marker posts appear to use the Surrey/"London" border as a reference point while travelling northwards, the Hampshire section appears to use the M27 as a reference point. The Hindhead Tunnel section is a continuation of the Surrey numbering system.
A clear demonstration that there is no master design for these things and they evolve on a scheme by scheme or area by area basis over time as different needs evolve for evolving or different Clients (DoT, DfT, HA, HE, TfL, maintenance agencies, county councils etc) as they all impose their own standards. Taking account of a hypothetical improvement or motorway is not on the list of considerations.
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

Post by Percy »

The first marker post in London (Kingston borough) heading North has, or used to have, "TfL" on it. The northbound posts in Surrey count down to zero, at least north of Guildford, so it must switch over somewhere (Surrey/Hants border maybe).
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

Post by nowster »

I've mentioned this before... the A55 within Wales has marker posts that count from Holyhead, but emergency phone reference numbers that count distances from Chester.
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

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I've been filling in some missing markerpost numbers on the CBRD Motorway Database in spare moments lately, using my whizzy new markerpost finder and the Highways England database.

It's thrown up a few more interesting cases that I can't really explain (or, if you prefer, it's thrown up more evidence that markerpost numbering is not logical at all).

The M23 counts southwards from a theoretical origin in central London towards the coast, but at Pease Pottage hands over to the A23 which counts northwards from an origin at the roundabout just south of the A27. But interestingly, there is still no duplication of numbers because the highest markerpost on the A23 is still lower than the first on the M23 - they just count in opposite directions.

The M181 has its own numbering scheme, separate to the M180, which counts from 8.8 at the southern end to 12.3 at the northern end. What's the purpose of the extra 8km? It's suggestive of a continuation towards Gainsborough on the A159; 8km would get you somewhere a bit south of Scotter. But the terminal junction at Bottesford Moor isn't laid out to allow for any continuation.

The M271 follows numbering on the M27, starting at an origin of about 10.6km at M27 J3. The M271's carriageways are lettered L and K, as though they are sliproads, and in a decidedly odd twist, count up from 10.6 in both directions away from junction 3, so that markerposts 10.6 to 11.8 occur twice on each carriageway.
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

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Chris5156 wrote:I've been filling in some missing markerpost numbers on the CBRD Motorway Database in spare moments lately, using my whizzy new markerpost finder and the Highways England database.

It's thrown up a few more interesting cases that I can't really explain (or, if you prefer, it's thrown up more evidence that markerpost numbering is not logical at all).

The M23 counts southwards from a theoretical origin in central London towards the coast, but at Pease Pottage hands over to the A23 which counts northwards from an origin at the roundabout just south of the A27. But interestingly, there is still no duplication of numbers because the highest markerpost on the A23 is still lower than the first on the M23 - they just count in opposite directions.

The M181 has its own numbering scheme, separate to the M180, which counts from 8.8 at the southern end to 12.3 at the northern end. What's the purpose of the extra 8km? It's suggestive of a continuation towards Gainsborough on the A159; 8km would get you somewhere a bit south of Scotter. But the terminal junction at Bottesford Moor isn't laid out to allow for any continuation.

The M271 follows numbering on the M27, starting at an origin of about 10.6km at M27 J3. The M271's carriageways are lettered L and K, as though they are sliproads, and in a decidedly odd twist, count up from 10.6 in both directions away from junction 3, so that markerposts 10.6 to 11.8 occur twice on each carriageway.
Very interesting finds, Chris. Is the northern part of the M271 definitely designated M271 in that case, as the DLS clearly isn't unique if there are two M271 L 11.0s (for example)
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

Post by ais523 »

That M271 case is just bizarre.

In other news, it turns out that there's actually a 99.9 DLS on the M69 (in the loop at the south end that connects it to the M6), so a need for "negative" numbers does actually exist there. That's one reason for starting at 100 rather than 0. (That said, it's weird that there's a 100.0 DLS, a 99.9 DLS, then no more for the rest of the relatively long slip road. You'd think that if having DLS for the slip road was useful, they'd be more evenly spaced. Either that or I just missed one; I was looking for them but it's possible that I might have missed one.)
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

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c2R wrote:
Chris5156 wrote:The M271 follows numbering on the M27, starting at an origin of about 10.6km at M27 J3. The M271's carriageways are lettered L and K, as though they are sliproads, and in a decidedly odd twist, count up from 10.6 in both directions away from junction 3, so that markerposts 10.6 to 11.8 occur twice on each carriageway.
Very interesting finds, Chris. Is the northern part of the M271 definitely designated M271 in that case, as the DLS clearly isn't unique if there are two M271 L 11.0s (for example)
My bad - I've looked more closely at this and spotted what was missing. Both sides of J3 are indeed referred to as M271, but the carriageway letters are different.

North of J3 they are J (northbound) and K (southbound). South of J3 they are L (southbound) and M (northbound).

It's still far from clear why you'd ever arrange it that way - surely the risk of having duplicated numbers on the same road would make it worthwhile to choose a different solution.
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

Post by Johnathan404 »

The M271 gets more messy when you consider it has a J1, with four slips.

I'm almost certain they have no marker posts at all, unless Chris can prove me wrong.
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

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Johnathan404 wrote:The M271 gets more messy when you consider it has a J1, with four slips.

I'm almost certain they have no marker posts at all, unless Chris can prove me wrong.
I wouldn't take this as ultimate proof, because it seems to be incomplete in places, but the HE database certainly doesn't record any marker posts on the J1 slips.
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

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Johnathan404 wrote:I believe the M18 is measured as a spur of the M1.
Lockwood wrote:Other example: The M271 uses the M27's , so as you head off the M27 towards Southampton you are met with 11.0, and the DLS from the M271 E/B towards Rownhams are in the 11.x range
And I believe the M271's exit slips are unnumbered.
On the contrary - see here!
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

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Chris5156 wrote:
c2R wrote:
Chris5156 wrote:The M271 follows numbering on the M27, starting at an origin of about 10.6km at M27 J3. The M271's carriageways are lettered L and K, as though they are sliproads, and in a decidedly odd twist, count up from 10.6 in both directions away from junction 3, so that markerposts 10.6 to 11.8 occur twice on each carriageway.
Very interesting finds, Chris. Is the northern part of the M271 definitely designated M271 in that case, as the DLS clearly isn't unique if there are two M271 L 11.0s (for example)
My bad - I've looked more closely at this and spotted what was missing. Both sides of J3 are indeed referred to as M271, but the carriageway letters are different.

North of J3 they are J (northbound) and K (southbound). South of J3 they are L (southbound) and M (northbound).

It's still far from clear why you'd ever arrange it that way - surely the risk of having duplicated numbers on the same road would make it worthwhile to choose a different solution.
These markings tell me that as far as the numbering merchants were concerned, the M271 does not really exist, but that it is really a set of M27 slip roads. (Slip roads are usually J, K, L and M).
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

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(re: M271 J1)

Well look at that! I take it back.

I have a feeling those signs were put in very recently, but I could be wrong again.

As a point of interest, much of M27 J12 is lacking DLS. I assume this is because it's owned by Portsmouth City Council (except the A/B mainline), but Highways England claim they own the link roads as far south as the river crossing. The notation of the spur would be particularly interesting.

(I apologise I can't work CBRD's map or Street View on my phone or I'd answer my own questions myself.)
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

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Vierwielen wrote:
Johnathan404 wrote:I believe the M18 is measured as a spur of the M1.
Lockwood wrote:Other example: The M271 uses the M27's , so as you head off the M27 towards Southampton you are met with 11.0, and the DLS from the M271 E/B towards Rownhams are in the 11.x range
And I believe the M271's exit slips are unnumbered.
On the contrary - see here!
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

Post by wrinkly »

Are there many cases where the numbering of DLSs or marker posts on a spur or less important road or motorway are projected from a main or more important route but in a decreasing sense?

The only example I can think of is the SE arm of the M5/M6 junction.
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

Post by Nicholas »

We've mentioned M271, but the M27 is also guilty of starting above 0. Oddly though, the next marker post does have a datum of 0, but for the A31 instead.
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

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Both the M3 and the M4 appear to have reference points somewhere in Central London (North/South Circular road?).
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

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Vierwielen wrote:On the contrary - see here!
Well, there you go! Sliproads appear to be lettered P, Q, R, S.
These markings tell me that as far as the numbering merchants were concerned, the M271 does not really exist, but that it is really a set of M27 slip roads. (Slip roads are usually J, K, L and M).
Yes and no. The M271 uses the origin point of the M27 and its carriageway letters are the ones usually reserved for sliproads, but the route number (as evidenced on the HE database and the DLS you linked to) is still M271. Otherwise there would be duplicated markerposts between the M271 and the M27's east-facing slips at J3, both of which must use numbers counting up from 10.6 and which are carriageways K and L of the M27. So the M271 does exist, but it's a motorway that does not have an A or B carriageway and on which the chainage counts in two different directions from an origin point partway along.
Jonathan wrote:As a point of interest, much of M27 J12 is lacking DLS. I assume this is because it's owned by Portsmouth City Council (except the A/B mainline), but Highways England claim they own the link roads as far south as the river crossing. The notation of the spur would be particularly interesting.
Interestingly the whole of the M275 appears in the HE database. I expect this is because the maintenance contractor used by the HE in this area, who maintain the M27, are also employed by Portsmouth City Council to maintain the M275, and so the local authority markerposts have made their way into the HE database.

The M275 counts southwards from a zero point at the M27 junction, but the spur northwards from J12 is a weird mix. It has carriageway letters J and K, and the chainage follows the M27 origin, but the route is listed as M275.

On the M275 itself, the A and B carriageways are the east-facing pair towards the A3(M) - this appears to be the mainline and starts at 0.0km. The west-facing pair, that join the spur, are J and K, and also share the M27's numbering.

By delightful coincidence, the northward spur from J12 is an example of exactly the thing Wrinkly asked about - it counts backwards from its connection to the M27, with 44.4km at its northern terminus counting along to about 44.9 at the bridge over the mainline. The numbers then count upwards towards Portsmouth and the main M275.
(I apologise I can't work CBRD's map or Street View on my phone or I'd answer my own questions myself.)
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Re: Motorway DLS starting above 0 (i.e. unusual datum points

Post by Johnathan404 »

Thank you very much for the breakdown, Chris.

I was wrong again: the M27 eastbound -> A3 slip has one DLS, labelling it H. The main spur and loop have marker posts but not DLS, probably for the same reasons as the M275.

Fans of anomalies may like M27 J9. Westbound you have two parallel roads: the mainline (B) and the sliproad (L). Eastbound you have two parallel roads: the maintenance road (signed as A) and the mainline (not signed). As the maintenance road isn't open to the public there is no harm in having one sign for two roads, but strictly speaking it's odd.
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