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

[ais523]

Travelling along the M69 recently, I was looking at the Driver Location Signs. The numbers go upwards as you go north, and downwards as you go south, which is not unreasonable. The confusing bit is that rather than reaching 0 at the south end of the motorway (which might be expected), the point at which the M69 crosses the M6 is approximately 100.

This indicates a datum point somewhere around 100 kilometres south or south-west of the M6 near Coventry, but I can't think of anything obvious. London is too distant. One possibility I considered was Strensham (because one plan for the Strensham–Solihull motorway connected to Coventry rather than Solihull and joined onto the M69), but it's too close (you can do it in under 100 kilometres along the existing motorways, and presumably a new route would be more direct). Perhaps the decision was "so we might want to extend the M69 into Coventry some time, let's just number this end 100 for future expansion".

Can anyone shed light on why this motorway has its locations numbered as they are? Any other examples of motorways with bizarre datum points?

[Lockwood]

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

[Steven]

Is it 100km to the other end of the M50?

[nowster]

The M6's datum point coincides with the M1's in being somewhere in the centre of London. The M62 has a datum point in the centre of Liverpool. The M56's datum point is the centre of Manchester. etc. etc.

The A74(M) for part of its length continues the M6's chainage numbering, then reverses direction to match the M74's. This is less obvious as Scottish motorways don't have DLSes.

The A1/A1(M) appears to have several datum points.

[Chris5156]

Perhaps the decision was "so we might want to extend the M69 into Coventry some time, let's just number this end 100 for future expansion".

That would be my guess. 100 is too neat for it to be a chance - I suspect that the existence of plans to extend south, without a firm line or end point, meant that hedging their bets was required.

The M55's datum point is the west coast, on the reasonable basis that if it were extended it could potentially go any distance eastwards from the M6, but was unlikely to go west of the beach at Blackpool.

(Gratuitous plug: the markerpost numbers at each junction are now marked on the Motorway Database almost everywhere, if you wish to see what the DLS numbering for a particular route looks like.)

[Bryn666]

What do you mean unlikely... that was no doubt reserved for the Douglas Link Tunnel!

[JohnnyMo]

See this thread: M69 Theoretical Origin The conclusion was to avoid negative datum points south of the M6

[Johnathan404]

I believe the M18 is measured as a spur of the M1.

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.

[Beardy5632]

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

As does the M48 and A48(M) with the M4.

[Chris5156]

I believe the M18 is measured as a spur of the M1.

You could argue it's numbered as an alternative mainline north of junction 32, which is of course what it is!

[wrinkly]

The A74(M) for part of its length continues the M6's chainage numbering, then reverses direction to match the M74's. This is less obvious as Scottish motorways don't have DLSes.

The reversal point is the bridge of J16. North of there the zero point is between the crossings over Eglinton Street and Pollokshaws Road, Glasgow.

[ScottB5411]

The DLS on the M69 are a direct continuation of those on the A46 at the southern end. I believe the datum point is the M5-A46 junction

[ais523]

Is it 100km to the other end of the M50?

I'm reasonably sure it's further (although it's hard to estimate distances which wouldn't go along existing roads, the difference is large enough that I doubt even a perfectly straight road could make it).

The conclusion in the previous thread seems to be that an extra 100km got added on the south end of the motorway when it got lengthened beyond its original south end, and that explanation does make a lot of sense.

EDIT:

The DLS on the M69 are a direct continuation of those on the A46 at the southern end. I believe the datum point is the M5-A46 junction

This works! It's 102km away by my measurements, close enough to be a measurement error.

[RichardA35]

Is it 100km to the other end of the M50?

I'm reasonably sure it's further (although it's hard to estimate distances which wouldn't go along existing roads, the difference is large enough that I doubt even a perfectly straight road could make it).
The conclusion in the previous thread seems to be that an extra 100km got added on the south end of the motorway when it got lengthened beyond its original south end, and that explanation does make a lot of sense.
EDIT:

The DLS on the M69 are a direct continuation of those on the A46 at the southern end. I believe the datum point is the M5-A46 junction

This works! It's 102km away by my measurements, close enough to be a measurement error.

Unfortunately it doesn't work as the evidence on the ground disproves this, as I wrote on the other thread -

The 100/0 point is at the southbound diverge of the M69 to M6 loop as stated and the gantry carries this reference.
On the A46, according to the structures references, the chainages count up in a northbound direction from 100/0 at the point where the "new" A46 alignment diverges from the A439 at Salford Priors and reach ~152/2 by the time they meet the origin for the M69.

DLS are an artifical construct that just happen to match up to the marker post chainages on most routes. Thee chainages on the M69 were laid out in 1977ish long before the decisions were made on routes of the Coventry Southern, Stratford, Evesham and Alcester Bypasses, and possibly the decision to reroute the A46 via the old A439, so any suggestion that there was a joined up "design" behind the chainage decision is a long way off the mark. The chainages on the A46 would have been laid out at the time of completion of the A46 via Evesham ~1987ish.

[ais523]

DLS are an artifical construct that just happen to match up to the marker post chainages on most routes.

Huh, I was under the impression that DLS were simply more easily visible versions of the markerposts, as opposed to having their own identity.

Thee chainages on the M69 were laid out in 1977ish long before the decisions were made on routes of the Coventry Southern, Stratford, Evesham and Alcester Bypasses, and possibly the decision to reroute the A46 via the old A439, so any suggestion that there was a joined up "design" behind the chainage decision is a long way off the mark. The chainages on the A46 would have been laid out at the time of completion of the A46 via Evesham ~1987ish.

I was thinking more in terms of "perhaps there was a design to extend the M69 to the A46/M5 junction that was 100km long" (we know that there were at least provisional plans to build a motorway in that sort of direction at the same time as the M69 was being planned). It wouldn't necessarily have followed the exact route of the A46, but it would be approximately the same length as the A46 is now.

Of course, all this is probably just coincidence, especially as given the uncertainty of the details, you'd probably have wanted to leave more than 100km of headroom in your markerpost numbering. I'm not sure if there were ever concrete plans for the precise nature of the "M5 to M69 connection that approximately follows the A46", and it would have been pretty unwise to assume you knew how long it would be before building it!

[Matthew]

I would suggest that there's a possibility that starting at 100 MAY be coincidental, in so much as those marker posts continue along the A46. Though I accept also that there's a further potential that I'm wrong.

Of course, the marker posts for the M6 are a continuation of the M1.

[wrinkly]

DLS are an artifical construct

I thought they grew out of the ground naturally like trees.

that just happen to match up to the marker post chainages on most routes.

Do you know of any routes where they don't?

[Debaser]

DLS are an artifical construct that just happen to match up to the marker post chainages on most routes.

From IAN 93/10 DRIVER LOCATION SIGNS - INTERIM REQUIREMENTS

1.2.3 This Guidance in Context
For more than 30 years, distance marker posts have been provided at 100 metre intervals alongside the hard shoulder of motorways. These are used for maintenance purposes, enabling contractors to identify where repair works are needed. They also show the direction to the nearest motorway emergency roadside telephone. If a driver uses an emergency roadside telephone, the Regional Control Centre operator will automatically know the precise location of the caller.

However, it was found that when a driver used a mobile phone to report a breakdown or incident, the caller often gave an incorrect location which caused a delay in attending the scene of the incident. Therefore, at a number of trial sites driver location signs were erected every 500 metres to enable drivers to identify their location. These use the same referencing system as the distance marker posts and enabled the emergency services and vehicle recovery operators to rapidly locate the scene of an incident without any confusion...

2.2.5 Project Sponsors should ensure that they are providing information to road users on distance marker posts which is consistent with information provided on driver location signs. Refer to section 2.7.3...

2.4.2 The location tolerance for a driver location sign must be plus or minus 20 metres (absolute maximum) from the associated distance marker post containing the same kilometerage information...

2.7.3 Kilometerage
There must be no tolerance for the kilometerage shown on a driver location sign. For example, if the distance marker post shows 248 over 80, then the adjacent driver location sign must show 248.8 on line 3.

[Debaser]

The A1/A1(M) appears to have several datum points.

Three, IIRC.

[JohnnyMo]

The A1/A1(M) appears to have several datum points.

Three, IIRC.

At least four, London, Huntingdon and either end of Yorkshire.