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Old November 10th, 2009, 12:21 AM   #1
wdw35
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Km/mile numbering on motorways

A technical question here.

In some mountain areas one can see cases where the two carriageways of a motorway have been constructed on different alignments. This can lead to significant differences in the length on one carriageway vs. another.

How are the km/mile posts established in such cases? (And, also, how are exit numbers established?)
Does each carriageway hold individual km/mi-posts?
Are there even cases where the distance listed (and actual) from A to B is different from the distance from B to A *on the same road*?

Thank you.
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Old November 10th, 2009, 01:27 AM   #2
J N Winkler
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There are several ways around this problem. The approach taken in Pennsylvania is to designate one of the two carriageways (I think the northbound for north-south and the eastbound for east-west highways) as the reference line for mileposting. If the other carriageway follows a significantly different alignment, each milepost is placed on that carriageway wherever it is crossed by a horizontal line drawn perpendicular to the reference carriageway at the corresponding milepost on that carriageway. The mileposts therefore always reconcile in the up and down directions. However, the spacing between mileposts may vary from one mile on the carriageway not chosen as the reference. The variation will, typically, be very small because the more twisty a road is, the less capable it is of supporting high speeds.

There is an Italian autostrada somewhere in the Piedmont region which has one, and possibly both, carriageways going through a spiral inside a mountain. It makes me wonder how km-posting is handled in the one-carriageway case.
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Old November 10th, 2009, 01:28 AM   #3
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Good question My only experience with split alignments is on the German A8 at Aichelberg and no special solution was implemented there. I think the difference between eastbound and westbound is actually quite small and no-one would really bother if the distance between two kilometer posts is actually only 900 meters.

In a way, your issue is not only relevant for split alignments, but for any road. Because distance is calculated on the basis of the median, there should always be some difference between the distance that follows from the km/mile posts and the distance you actually drive. After all, in your own lane, a curve will be shorter or longer than the curve in the median, based on whether you're on the inside or on the outside of the curve. But that difference will be even smaller than the difference on a split alignment.
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Old November 10th, 2009, 03:37 AM   #4
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Quote:
Originally Posted by -Pino- View Post
In a way, your issue is not only relevant for split alignments, but for any road. Because distance is calculated on the basis of the median, there should always be some difference between the distance that follows from the km/mile posts and the distance you actually drive.
Yes, but this is totally insignificant, since here the horizontal curves compensate each other.
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Old November 10th, 2009, 03:43 AM   #5
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Quote:
Originally Posted by J N Winkler View Post
There is an Italian autostrada somewhere in the Piedmont region which has one, and possibly both, carriageways going through a spiral inside a mountain. It makes me wonder how km-posting is handled in the one-carriageway case.
Yeah, that's the case I was thinking of too.
Milepost this!

http://maps.google.com/maps?f=q&sour...91231&t=h&z=13

What is your proposal here, J N Winkler?
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Old November 10th, 2009, 11:13 AM   #6
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Quote:
Originally Posted by wdw35 View Post
Yes, but this is totally insignificant, since here the horizontal curves compensate each other.
It is not so insignificant. Here in Slovenia officialy every half of motorway is counted for itself. All roads in exits and interchanges are also counted for itself, separating them with distinctions as "arm A", "arm B" etc. Also every tunnel or large viaduct can add difference in distances, because it is often made as small split alignment. This can result even in one or two hundred meter difference between here ant there and reverse.

In case of this split alignment there is about 500 m difference in distance between A-B and B-A.
http://maps.google.com/?ie=UTF8&ll=4...,0.164795&z=14
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Old November 10th, 2009, 12:51 PM   #7
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Keber, you should read more carefully what other say before saying something yourself. The "insignificant" remark was about -Pino-'s reference to "any road", i.e. non-motorways. In this case, it is truly insignificant, unless the road is on a huge spiral route.
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Old November 10th, 2009, 09:30 PM   #8
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On a 100 km route, the difference between the left and right sides of the road will be close to zero, since no route runs in circles. And besides, what's 100 meter on 100 kilometer? But on the scale of one kilometer -when the route runs in one long curve to the left or to the right-, you will get a difference that is not evened out.

Not that it is a difference that anyone should worry about, but the difference is there.
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Old November 10th, 2009, 11:29 PM   #9
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Quote:
Originally Posted by -Pino- View Post
On a 100 km route, the difference between the left and right sides of the road will be close to zero, since no route runs in circles.
Except if you're interested in beltways.
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Old November 11th, 2009, 12:05 AM   #10
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Quote:
Originally Posted by -Pino- View Post
Not that it is a difference that anyone should worry about, but the difference is there.
There is notable difference between various sections of a route (it measures only in few percent, but still). And those can differ quite fast. Also in 100 km of a particular route there are rarely two halves of a motorway exactly paralel through whole, especially if you have interchanges or demanding terrain.
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Old November 11th, 2009, 03:41 AM   #11
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Anyway, the point of my question was to see how situations like this impact practical things, like:
- route marking (and hence route lenght, and hence the agency knowing exactly how many km or sqm a road has when, say, it bids its resurfacing);
- exit numbering.

We see that apparently Slovenia has a rather systematic approach. In this respect, keber, would you please elaborate for us:

1). The individual numbering for carriageways each start at the same end of the motorway (i.e. the carriageways share the same km 0) or at different ends of motorway (to allow the user to always travel from km posts with smaller values to larger values)?

2). How (if) are exits numbered in Slovenia? Presuming they are numbered km-based (not sequential), how is the issue described at number 1 dealt with?
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Old November 11th, 2009, 11:55 AM   #12
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well, we have some members from that part of Italy. JB Colbert is from Savona, Genius is also not that far, i saw somwbody from Pavia... JB could really check the situation at A6.
but i think that left and right lane at A6 don't have significant difference since both are curvy. the largerst difference is at A7 between Novi Ligure and Genova where southbound lane follows route of old road, and northbound rout is built as upgrade to motorway. there are definitely few kilometres of difference.
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Old November 11th, 2009, 12:13 PM   #13
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Interstate 8 east of San Diego and Mexican toll road between Tijuana and Mexicali both have this problem.
[IMG]http://i33.************/28brz1j.jpg[/IMG]

As well as the Anchietes / Imigrantes freeways south of Sao Paulo
[IMG]http://i36.************/33f9jzk.jpg[/IMG]
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Old November 11th, 2009, 10:00 PM   #14
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Quote:
Originally Posted by wdw35 View Post
We see that apparently Slovenia has a rather systematic approach. In this respect, keber, would you please elaborate for us:

1). The individual numbering for carriageways each start at the same end of the motorway (i.e. the carriageways share the same km 0) or at different ends of motorway (to allow the user to always travel from km posts with smaller values to larger values)?

2). How (if) are exits numbered in Slovenia? Presuming they are numbered km-based (not sequential), how is the issue described at number 1 dealt with?
1. At the same end;

2. sequential (1, 2, 3 ...).
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Old November 12th, 2009, 12:54 AM   #15
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Thanks man.
Btw sequential numbering of the exits sucks.
Only worse than that is no numbering whatsoever (as we have in our country ).
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Old November 12th, 2009, 01:21 PM   #16
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You can't just say that something sucks, explain why..
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Old November 12th, 2009, 01:47 PM   #17
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I think both are ok. I like sequential more because it's in order, while distance-based gives you the information how far you are. I can't decide, which one is better.
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Old November 12th, 2009, 02:53 PM   #18
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Quote:
Originally Posted by Verso View Post
I think both are ok. I like sequential more because it's in order, while distance-based gives you the information how far you are. I can't decide, which one is better.
i think similar. i have more got used to sequential one, so i prefer it. smaller numbers are generally easier to remember. so when you need to explain somebody which exit to take, it is easier to remember 18 than 263. i actually don't understand distance-based scheme because you allways have distance marker near exit, so if you prefer to name exit after distance, you allways can do it.
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Old November 12th, 2009, 03:45 PM   #19
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Quote:
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i think similar. i have more got used to sequential one, so i prefer it. smaller numbers are generally easier to remember. so when you need to explain somebody which exit to take, it is easier to remember 18 than 263.
True. Also, if you're at exit #17, you'll immediately know you have to leave the motorway at the next exit, while you can't be sure by the distance-based one.
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Old November 12th, 2009, 04:01 PM   #20
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If exits are numbered sequentially, what happens if you add a new exit between two others? It'd be very difficult to renumber all the exits, and if it was placed between exit 3 and exit 4, calling it exit 3A would imply that it's related to exit 3, when it may be miles away.

It's much easier to deal with this by numbering them by milemarker. This way, if you add multiple exits within one mile, they can be numbered with a letter after the mile number - for example, exit 118-A, 118-B, 118-C - because they're close together, this makes more sense.
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