Two separate but related questions which popped into my head driving to and from Iowa for Xmas.
Those little green mile markers…from where are they measuring?
Highway signs that list how many miles from a city one is…from what point in the city are they measured?
The state line.
I’ve always been told it’s measured from “dowtown,” but I’ve no idea how they determine what point that’s at.
(1) Generally, the convention is measured from the southern or western terminus, depending on orienation of route. That terminus may or may not be a state line.
(2) It used to be pretty standard that the measurements were to city center but there are some agencies that may post distance to the nearest city limit line.
This holds for most states, but (and I’ve got to say this), YMMV.
Just to expand on BLD, it’s where the road/hiway starts but it resets at the stateline if the hiway crosses it.
Traditionally the measurementys were to city/town hall. Some cities, such as Boston, had a stone marker from which dstances were (are?) reckoned.
When I’ve bothered to check them, the mile markers seem to be fairly accurate. But the distance signs vary. On a trip last week I noticed several instances of the following - a sign said Albequerque 70 and six miles later another said Albequerque 54. I’ve noticed other moving targets, as well.
What really gets me wondering is every now and then, you’ll come across a sign listing distances to two or three cities and one of the numbers has been “patched.” Did the sign shop goof the first time around and just needed to correct one city’s number, or did the city move?
No doubt it was a “city on the move.” 
http://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=“city+on+the+move”
Sorry to Hijack… but this thread just got me thinking. Why are the signs placed where they are? Who chooses to put a sign at 104, 87, 33 etc miles away from the city?? There seems to be no kind of pattern or standard for where they put signs? Why not at least put them where the closest city is at a value of ten. Or something that at least makes sense? Anyone know why they are placed where they are?
Well, there’s a sign on the Queen Elizabeth Way just outside Toronto that says “Niagara Falls 114 km”. This distance is not a round number.
The sign is, however, just after a major freeway merge.
These signs confirm to drivers that they are on the right road (or not), just after they have passed through an interchange or road juction.
I have understood for decades that the distance to a town is measured to the main post office. That could be the Canadian convention however.
This Iowa DOT FAQ could answer your question in Iowa.
As Sunspace noted, they are frequently placed near entrances to let drivers who have just entered (or who have just passed an exit) know how far to the next exit or how far to the next major city.
Her in South Africa, the convention is:
Mile markers are measured from the last major intersection if they are on your side of the road, or to the next major intersection if they are on the opposite side of the road. There is no hard and fast rule that defines “major intersection”
Distances to towns are measured to the central post office.
As to the odd numbers, this might be clue:
In the old days we used imperial units, and we typically had distance markers near major intersections. These obviously confirmed on which route one was and how far the next town was, thus could be any number, e.g. 63 mile. Further down the road one used to find them at 10-mile intervals and nearer town, 5 mile. When we switched to the metric system, the miles were simply converted to km (about 8km = 5miles) thus, 70 miles suddnely became 113km or some odd number like that, so one suddnly found markers at 113km, 97km, 81km etc. Later on when the signs were replaced or the road resurfaced, the signs were placed at 20 or 10km distances.
I’d also like to know how far out the highway administrators are willing to go, to inform you what’s coming up.
To illustrate what I mean: Once on a road trip from Washington to Boston, I was amused to see a sign on I-95 saying “New York 320” (or somewhere thereabouts). This is by far the largest distance notification I can recall witnessing in all my many years of American highway travel. Maybe I’m too easily amused.
As I drove past this sign, I kept myself further amused by fantasizing about even sillier notices. Perhaps, after you join I-40 westbound in North Carolina, there should be a sign saying “Los Angeles 2873”. (Or whatever the real distance is.) And when you leave Alaska heading south on the Pan American highway, they should tell you with a friendly green sign that Santiago, Chile, is only 9274 miles down the road. (So why not hold out just a bit longer for that next rest stop?)
Anyway, what do you people suppose is the largest mileage notice for any city you can reach by highway? Personal anecdotes are good enough answers for me, at least to this question.
Even better, let’s define the absurdity of a highway distance sign to be the ratio of the stated distance to the size of the stated city’s population. So, the larger the distance and the smaller the city, the more absurd is the sign. Given this definition, which is the most absurd distance sign you’ve seen? (My New York example isn’t all that absurd actually, since New York is so large.)
As mentioned, I’m too easily amused.
If that’s how you define absurdity, I’ve seen half-mile notices that are more absurd (ie, any town with <12500 people).
As I pulled out of El Paso the other day, I saw a sign that said Dallas 581.
I was going to mention that in Texas as well. I also think there are some signs with miles to Albuquerque in at least Western AZ if not SE CA on 40 that would have pretty large mileages.
Signs on I-5 between San Francisco and LA have numbers over 300 I believe.
I remember seeing some big numbers on my drive through WY and the plains states through OH.
In Australia mileages are measured from the Post Office as well. The big signs on the highways are sometimes wrong. There used to be one near Mittagong where you drove 30 kilometres or so and according to the next sign were only 8 kilometres closer.
I have read that, on the Trans-Canada Highway, at the eastern end in St. John’s, Newfoundland, there’s a sign that says something like ‘Victoria BC 7000 km’. Likewise, in Victoria, there’s a sign that says ‘St. John’s NL 7000 km’. Buy I have never been to either of those endpoints, so I cannot confirm this.
I seem to remember at one time seeing a sign on Highway 17 just north of Sault Ste. Marie saying ‘Thunder Bay 700 km’, but the last time I was there, I looked for it and didn’t see it.
I’ve seen quite a few signs in Europe listing destinations over 1000km - almost always in a different country