And the OP is or was?
I’ll take a guess:
They begin at the Western or Southern border where the Interstate crosses the line and are numbered from West to East or from South to North.
Exits are then numbered according to the closest mile marker, although very large, complicated interchanges that extend over long distances may use either an “A” “B” system or may actually be given a different number for each departure lane, depending, apparently, on the whim of the engineers that laid it out.
The OP probably took the last service exit. We’ll wait while he fills up at McD’s.
Excellent job of answering the probable question, Tom.
I just want to add that some states (New York being one example, and Pennsylvania was another until – IIRC – this year), number exits not by the mileage signs but sequentially, so that the exit nearest mile marker 143 is not Exit 143 but Exit 54, meaning that there have been 53 separate interchange locations prior to this point as one progresses north or east.
However, there is one other possibility on what the original question might have been, and I confess that I don’t know the answer.
Every tenth of a mile or so in some states, there are little green signs which say something like:
The bottom number changes on each sign but not according to any sensible pattern where it’s measuring miles or tenths of miles.
Does anyone understand this coding system?
I could also say that the distance to a city, as listed on a sign, is the driving distance from a specified point in that city’s downtown, and not the city line. A sign reading …
Houston 35
… means that downtown Houston, Texas is 35 miles away from the sign. The actual Houston city limits might be about 10 miles from the sign.
Yeah, the Ohio Turnpike was originally numbered serially by exit. But, as Toledo, Cleveland/Akron, and Youngstown/Warren kept getting bigger and they kept having to “insert” exit numbers, about the time that they were going to need an “Exit 11 D (iv) c” they decided to look at reasonable states and switch to a mile-based system. They currently use a dual numbering system as the regular users adjust to the new system.
(Ohio took dumbness pretty far, originally, numbering their non-Turnpike roads from each county line, so that a trip across the state was a surreal journey that avoided any genuine notion of location. They finally started using the state boundaries around 1990, I think. I’ll have to look, tonight, to see whether they ever fixed that on their two-lane state routes.)
This was actually the answer to my question, that somehow got ate up by the message board somehow (my connection timed out when I posted it and was getting really slow connection times to this site yesterday for some reason.)
Anyway, yes, my question was to be: What point are they measuring to when you see a sign on the interstate that says “Foo 12 miles”? It obvioulsy isn’t the city limits, since a) they’d have to change the signs every time the city expands b) these are never seem to be accurate when I’m in a car on a trip and c) When driving between San Antonio and Austin once I saw a sign that said “Austin City Limits” (no, not the TV show), followed 1/2 mile down the road by a sign that said “Austin, 12 miles”.
Now, I have heard that that the milage on these signs is actually the number of miles to the central post office for each city, rather than the “center” of the city. Has anyone else heard this? I can’t seem to confirm it anywhere.
I think some of these signs are still around in OH.
Last summer, I was driving from New York City to Columbus, OH. When I crossed the Ohio border on I-80, I noticed these bizarre mileage signs that apparently started over again for no apparent reason. These were a different color.
Once I got past Akron, onto I-71 (the most charmless stretch of interstate in the lower 48, I might add), the mileages started to make sense again.
I seem to recall the state of NY installing signs like that about 30 years ago. They were used to gather data about things like accident frequency, maintenance needs, etc. The numbers corresponded to some kind of grid system. In other words, every tenth of a mile stretch of state highway had some unique number to identify it in a database. So, the numbers, other than the route number at the top, didn’t mean anything, per se.
Is Southern California the only area in America that doesn’t number freeway exits?
– Dragonblink, who had to have numbered exits explained to her by her dad a few months ago.
According to this article, California is the only state in the country that doesn’t do it.
However, the article also says that state has finally decided to bring itself out of the dark ages in this regard, a process which will take some three years.
The obvious question is which format the numbering will take on (sequential, or mile-based). This quote from the article seems either confusing or contradictory:
Huh? Unless the average state-wide is close to one exit per mile that can’t possibly work (unless I’m mis-reading that sentence).
From the NY State DOT website
[QUOTE]
What do those small green signs mean?
Reference markers are small green signs with three rows of numbers provide a unique locator for every state road. The top number is the route number. The second line identifies the DOT region, the county, and the sequence of the county along the route. On the third line is the mileage of that route from the county line, going west to east, or south to north in tenths of a mile.
Police agencies and motorists put these numbers on accident reports. When this data are analyzed, safety engineers can pinpoint trouble spots on the highway.
QUOTE]
I had always assumed that it was based on whim. On the west side of Cleveland, I-480 has a sign that says “Toledo 107.” Three miles later, after I-480 has merged directly onto the Turnpike, there is a sign that says “Toledo 90.” I used to notice the same thing when I traveled SR-2 and I-90 across the top of Ohio: the mileages to Cleveland were not consistent along the same stretch of highway. Lest I be accused of slamming Ohio signage, I will note that I encountered the same problem, this summer, traveling from Spokane to Seattle: I marked my odometer at the first sign giving a distance to Seattle so that I could track the distance to my (pre-Seattle) exit. The miles rolled off in regular fashion for a couple of hours, until there was an abrupt change that added several miles to the expected distance. It seemed as though I encountered the same thing around Chicago, but I was not watching as closely, so I cannot testify to that.
Sequential numbered exits never made much sense to me. The mileage based exits at least give the driver some sense of how many miles until the next opportunity to potty. 
Most mileages I’ve mapped from Mapquest,or the like,seem to use the center of the mapping grid in a town or city for their E/W,N/S numbering systems.,eg,main and central,or whatever your town’s street names are.
Some I know like NYC uses BWY/42nd,Philly Broad & Market,etc.
These mileages seem to jibe pretty much with hiway signs I’ve encountered.
I’d suspect,tho,that most interstate hiway mileages coincide with the center city off/on ramps.For NYC coming from the south that would be the Lincoln tunnel exit on the Jersey Tpike
No cite,tho.