I don't understand the mile markers/exit numbers system for I-87 in New York

A few months ago, I was driving on I-87, and I needed to find a truck stop or rest area to use the restroom. I think I was in the state of New York. The Pocket Truckstop Guide said that there was a truckstop called the Exit 11 Truckstop at exit 11. I exited the interstate at exit 11, and I could not find the Exit 11 Truckstop or any other truckstop. I got back on I-87, and I went over a bridge called something like "The Zia Taporra Bridge" about ten miles past exit 11. I would estimate I got on I-287 and crossed into Connecticut within 50 miles after I crossed the Zia Taporra Bridge. Within a few days after this experience, I looked up the Exit 11 Truckstop in the Pocket Truckstop Guide, and it said that the Exit 11 Truckstop is in the town of Round Lake, New York. Round Lake, New York is up in northern New York, north of Albany, NY.

I-87 runs north-south like all digit interstates that end with an odd number. The exit numbers on north-south interstates usually correspond with mile markers (i.e. exit 214 is at mile marker 214), and the numbers on north-south interstate usually start at mile marker 1 at the southernmost extreme of the interstate within a state, and the numbers usually increase as one moves northward on the interstate. This does not seem to be the case on I-87 in the state of New York. According to my atlas, exit 18 on I-87 in the state of New York is at New Paltz, NY, which is about sixty miles from the southern extreme part of I-87 in NY. Furthermore, exit 20 on I-87 is about forty miles north of exit 18 (rather than two miles north of exit eighteen). Exit 9 is about sixty miles north of exit 20! Clearly I-87 has some sort of different exit number marking system than most north-south interstates.

How are the exit numbers marked on I-87 in the state of New York?

It looks like the numbers actually run down from the southern most point on I-87 until you get to around Albany, NY. And then from Albany, NY they exit numbers run up as one goes north until you get to Canada.

 

I just examined my Atlas and the Pocket Truckstop Guide more thoroughly, and I figured it out. I-87 becomes the NY State Thruway at Albany, NY. NY DOT numbers the exit markers going up from South to North on I-87 north of the NY State Thruway, and NY DOT numbers the exit markers going up from South to North on the NY State Thruway. However, the mile markers don't correspond with the exit numbers on I-87 or on the NY State Thruway.

 

Balakov, I like how you added this part:
"1st Exit is Exit 1.
The 20th exit is exit 20.
Even if it's 200 miles from Exit 1"

I like how you added what you did because before you added that, I did not know what you meant when you said "not by highway mileage." I understand it all now.