Highway 27 is NY State Highway 27, so that has nothing to do with national roads. NY-25 is also east-west, as is NY-17 and many others.I-495 is a three-digit Interstate. Three-digit Interstates are used as extension or loop routes connecting to one- or two-digit Interstates. I-495 was intended to be a spur route connecting to the north-south road I-95. It never actually got there to connect, but the numbering stuck. You ll notice a NJ-495 on the other side of the Hudson River. If an expressway got built along 34th Street and/or through the Empire State Building as Robert Moses envisioned, that 495-to-95 connection might have happened. Similar situation with the Clearview Expressway, I-295, and its awkward end at Hillside Avenue. Moses envisioned that road continuing farther and getting back to I-95.You ll notice that the numbers for many three-digit Interstates are re-used in different parts of the country. There s I-295 in New York, then another I-295 carries the Delaware Memorial Bridge farther south. There are a few I-495s up and down the east coast as well.So, you re correct on the Interstates - one- and two-digit Interstates that run north-south are odd, and east-west are even. Three-digit ones connect to the one- and two-digit ones. The one- and two-digit ones are even laid out in sort of an ordered pattern, except for I-99, which is a story for another day.On the concept of Interstates crossing state lines - the Interstate Highway System was intended to create a network of highways that spanned the country, though each highway wouldn t necessarily cross state lines. In fact, many Interstates, especially many (most?) of the three-digit connector ones, stay within a single state. Some are under ten miles in length. Actually, the unsigned I-878, the Nassau Expressway (on the Interstate logs, but with signs indicating NY-878), is under a mile long. A few one- and two-digit Interstates serving major corridors, such as I-4 (FL), I-12 and I-49 (LA), I-16 (GA), I-17 and I-19 (AZ), I-27, I-37, and I-45 (TX), I-43 (WI), I-96 (MI), I-97 (MD), and yes, I-99 (PA), lie entirely within a single state.State roads do their own thing, such as NY-27.Good question, good observation.
I don t think that s an actual rule...we live in southern TX, and State Hwys 281 and 83 are definitely odd (in more ways than one).Plus, the LIE is also one of the few (if not the only one) of the Interstates that actually does not cross into a second state, so again, not that odd when you think of the LIE.