State Highways

• Primary Highways: (usually one- and two-digit numbers)
  • In a geographical system, the highways are numbered in a way similar to interstate and US highways. The numbering starts on one side of the state, and increases as you go across (ie Florida). With this system, there is also some sort sort of rule where odd numbers are one direction and even the other direction, but not always.
  • In a clustering system, highways in the same area of the state are around the same number. However, the "areas" are probably arbitrary, and cross-state highways have to, by definition, violate this system.
• Secondary Highways (usually three-digit numbers):
  • When a secondary highway is numbered with the parent route system, usually two numbers from the parent route are used, and then another digit is slapped in front or behind of those two digits.
  • Some states also use clustering for secondary highways.
  • Secondary highways can also be numbered by county.
  • And then there are states which also number secondary highways geographically.

• Some states do it in a legislative manner. This can be done a number of ways, either with the legislature asigning each route a legislative number, or similar method. Sometimes, this is the reason you will see clusters of numbers.

To find out much more detailed info about numbering in a specific state, try looking at the highways page for that state.

Other State Highway Notes

Most of the time, a three digit state highway is just some sort of spur, link, or loop. However, there are MAJOR violations of this rule. The one that comes to mind immediately is CO 470, which is freeway for its entire length. This is a case where a three-digit interstate was proposed, or it existed, but the state took over, and the state highway took on the number of the three-digit interstate.

A lot of state highways get their number by being a decommissioned US highway. Decommissioning is what it's called when AASHTO decides that a certain US highway should no longer exist along a certian route, so the number is decommissioned, leaving the state to do what it wants to. Examples of US -> State decommissioning abound in every state. For example, MN 16, 61, 210, and 371 are all the old routes of US 16, 61, 210, and 371 in Minnesota.

To get more info on decommissioned US highways, go to James Sterbenz's US Route list or Robert Droz's Unofficial US Highways Page and look for the italicized routes.
 

Some Random Thoughts from E-mails

Scott "Kurumi" Oglesby:

Arizona, Indiana and Pennsylvania number most 3-digit routes as spurs off the 2-digit ones (i.e. 16 -> 116, 216, etc.) PA's 9xx routes are mostly unrelated to 2-digit routes.

Washington also does this, but as a prefix: I-82 gives you 821, 3 gives you 304, 305, etc; I-5 gives you all the 500-series numbers in its corridor.This is much better than WA's old system where Route 5 had about six different branches, all labeled route 5! I'm not kidding, that road had more than two ends.

Hawaii has sort of a Washington system going; Maui has roads in the 30's and 300's for example.

Hawaii and Florida have 4-digit roads that follow the 2- and 3-digit roads. You've seen 9336 in Florida, but there's also some 2000-series routes.

NARN Home | MDRoads Home