Odd routes are north/south and even routes are east/west. No duplication of numbering is allowed. Violations of these rules include MO 47 (over half this route is east/west and may be signed as such), MO 64 (it duplicates I-64), MO 7 (it runs east/west for the most part), MO 148 (it runs north/south), and MO 91 (it runs east/west).

Some oddities have occured in the system and they include:

• US 54: This highway runs east/west until Camdenton, where it turns north/south and is partially signed that way. There were "North US 54" signs near the Lake of the Ozarks, but that was in 1992.
• I-64: This highway is not refered as I-64 in the St. Louis area, but as Highway 40, the number prior to I-64. All St. Louisians call I-64 either "US 40" or 40/64, always with the "40" first.
• MO 725: This highway (now I-170) was maintained by the county of St. Louis before the state took over in the late 60's and gave the Innerbelt (then only from US 40 to Route D) this number. It became I-170 in 1983.

Secondary highways are letter-apprended routes that are maintained by the states, but the letters are unique to the county. There are really no rules behind them, provided that the same letter is not used twice in the same county.

Some oddities include:

• Route D (St. Louis County): This route goes from the city limits of St. Louis to I-270 and not only has two state highways terminate at Route D, but this route is getting extended into St. Charles County. Expect this route to be given an number when the extension gets completed even though there is some opposition to it.
• Route A (Douglas County): The route from MO 76 to the point it turns north/south is gravel and not paved. All other secondary routes are paved.

The original Nevada State Route system was renumbered around 1976 to its current system.  This system clusters primary and secondary routes by county, with some exceptions.  In the primary state system: Churchill has Routes 115-121, Clark has Routes 142-171, Douglas 206-208, Elko has Routes 221-233, Esmeralda has Routes 264-267, Eureka has Route 278, Humboldt has Routes 289-294, Lander has Routes 304-306, Lincoln has Routes 317-322, Lyon has Routes 338-342, Mineral has Routes 359-362, Nye has Routes 372-379, Pershing has Routes 396-401, Washoe has Routes 425-447, White Pine has Routes 487-490, and Carson City has Routes 509-531.

In the secondary state system: Elko has Route 535 (irregular), Clark has Routes 562-612, Washoe has Routes 646-686, and Carson City has Route 705

In the tertiary state system: Churchill has Routes 715-727, Clark has Routes 738-745, Douglas has Routes 756-760, Elko has Routes 766-767, Esmeralda has Routes 773-774, Eureka has Routes 780-781, Humboldt has Routes 786-794, Lander has Route 806, Lincoln has Route 816, Lyon has Routes 822-829, Mineral has Route 839, Nye has Route 844, Pershing has Routes 854-860, Washoe has Routes 877-880, and White Pine has Routes 892-895.

Exceptions to the rule: three state routes that carry their route numbers from adjoining states: State Routes 28, 88, and 140.
 
 

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New Jersey
Methods:
Primary: Clustering?; Secondary: Geographical

New Jersey's early primary state highways used low numbers in the north and high numbers (up to 52) in the south. If you look at a map, you will see evidence of this. In the 1950's, many of these numbers were changed, with new numbers being above 52.

• If a state route was combined with a US route, the state number was dropped, e.g. US 46 was also NJ 6 - NJ 6 was dropped. An exception would be to maintain route continuity, e.g. US 130 and NJ 33.
• If a state number duplicated a US number elsewhere in the state, it was changed, e.g. NJ 46 became NJ 77.
• If a state number had an 'S' prefix (for 'spur') or a letter suffix, it was changed, e.g. NJ S4B became NJ 208.
• If a state route crossed a state line, the neighboring state number was usually adopted, e.g. NJ 2 became NJ 17 (continuation of NY 17).
• If a state route was rerouted, the old road would be renumbered, usually as a 100-series number (old NJ 24 became NJ 124). Some of these routes are not related to the original number, e.g. old NJ 15 became NJ 181.

In addition to primary routes, NJ has "500-series" or "New Jersey Secondary" county maintained routes. They either supplement the state highway system or serve as feeder routes. Many of the routes are scenic and long, and make travel easy over circuitous routings in suburban areas.

The routes are numbered from 501 to 585, with even numbered routes running east-west and odd numbered routes running north-south. The east-west routes begin with 502 up north, and go to 540 in the south. Many of these routes cross the state. The north-south routes in northern New Jersey begin with 501 in the east, and go to 521 in the west. In north-central New Jersey, they start with 523 in the west and go to 531 in the east. There is a loose pattern in mid-Jersey of north-south routes, but in the extreme south they begin with 551 in the west and go up to 563 in the east. North-south routes 565 and above seem to be scattered throughout the state, as are some east-west routes above 540.

Some of New Jersey's 500-series routes have special designations of Spur, Alternate or Truck. There are sections of 500-series routes which use state highways or municipal streets for continuity purposes.

In addition to the 500-series of secondary highways, there is a "600-series" of county numbered routes. These routes are not unique in the state, as 500-series routes are. For example, a number of New Jersey counties could have a distinct "County Route 601". If a 600-series route approaches a county boundary, the neighboring county will try to continue the same number. Many counties have patterns of 600-series routes and some have no pattern at all. Some counties run out of "600" numbers and go into the "700-series". 600s and 700s make up county roads not included in the 500s.

Some counties do not use the 600- or 700-series but have their own systems instead. There are counties which have numerous county roads and do not number them all. Some counties have numbers such as 5, 33, 40-A, S-85 or 134. These numbers are generally very local and rarely have EAST-WEST NORTH-SOUTH designations. Other counties may number their roads on paper without posting signs. In urban areas, street names are more familiar to people than are route numbers. All counties use the 500-series.

The best way to research this is to look at a New Jersey map. Most Interstate, US, NJ and 500-series routes are shown. Some maps include significant county-numbered routes.

For an even more detailed description of New Jersey highway numbering, see Daniel Moraseski's page.

North Carolina numbered its highways in 1921 when it passed a "good roads" bill. Two-digit multiples of ten were used. The most important highway, the Central Highway, was given the lowest available number: NC 10. The state's other major east-west route received 20. Routes 30 through 80 were assigned to north-south highways; 30 was the easternmost such route and 80 was westernmost. NC 60 was a "diagonal" route, but still fit into the 30-to-80 pattern well. NC 90 was an east-west route that didn't go through as many important cities as 10 or 20.

The network of these primary routes was used as a base to number other highways of less importance. Such roads were given 2-digit numbers, with the first digit matching that of the parent highway. Off route 20, for example, came NC 21, 22, 23, all the way up to 29. Numbers were generally assigned in ascending order from east to west or from south to north based on where the spur routes hit the parent route. This rule wasn't perfectly obeyed, though. Three-digit state highways were almost always short; their first two digits usually denoted one of the 2-digit routes which they touched. For example, off NC 27 came 271, 272, 273... et cetera.

This system lasted up through the 1930s, when renumberings took place to eliminate state-US duplications. In the 1950s, more renumberings took place to eliminate duplications with Interstates. The upshot is that the remnants of the original system can still be seen in some places, resulting in the clustering.

This is a condensed version of the writeup on Matt "JVincent" Steffora's Highways of North Carolina site.
 

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Ohio
Methods:
Primary: Clustering; Seconday: Clustering and parent route

The short explanation is that Ohio uses a "clustering" system, in which groups of highways within an area are assigned sequential numbers. However, the numbering system in use today has evolved since 1924, when it was first put into use, and there have been many route numbering changes since then. Even so, it is very easy to see the clustering in many areas. For example, look in southwest Ohio, and you will see the 13x routes. Similarly, in northwest Ohio, you will see routes 11x.

The first highway numbering system in Ohio went into effect in 1912, when a series of "inter-county highways" were assigned numbers. This numbering system appears to have been for inventory purposes only, and there were apparently no signs marking these routes. Except in a few rare cases, the inter-county highway numbering system bears no resemblance to the current numbering system.

After the creation of the U.S. highway system, Ohio continued to add new state routes at a fast pace. The new routes were simply assigned sequentially, beginning with route 230, and going up into the 300s. Over the years, many routes were eliminated, rerouted, or combined, so that some numbers disappeared or were reused. There is no policy about reassigning unused numbers. The state simply maintains a list of unused numbers, and when there is a need to use a new number, the state picks one from its list as a proposed route number, subject to public comment.

In 1938, Ohio created a large batch of new state routes. For some reason, the entire 400 series was skipped. The new routes were assigned numbers in the 500, 600, and 700 series, with the 500 series in general being the most important and the 700 series in general being the least important.

Most highway numbering changes since 1962 have been relatively minor. The 400 series began to be used in the 1960s. The 400 series numbers were given mostly to routes which previously carried other numbers. Beginning in 1970, 800 series numbers began to be assigned, starting with route 800, a newly renumbered segment of the old route 8. For the most part, routes in the 400 and 800 series are not assigned sequentially. Instead, they are assigned according to the numbers of the routes they intersect or are near. For example, route 835 intersects US 35 on both ends.

An extended version of Ohio's numbering pratices can be found at John Simpson's site.
 

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Pennsylvania
Methods:
?????

Doesn't seem to be any method to the madness here, either. Numbers used in two distinct places include 29 (Philadelphia to Allentown and Nanticoke to Sayre), 97 (Gettysburg, continues to MD and near Erie), plus 17 (in Perry County and from I-90 into NY State and then NJ; it's a juggernaut). There is no duplication between other numbers, whether US or interstate.

The numbering system starts at 3 and goes to 999. Roads above 999 are secondary or township routes. County numbers exist, and generally use a clustering system. County numbers are 4-digit and usually are numbered 1xxx to 4xxx, and change at county lines (not continuous). They are marked with small white rectangles roughly 1/2 mile apart as SR XXXX and a mileage marker system, that works like this:

SR 0322
        60

This means survey segment 60 on SR 322 (actually U.S. 322, but regardless of Interstate, U.S., or PA designation, it always says SR).  On Interstate routes, the survey segment usually corresponds to milepost times ten, but not always.  These segments are used, among other things, to number bridges with a fourteen-digit BMS (bridge maintenance system) number.  The first two digits are the county number (01-66 alphabetical, but Philadelphia is 67); the next four are state route number, although I've seen some 7xxx numbers on city-owned bridges; the next four are survey segment; and the final four are the offset, in feet, from the beginning of that survey segment.  For example, 36-1010-0050-1342 is located 1342 feet from the beginning of survey segment 50 on State Route 1010 in Lancaster County (county number 36).  Incidentally, the same county numbers were used to form the first two digits of the five-digit legislative route (LR) numbers.

Recently, PA started numbering freeway exit and entrance ramps with a 8xxx number.

State Highways are either traffic routes (a very OLD designation) or secondary routes. Both types have little white square signs with the road number SRxx and a number. Except on interstates, this number indicates a maintenance section (C.C. Slater got that from a guy at PennDOT). This is a fairly new system -- implemented in the 80's to replace 5-digit "Legislative Routes".
 

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South Dakota
Method:
Geographical

Even numbers go east and west; odd numbers, north and south. For the most part, the lower numbers are on the north and east sides of the state, and the higher numbers are on the south and west sides. For example, it ranges from SD-10 on the north, to SD-52 in the far south. Also, SD-11 on the east to SD-89 in the west.

There is no duplication of numbers, although some state numbers have been recycled over the years.

There appears to be only a couple cases where there was a 1-digit number, and these were before 1950. SD-8 was once used on the current SD-20 segment west of the Missouri River, and SD-9 was once used in the northeast corner of the state. There appear to be no even-numbered, 2-digit routes higher than 54 that have been used (except I-90, of course).

The 3-digit routes appear to follow the even/odd pattern, but there appears to be no real method of how they are assigned. Most appear to have been assigned after 1950.

A special case is SD-1804 and SD-1806. These run along the east and west sides of the Missouri River, respectively, and reflect the travel periods of Lewis and Clark in the area. They generally run north-south, but whether they are signed that way is unknown. These were assigned around 1976.
 
 

Sources:

• Al "alt" Tossoonian: NJ
• William F. Yurasko: More NJ 600-series, PA
• C.C. Slater: More PA info
• S.D. Rhodes: NC
• Andrew Field: NV
• John Simpson: OH
• Kim "Harv" Harvey: MO
• Scot C. Benoit: PA county info
• Chris Geelhart: SD
• Dan Moraseski: NJ
• Justin Spivey: PA

If you have information on how a state numbers its highways, don't hesitate to e-mail me.

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