It is useful to think of SRs as either "long" or "short" (my terms). Long SR's are connecting roads, funneling rural traffic to and between primary highways and smaller villages. Short SRs provide access to residences. They usually either dead-end or lead to other short SRs that dead-end. 

Long SRs are usually paved. Although they are generally designed for slightly lower travel speeds than highways are, many long SRs are still suitable for travel at 50-55 mph. They tend to be narrower than highways (16-18 feet for SRs versus 22 feet for highways), with lower load limits and visibility. They usually carry only a few hundred vehicles per day, whereas most rural highways carry well into the thousands per day. 

Short SRs tend to be unpaved and lack most modern design features. They typically carry fewer than 100 vehicles per day, almost all of which are local residents.

SRs in most counties follow a rather rigid numbering scheme. Here we'll look at Burke County, which is situated in the foothills and contains part of the Blue Ridge. Burke is not overwhelmed by large cities and it contains no numbering "violations" I know of, so it's good to look at. Image #2 below illustrates the three stages of SR numbering: 10xx, first-pass and later-pass roads. 

10xx roads: A handful of long SRs are assigned numbers that start with 10. Numbering always starts at 1001 -- 1000 is never used -- and proceeds with 1002, 1003 and so on. (An exception is an SR 1058 which is an old alignment of N.C. 58.) Very few counties have 10xx routes above 1010. The 10xxs tend to be the more frequently traveled SRs in their counties, and often run between two or more "sectors" as explained below. 10xx roads often keep their numbers or at least receive a different 10xx number upon crossing county lines. 

First-Pass SRs: SRs are assigned clockwise around each county, always starting with 11xx roads in the southwestern corner. A county is divided into several "sectors" (my term again), with primary highways serving as the sector boundaries. In Burke, sector boundaries are cut by U.S. 64, U.S. 70, N.C. 18 and N.C. 181. Occasionally a 10xx road is used to divide sectors, as SR 1001 in Burke separates the 15xx and 16xx sectors. Some primary highways, such as N.C. 126 in Burke, are not used as boundaries. Some counties, especially those which have strange shapes or primary highways that do not go through the county seat, have sectors that are not wedge-shaped. But the majority of counties are divided simply, as Burke is. 

Each sector contains one or more blocks of 100. For example, the southwest sector of Burke (U.S. 64 clockwise to 70) contains SRs 1100 to 1199, and the northwest sector (70 clockwise to N.C. 181) contains 1200 to 1399. Consecutive SRs are often clustered within a sector, as 1129 through 1132 in Burke are near the McDowell County line. But there are no steadfast numbering rules beyond the sector level: any 11xx can be located anywhere with the 11xx sector, and on any type of road. 

The first-pass routes often get into the 2000s in the southern parts of counties. Some blocks of 100 are skipped in some counties. Low-population counties have as few as two sectors, but most are like Burke with several. At least two counties, Lincoln and Yancey, do not have a sector scheme. (Yancey can be forgiven because it's mostly mountains and its SRs still seem to follow a rough clockwise pattern. Anarchy seems to rule in Lincoln.) 

Later-Pass SRs: The original sector system did not accommodate growth well, with as few as 100 available numbers in any given sector. In time, additional blocks were assigned to faster-growing sectors. The 11xx sector in Burke got 21xx roads later on, and the 15xx sector was given 22xx roads. (The 20xx roads in Burke may have been part of the first pass or the second pass.) Although the second pass of SR numbers in Burke was assigned clockwise, it's not a rule. The blocks of numbers can be thought of similar to telephone area codes, with the 21xx numbers in Burke assigned as an "overlay" of the 11xx area. 

Some more populous counties have been assigned numbers in three or more passes. At least three counties, Mecklenburg, Wake and Guilford, have SRs running well into the 4000s. Mecklenburg apparently has 5xxx roads, but I haven't seen any. 

With the exception of a few 10xx roads, no effort is made to keep the same SR number on a road as it crosses county lines. in fact, the numbering system precludes it.

The first style is to indicate the SR number with an arrow on a fair-sized black on white sign (Image #3). This method seems to be limited to longer SRs, especially 10xxs, in the Piedmont. Although undistinctive in shape, these signs are adequately sized for what they do. The problem is that you can't count on finding a sign when you need one, and some parts of the state seem to completely lack them. Most of the larger signs I've seen look old, and the state may be phasing them out. 

The second style of SR signage is to attach numerals to a post at junctions (Image #4). While this format is common and consistent throughout the state, the numerals can be hard to read from vehicles and can come off easily. 

Under the third style, standard white-on-green street signs showing both the street name and the SR number are erected at rural intersections (Image #5). This method became popular in the early 1990s, when counties made SR names conspicuous for emergency 911 services. The apperance of the signs varies slightly by county (differences in size, lettering, placement of the SR number, etc.), but in many counties this method is rather reliable. Drawbacks are that not all areas have fully implemented this style of signage, the SR numbers can be small and hard to read, and occasionally the SR numbers are missing from signs. 
 

Once an SR number is assigned, it is rarely changed. Exceptions occur when long SRs are straightened out, bypassing older sections. Some roads are named, say, "Old SR 1223" but numbered SR 1289 (that is an actual example from Burke County). Occasionally SRs are upgraded to state highways, with an upgrade to the actual roadway usually accompanying the change in status. 

When cities grow, responsibilty for SRs that were previously outside the old city limits changes hands. The SR numbers of these roads are "retired" in the sense that they're forever associated with their original road and not reused. As an example, I once lived just inside the city of Charlotte, and while my street had been annexed in the 1970s, it still has an SR number today (SR 4320). Other streets in the subdivision had been annexed in the late 1980s, but in the early 1990s were still signed with SR numbers on posts. Also, Sharon Road in Charlotte is still SR 3600, even though it's been maintained by the city for decades. 

In 1989, the state passed a road bill that for a short time gave North Carolina the highest gasoline tax in the nation. The bill was opposed by legislators from rural districts until a provision to pave many SRs for the first time was included. 

City streets. Cities have responsibility over all roads within city limits which are not part of the state's primary highway system. Cities cannot widen or realign state-numbered highways without state approval. 

National Park Service roads. The two most prominent NPS roads in N.C. are the Blue Ridge Parkway and Newfound Gap Road in the Smokies. Also included are various spur routes from the BRP, secondary roads in the Smokies and access roads to other national monuments such as the Wright Brothers Memorial on the Outer Banks. These are maintained by the National Park Service, even to the extent that Newfound Gap Road is technically not part of U.S. 441. Roads within state parks have N.C. or SR numbers. 

U.S. Forest Service roads. Located within the state's four National Forests and numbered with one to three digits. Often indicated by the interchangeable prefixes FR, FS or USFS. Sometimes letters will indicate spur routes, such as FS 123B from FS 123. Usually signed with small brown markers at road junctions. Almost all are unpaved; condition varies greatly, and some have fallen into disuse. 

BIA roads. Some roads within Cherokee Indian territory are managed by the Bureau of Indian Affairs and the Cherokee Nation. 

Private roads. In addition to the obvious here (roads around commercial developments), at least one private university maintains a sizeable road network. Most traffic laws are enforceable on these roads; don't say you weren't warned. 

Put it all together and you get more than 100,000 miles of roads criss-crossing the Tar Heel State. To get any more in depth would require discussing each SR one at a time. That's beyond the scope of this site, but I hope you learned something here, anyway.