Clustering is prevalent (x9 routes near US [now AZ] 89; US 80 near AZ 84, 85, 86). Then there are route continuations (old AZ 93, current AZ 180A, and technically AZ 89). State routes can have alternate routes (old AZ 86A and current AZ 89A), and are designated xxA instead of Alternate xx. There are three-digit spurs off all types of routes (AZ 210, AZ 360, AZ 188). Some highways are numbered by association (AZ 95 created because it parallels US 95, AZ 51 created out of former I-510 designation). Lastly there are the Loop Routes x0x (101, 202, 303) in metro Phoenix.
 
 

Before 1964, California routes had two distinct numbers: the number with which the route was signed, and the number with which the legislature identified the routes (LRN). A single LRN could be used over different sign routes, and vice-versa.

Legislative routes were initially established with legislation in 1909, when routes 1 through 25 were defined. They have been extended by legislative acts over the years.

Before 1934, the only routes that were signed in California were the US Highway routes (signage of these started in 1928). In 1934, the state began assigning sign route numbers to routes in the state. These numbers were assigned as follows:

• The odd/even to direction relationship is the same as for US Highways (i.e., odd is North/South, even is East/West).
• Route numbers are not duplicated with US Highway numbers signed in 1934.
• Smaller numbers are used primarily in major metropolitan areas.
• Route numbers are assigned in groups of two: two for Northern California, two for Southern California, two for Northern California, etc.
• Space has been left in the numbering plan for future routes.

In 1964, all state routes were renumbered to bring their legislative route number in sync with their signed route number. There was also a renumbering to retire some US highway numbers, and to sign with Interstate route numbers. This removed any remaining semblance of order in the numbering of state highways.

This was a direct verbatim cut-and-paste from Daniel Faigin's site.
 
 

Connecticut Began posting numbers in the early 1920s, and had three conventions:

• 1-99: US routes or New England interstates
• 100-199: major state routes
• 300-399: minor state routes
• There were a few routes in the 200s.

In 1960, the state abolished its county-level governments (Connecticut counties are now little more than lines on a map). In the few years following, it reallocated several hundred miles of road between town and state control. Some routes were deleted; many more were added. A good rule of thumb is that any route number above 220 (note the exceptions above) was created circa 1963. About 1/3 of the state's highways were affected by adding, deleting, or rerouting; however, many highways on a 1940s map will look familiar today.

There are several loosely clustered areas of consecutive numbers, e.g. 41, 43, 45, 47, 49 (since moved) in the NW corner, and 83, 85, 87, 89, 91 (moved), 93 (moved), 97, 95 (moved) going west to east in the east half.

Also created in the 1960s was the "secret" route system: short roads that the state maintains but doesn't signpost. They have route numbers from the 400's up into the 900's. You might notice that the secrets sometimes leak out: Rand McNally has been marking Route 664 near Killingly for years, though you'll never see a sign for it.

Connecticut has the "A" suffix on five alternate routes. There were many more in the 1940s and 1950s (about 20).

The quick stats:

• number of signed route numbers: about 205 existing; over 30 no longer used
• number of secret route numbers: 225 existing; about 45 no longer used
• lowest and highest signed numbers: 2, 372
• lowest and highest secret numbers: 401, 919

Delaware
Methods:
Primary: Geographical; Secondary: None needed

State highways running east and west generally are even. Starting in the northern part of the state with 2, even numbers generally increase in pattern. The major exceptions to these standards come from routes that enter from another state, such as 273, an east-west route coming from Maryland.The highest numbered Delaware route to not leave the state is 72. 100, 273, 299, and 404 all come from other states.

North-south routes are numbered odd and start at 1, along the ocean. They are much looser in pattern, but the state is very narrow east-west. Exceptions are roads from other states like 100 and 896.

US routes may be duplicated, as the case with 9. There are not duplications of interstates, but it is doubtful that there is anything on the books about that since there are only three.
 
 

(Hey, it may not be a state, but it is still a jurisdiction that can create highways.)

Washington, DC, has one numbered route, DC 295. It's an extension of I-295 and was created about 1990.
 
 

Florida follows a pattern with low numbers in the north and east and high numbers in the south and west. Starting with 2, the even numbered east-west routes go to 84. North and south routes start with US 1; 3,5,7 etc.There is no Florida 1, but A1A which is follows US 1 could also be considered an informal beginning of the system.

Three digit FL routes are numbered to coincide with the set of ten they are in. For example FL 786 is between 70 and 80. Secondary routes in Florida are continuations of primary routes, i.e. 210.

Florida has a "hidden" route system where all highways have a state number. I-10 is also numbered as FL 8. Others: I-95=FL 9, I-75=FL 93, I-4=FL 400. US roads also have "hidden" numbers.

Before 1946, Florida had a system that was chaotic at best.

Robert Droz found the official description of the current system on a Florida State Road Department July 1946 map:

Pursuant to a Legislative directive to follow a systematic plan of numbering, reduce excessive numbers and eliminate duplication, all State Roads have been assigned new numbers.

EXPLANATION OF STATE HIGHWAY NUMBERING

ALL ROADS leading from north to south bear odd numbers with the number one assigned to the extreme easterly road. All roads leading from east to west bear even numbers with the number two assigned to the extreme northerly road.

Certain control roads have been selected for the purpose of dividing the state into segments within which an appropriate numbering plan may be effected. The north and south control roads are one and two digit numbers ending in 5--- 5-15-25-35-45-55-65-75-85-95; the east and west roads are two digit numbers ending in zero --- 10-20-30-40-50-60-70-80-90.

There are four important diagonal road ; two leading from northeast to southwest, and two leading from northwest to southeast --- these are numbered 100-200-500-600.

Main connecting roads leading from north to south are assigned one or two digit numbers between the limiting control routes. For example, north and south main connecting roads between routes 15 and 25 are numbered 17-19-21-23. Main connecting roads leading from east to west are assigned numbers in a similar manner. For example, east and west main connecting roads between control routes 50 and 60 are numbered 52-54-56-58.

Secondary or short length roads are assigned three digit numbers. The first number of the three digit number places the location with relation to the east and west control routes as follows:
Between Fla. and Ala.-Ga. Line and Route 10 --- Numbers 101 to 199
Between Route 10 and Route 20 --- Numbers 201 to 299
Between Route 20 and Route 40 --- Numbers 301 to 399
Between Route 40 and Route 50 --- Numbers 401 to 499
Between Route 50 and Route 60 --- Numbers 501 to 599
Between Route 60 and Route 70 --- Numbers 601 to 699
Between Route 70 and Route 80 --- Numbers 701 to 799
Between Route 80 and Route 90 --- Numbers 801 to 899
South of Route 90 --- Numbers 901 to 999

For example, Secondary Road 505 would be located between control routes 50 to 60 and would be found near the easternmost portion. Secondary Road 196 would be located between the Fla.-Ala. Line and Route 10 and would be found near the westernmost portion.

Hawaii
Methods:
Primary: Clustering

The method is clustering by Island:

• Hawaii (the Big Island) has 1x-2x
• Maui has 3x
• Molokai and Lanai have 4x
• Kauai has 5x
• Oahu has 6x-9x

The original Illinois state highways were simply numbered in order of origination, and consisted of little more than numbers placed on poles of existing trails. Before that important routes had names, such as the "National Home Trails" or the "National Road"

An organized system of numbering state highways started in 1918 with the first State Bond Issue (SBI) Routes, 1 thru 46. SBI Routes 47 thru 185 were authorized in 1924. Bonds were floated to pay for specific routes. SBI # 1 paid for Route 1, and so on. Remarkably, many of these numbers still exist on the original or nearby alignment. As the highway system grew the numbers were altered to accommodate new roads or extensions of older roads. When the US Highway System of Interstate Roads started to be posted in 1926, the US numbers were just tacked onto the existing IL/SBI number unless the US route was routed along a new route.

Eventually the IL/SBI numbers were dropped when it was practical to do so. Other IL (and US) routes were dropped over the years as the Interstates were built, and traffic patterns changed.

SBI Numbers are still used in several instances. IDOT District maps still refer to SBI numbers on the various roads it maintains, along with other non-posted designations that refer to how the route was authorized. Bridge weight plates refer to SBI numbers instead of posted route numbers as well. If you look at a bridge plate along old US-66, you will see the route referred to as "SBI-4". Some IL routes had letter appended to them. These routes have been eliminated over the years by changing to regular numbers or removing the number altogether. The last one was IL-116A which became IL-117 in the early 1990's. These are considered Spurs or Alternates of the parent route, and letters used have been A,B,C, E (East), N (North), S (South), and W (West). These should not be confused with the US appendages, as different rules were used.

IL Numbered routes do not follow any other known pattern. The "Even/Odd" configurations of the Interstate and US systems do not apply for IL routes. Some routes in IL are multidirectional, in that they do not have ANY posted direction. Sometimes numbers are assigned because they sound good, or the route is related to another route, or because the number had once been assigned to a portion of the route.

Here are a few of the IL Route Number Oddities:

• IL-137 in Lake County runs N-S from the WI line thru Waukegan and North Chicago, then heads due West to the Round Lake area. None of the IL-137 signs have a directional indicator on them. This had been an EW Route until it was extended north in the 1990's to cover a small freeway in Waukegan. Portions of the non-freeway N-S leg was originally IL-42.
• IL-172 today is weird enough in that it is an "L" shaped route from south of Rock Falls to Tampico and IL-92. Prior to 19XX this route was "T" shaped and extended west to Prophetstown!
• IL-38 was numbered that since it had a good sound! The route was the original US-30, also known as the Lincoln Highway west of Geneva. It became US-30-A (or Alternate US-30) after US-30 was rerouted south of the original road. After the Great Purge, the portion of US-30-A from Dixon to Bellwood became IL-38. Sound this out...
• IL-40 now runs from Mt. Carroll to Peoria. This had been IL-88 for many years. When IL-5, the East-West Tollway, was changed to I-88, IL-88 had to change since it crossed I-88 near Rock Falls. In the 1930's, IL 40 existed from Sterling to Mt. Carroll. IL-40 disappeared when IL-88 was extended to Mt. Carroll in 1938. When I-88 was placed on the Tollway, IL-88 had to go.
• IL-54 had originally been where part of IL-83 is now. After US-54 was removed from Pittsfield to Chicago IL-54 was applied from Springfield to Onarga.
• In 1964, IL 43 and 112 were decomissioned as with US 67A. IL 3 was returned to the old routing in Madison County, IL for the first time since the mid 30's. (Most of the originial route has been replaced by a four lane highway and part of it is ironic because it was first marked IL 3 until the Berm Highway (now 143) was opened in the mid 70's. When the "Switch" happened in the Alton area during Thanksgiving 1987, that one section from new 143 to old 143 was remarked IL 3.)
• IL 112 was dumped in favor of IL 159 (today a massive 10 mile bottleneck from Fairview Heights to Edwardsville). 159 once turned west at Edwardsville going on the current routing of 143.

• IL 43 was the old number for IL 4 from Marine to Murphysboro. That highway got decomissioned in 1964.
• IL 11 is retired highway number in IL and will never be comissioned again. It was the original number for US 40.
• IL 2: Before the East-West Tollway was completed in the late 70's, the old IL 5 route from Quad Cities to Dixion was marked IL 2.

Once again, a pretty neat system. It is a geographical system like the interstates, where the numbers start on one side of the state and increase as you go across. It works better here than Florida's system, because Florida is so strangely shaped, but Indiana is rectangularish to begin with. Ever notice how Indiana 37 is exactly where a hypothetical U.S. 37 could go?
 
 

Sources:

• William F. Yurasko: DE, DC, and FL
• Matthew "JVincent" Steffora: IN
• Andrew Field: CA
• Steve Riner: More CA info
• Rich Carlson: IL
• Kim "Harv" Harvey: More IL info
• Dan Moraseski: More FL info
• Daniel Faigin: More CA info
• CC Slater: HI
• Scott Oglesby: CT
• Robert V. Droz: Yet more FL info
• Chris "Calvin" Sampang: AZ

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

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