Florida State Highway System
The State Highway System of the U.S. state of Florida comprises the roads maintained by the Florida Department of Transportation (FDOT) or a toll authority. The components are referred to officially as state roads, abbreviated SR.

Numbering
State Roads are always numbered; in general, the numbers follow a grid. Odd numbered roads run north–south, and even numbered roads run east–west. One- and two-digit numbers run in order from 2 in the north to 94 in the south, and A1A (formerly 1) in the east to 97 in the west (99 used to exist but is now a county road). The major cross-state roads end in 0 and 5.

Most routes of the form X00 are major diagonal routes; an even first digit indicates a southwest-to-northeast direction, and an odd first digit indicates a northwest-to-southeast direction.

Other three-digit numbers are placed in horizontal bands based on the first digit:

Three-digit numbers increase from east to west across the band; 30 is skipped because it runs along the Gulf Coast in the panhandle and doesn't go all the way across the state.

When the grid was first laid out in 1945, the rules were almost perfectly followed.  However, over the years, as routes have been added, there has not always been room to follow the grid.  Placements such as 112 (in the 8 band), 752 (in the 2 band), and 602 (in the 1 band) are the most notable violations of the grid system. The Pensacola area has a collection of these "misplaced" street numbers.  When FDOT added route numbers to a collection of Miami-Dade County streets in 1980, most of them received 9## designations regardless of the band that they occupied.

Every section of U.S. Highway and Interstate Highway has a State Road number assigned to it, usually unsigned (for example, Interstate 4 is also unsigned SR 400). In addition to some named toll roads (for example, 91 and 821, which make up Florida's Turnpike) some minor State Roads are also unsigned (like SR 913 and SR 5054).

History

Prior to the 1945 renumbering, State Roads were given numbers in the order they were added to the system.  The 1945 renumbering removed many roads that were never built and added some that had not existed prior to 1945.

In 1955, the Florida Department of Transportation slowed down the addition of new state roads and began to classify roads into primary, secondary, and local roads.  Primary roads would continue to be state-maintained, while Secondary roads would have an S before the number, and would only be state-maintained during a construction project.  Local roads would be completely removed from the system.

In 1977, FDOT changed the division of roads into state/county/local.  Most secondary roads and some primary roads were given to the counties, and occasionally a new state road was taken over; some main roads in incorporated areas were given to the localities.

The secondary signs had the S changed to C (for county) and a small COUNTY sticker added to the bottom.  As signs grew old, they were replaced with the standard MUTCD county road pentagon.  While this occurred throughout the State of Florida, the part of the state south of SR 70 was hit particularly hard by the transition from State to County control and maintenance.

In the early 1980s several state roads were renumbered; in the latter half of the 1990s, budget cuts and other factors prompted a series of truncations of several state roads, primarily in urban areas and the Space Coast and the Treasure Coast.  The trend seems to have been reversed since 2002 as new state road designations have been added as a result of construction of new highways, most notably in the Jacksonville, Orlando, and the Tampa-St. Petersburg metropolitan areas.

Interrupted state roads
While most state roads are contiguous, there is a relative handful of routes that have interruptions in their designations. 

The most famous of the set is SR A1A, which exists in seven separate pieces along the Atlantic coast from Fernandina Beach to Key West.
State Road 2 has two sections separated by the State of Georgia. The western segment extends westward from Georgia 91 as it crosses the Chattahoochee River and has its western terminus at SR 81 near Sweet Gum Head; the eastern segment crosses the Okefenokee Swamp to connect separated segments of Georgia 94.
State Road 5 temporarily ends leaving Lake Worth, FL as its segment in West Palm Beach was relinquished to the city in the mid 2000s. The route resumes at US 1's junction with Belvedere Road, where it runs concurrent with US 1 northbound.
State Road 15 has two sections bridged by County Road 15 and US 192/441.  SR 15 is only signed in the Orlando area and in Palm Beach County. For most of its route, SR 15 is an administrative FDOT designation for US 441 south of Holopaw, US 17 between Orlando and Jacksonville, and US 1/23 north of Jacksonville.
The two separate sections of SR 17 formed when US 27 was rerouted in Highlands County, where it passes through Avon Park and Sebring, and in Polk County, from Haines City to Frostproof. Originally signed Alternate US 27, it is now signed as just SR 17.
State Road 25 cosigns with US routes throughout most of its length, but briefly departs and travels on its own road in Lake and Marion counties. However, all but less than half a mile of this road has been relinquished to the counties, interrupting SR 25.
State Road 30 is gapped by Bay County Road 30 on Front Beach Road west of the Panama City Beach limits to the road's westbound cosign with US 98.
Three sections of State Road 44 exist. Two are connected in Lake County by US 441 and County Road 44. The third is isolated over the Halifax River in New Smyrna Beach due to a route relinquishment to the city. Trailblazers exist down the former route to direct motorists to the continuation of SR 44.
State Road 54 has a gap in eastern Pasco County, between the western terminus of State Road 56 and Bruce B. Downs Boulevard in Wesley Chapel. It also contains a former segment between 301 in Zephyrhills and US 98 in Polk County.
There are two separate segments of State Road 61 in the Tallahassee area.
State Road 78 spanned southwestern Florida from the northern tip of Lake Okeechobee to Pine Island on the Gulf Coast, but during the late 1970s, the FDOT attached an S-prefix on the designation of the section between SR 29 and SR 31... and started a process that converted the segment into County Road 78 connecting the remaining pieces of SR 78.
State Road 80 has a gap in downtown Fort Myers after a segment of the route was relinquished to the city in January 2006. The continuation, less than 1/5 of a mile, connects the west end of SR 82 to US 41.
Until 1993, State Road 84 traversed the state from the Atlantic Coast to the Gulf Coast. When the upgraded Alligator Alley became part of Interstate 75, the unsigned FDOT designation of SR 93 was applied to the toll road, and the end pieces of SR 84 retained their number — and their signs. SR 84 is also a pair of frontage roads that run parallel to I - 75 and I - 595 from Glades Parkway exit to just west of Florida's Turnpike, with a short gap before the eastern end piece emerges from I - 595 as its own road east of US 441.
There are two separate roads in Okaloosa County with the SR 189 designation: a bypass around Fort Walton Beach and a road going from Baker to the State of Alabama. Both were at one time planned as a single continuous route.
SR 211 in Jacksonville is broken into two segments. The southernmost segment consists only of the Ortega River Drawbridge. The second segment connects SR 128 to Interstate 95.
SR 228 is separated into two segments in Duval and Baker counties by a stretch of former SR 228, now maintained by both counties.
At one time, SR 231 extended from US 90/SR 10 in Olustee to SR 121 near Gainesville. FDOT relinquished most of the road to the control of the counties it runs through, except for a segment in Union County, and another running through Bradford and Alachua counties.
SR 406 in Titusville has a maintenance gap on A. Max Brewer Memorial Parkway between US 1/SR 5 and the bridge over the Intracoastal Waterway. Brevard County, who maintains the road gap, instead chooses to sign it as County Road 402. The county also assigns this number to the road's continuation after the eastern terminus of SR 406.
SR 527 exists in two segments in Orange County. A former segment of the road in downtown Orlando was relinquished to the city.
In Winter Haven, US 17 connects the two separate sections of SR 540.
SR 590 has a 2-mile gap in Safety Harbor. After the segment was relinquished to Safety Harbor in 1995, the city added "TO" trailblazers to the existing SR 590 shields to direct motorists to the continuation of the state route.
SR 732 in Jensen Beach exists in two segments. The original SR 732 on Savannah Road was relinquished to Martin County and renamed CR 732. Meanwhile, FDOT moved the SR 732 designation to the Jensen Beach Causeway. In 2007, FDOT adopted Jensen Beach Boulevard (CR 732, former SR 707A) as part of the route. Unlike most relinquished segments, CR 732 does not connect the two segments.
A segment of SR 806 on Atlantic Avenue in Palm Beach County was relinquished by FDOT in downtown Delray Beach, creating an official gap in the route.
Two sections of SR 811, both of which run roughly parallel to U.S. 1, are found in Fort Lauderdale and near Palm Beach Gardens and Jupiter.
A segment of SR 820 on Hollywood Blvd was relinquished by FDOT in downtown Hollywood after requests from the city to gain control over the road. This has split the official route into two segments. No shields exist on the Hollywood maintained road.
SR 825 has existed as an uninterrupted state road in Kendall, but FDOT adopted a locally maintained segment of its host road further north in Tamiami to accommodate the Dolphin Expressway (SR 836), and kept the SR 825 designation for the separate northern segment.
At one time, SR 865 extended from Estero Island (near Fort Myers Beach) to Tice. Today, SR 865 consists of two separate sections (one running north–south and the other running east–west) connected by a stretch of a county road that was originally part of SR 865.
State Road 909, also known as West Dixie Highway to the residents of North Miami, has a two-block-long gap, where drivers are greeted with signs saying "TO West Dixie Hwy/TO 909" while they drive a short stretch of Northeast 125th Street (SR 922) connecting the two pieces.
State Road 951 used to run from Marco Island to Big Cypress Swamp. FDOT relinquished most of the route to Collier County and Marco Island, leaving only two segments: one from Marco Island to US 41, and a smaller piece connecting SR 84 to I-75.