27 May 2008

Facts about Louisiana's highway system


  • Louisiana has 6 primary interstates or 2dis: 10, 12, 20, 49, 55, 59; and 6 secondaries or 3dis: 110, 210, 310, 510, 610, 220.
  • IH 12 and 49 are “intrastate” interstates.
  • The longest interstate is I-10: 274.42 miles long.
  • There are 893 miles total in La.’s Interstate highway system.
  • The first section of La.’s interstate system to be let for construction was I-59 from Slidell to the MS state line. Construction began in late 1956.
  • The last section to be completed was I-49 through Alexandria, opened on 1 May 1996.


  • The current state highway numbers were assigned in 1955. This replaced a previous system which had its origins in 1921.
  • State roads are designated in ranges 1-191, 300-12xx, and 3000-32xx (the last two ranges are constantly increasing). The routes up to 185 were classified in 1955 as belonging to the primary and secondary systems. LA 191 was added much later, around 1980. Portions of these lower numbered routes and all routes 300-1241 belong to the tertiary system. Routes with numbers higher than 1241 can belong to any system as they are post-1955 additions.
  • Longest road: LA 1, 436 miles
  • Shortest road: Spur LA 3066, a whopping 0 miles (!), according to the route log; LA 364, 0.05 miles, a bridge over Bayou Lafourche (which intersects LA 1) – this ties with many other 0.05-mile routes
  • Longest bridges:
    • Lake Pontchartrain Causeway: 23.87 miles
    • IH 10/55 viaduct, St. Charles/St. John the Baptist Parishes: 23.08 miles
    • IH 10 over Atchafalaya Floodway: 17.9 miles

Useless Trivia:

  • There are ~1292 state highway routes (SRs) in La. (as of 2002), not counting interstates or US highways.
  • Of the 1292 SRs for which length is known:
    • 213 are shorter than 1 mile in length
    • 368 are shorter than 2 miles in length
    • 486 are shorter than 3 miles in length
    • 606 are shorter than 4 miles in length
    • 676 are shorter than 5 miles in length
    • 751 are shorter than 6 miles in length
    • 950 are shorter than 10 miles in length
    • 1057 are shorter than 15 miles in length
    • 1126 are shorter than 20 miles in length
    • Only 12 routes are longer than 100 miles in length
  • The longest tertiary SRs (numbered routes 300 and above):
    • LA 308: 82.26 miles
    • LA 577: 61.43 miles
    • LA 507: 60.69 miles
  • The shortest non-tertiary SR (numbered routes 1-299, excepting spurs of these routes) is LA 180, all of 0.12 miles in Pineville.
  • The longest SR entirely within one parish is LA 103 in St. Landry Parish, at 39.91 miles in length.

An Explanation of Louisiana’s “Hyphenated” Routes

The Louisiana state highway system’s most ubiquitous and unique anachronism is the infamous “hyphenated” routes. These routes were created with the 1955 renumbering, and are a legacy of the assumption by the state through the years of many otherwise local streets in cities and towns throughout the state. The state-maintained city streets were/are often short sections of road, usually interconnected with other state-maintained local streets in the vicinity. Because of the interconnectedness of these state-maintained streets, as well as their close proximity and extremely short length, it was decided for practical purposes not to separately number each and every street in a town as a separate state route. Instead, each street was deemed a ‘section’ of a larger whole, with the aggregate comprising a single state highway; this becomes obvious when reading the 1955 statute that defines the newly redesignated SRs. The separate sections are denoted by numbers in the statute, which correspond in signage to the last digit in a hyphenated SR number. For example, LA 560-3 is section 3 of LA 560.

Similar as it may sound, this method is different from SR legislative definitions in states such as California, where SRs are often defined as existing in disjoint sections; but in these cases, there is often a linear continuity of a route through cosigning or implied connections made via other routes. In Louisiana’s case, the base “route” usually resembles a web-like or disconnected pattern; thus the distinction between sections in signage - or else there would be multiple, often intersecting routes with the same number, and real confusion would ensue.

Over the years many of these local streets/hyphenated SRs have (rightfully) been returned to local control, thus decommissioning parts of, or entire “families” of, hyphenated routes. For example, the LA 466 family originally had 17 sections, all within the city of Gretna. Most of these sections have been killed; the remaining section lost its “hyphen” and was renumbered as plain LA 466. In Baton Rouge, the LA 950 family had 17 sections; all are extinct. Some families still survive intact, such as the six sections of the LA 830 family in Bastrop. A few have been renumbered, almost always to 3000-series routes: LA 3155 in Metairie was once LA 611-13, and former portions of LA 611-3 and 611-4 that were severed from the rest of their routes were redesignated LA 3261 and 3262, respectively.

Most, if not all, of these hyphenated routes comprise the true inanities of the state system. Invariably they are short, local streets that serve no state purpose whatsoever; many are dead ends, residential side streets, etc., and sometimes end in arbitrary places. The majority of these routes can be found in urbanized areas, though there are a few that exist in rural surroundings. All of the hyphenated routes in the nomenclature are found among the original secondary SR system (routes 300 to 1241). There are no 3000-series hyphenates, due to the fact that all hyphenates were created with the 1955 renumbering (the 3xxx routes were later additions/renumberings).

Well, at least these make more sense than “LA 99½.” :-)

A History of Highway Numbering in Louisiana

Highway numbers in Louisiana first appeared in 1921 with the creation of that year’s reformed state highway system. Routes 1 through 98 were defined that year. These first 98 routes remained consistent throughout the pre-1955 era. The lowest numbered routes seem to have followed major auto trails; for instance, LA 1 was the Jefferson Highway, LA 2 was the Old Spanish Trail, etc. The remainder of the numbering system seemed to work on a lower-number, higher-order principle, with some clustering; for instance, LA 61 and 62 both existed in St. Bernard Parish. When US highways were added in 1926, the US designations were simply overlaid over the preexisting SR designations in a method similar to modern Georgia (the SR was included in signage as well).

Other routes were added as time went on, numbered in consecutive fashion, starting with LA 99 in 1924. By 1926 there were 162 defined routes; by 1929, 490. The number of designated routes increased precipitously during the Huey Long era, with 1325 routes defined by 1930 and more to come. A few routes were given “half” numbers, such as LA 99½ and LA 1315½. I have no clue as to why. (LA 99½ was actually redesignated in the pre-1955 era, as LA 2203. I guess that ‘half’ business got old after a while.)

The pre-1955 system eventually reached the 22xx numeric range (or so) at its zenith. There were also strange “C-xxxx” roads, generally in the 13xx and 14xx range, the purpose of which is unclear. All roads were seemingly numbered in the order that they were taken into the system, which led to anarchy, inconsistency, and disorder prevailing among the system of numbered routes. There was no pattern of clustering of the higher numbers as is demonstrated with today’s system. Route designations were sacrosanct; apparently they could not be rerouted to take advantage of a newer alignment, though for the most part the major routes by and large retained consistent numbers. Not all numbers were assigned to existing roads; some roads were merely “projected,” which is to say they were only lines on paper. Apparently there was no such thing as decommissioning, though state roads were often improved only “if funds were available.”

The 1955 renumbering renumbered all routes roughly based on the A-B-C system of route classification: A was primary, B secondary, and C farm-to-market (or as I call it, tertiary). The A routes mainly comprised one and two digit highways, the B routes three digit routes below 300. All routes 300-1241 were classified C routes, and clustering was and is still apparent in their ranks, especially with routes 700 and above. The A and B “primary” route range was 1 to 185. For some reason no 2xx numbers were used; this range may have been intended as an expansion area for future primary route designations (this was never done). LA 191 was added around 1980 as the Toledo Bend Scenic Drive, and is the only primary route designation to be added after 1955.

3xxx designations were given to all new post-1955 SRs, and were the only new type designations, until a few years ago when the state apparently decided to begin assigning (in addition to 3xxx numbers) 12xx designations to new routes, starting with 1242 and working up (1241 is the highest-designated 1955 route). State routes in any range can be decommissioned, and there are many corresponding gaps in the sequence. I do not know of any instance where a route number has been recycled (aside from the general 1955 renumbering); generally the state will assign a new 32xx or 12xx number when a new number is needed. A good many newer designations are given to roads long having been on the state rolls, but which have been severed or isolated from its former designation by newer construction.

Official US-SR duplications have been disallowed since 1955, but Interstate-SR duplications are permitted (Interstate highways in the modern sense not yet existing in 1955), and all Interstate routes except 210 and 220 are duplicated in the rolls of SR numbers. Most SRs with the same numbers as Interstates are a comfortable distance away, with one exception: LA 59 and I-59, which exist within 10 miles of each other in St. Tammany Parish, and which both intersect I-12.