of Highway Terms


What is "Daylighting?" What's the difference between an "Interstate" highway and an "interstate" highway? I have included this glossary to answer such questions by defining and clarifying ambiguous terms. While I am sure that many people reading this page are familiar with most of the terms here, I want to make sure some of the technical terms I use in my page are defined and understood.


As used on this page, an alignment is the specific location a route traversed. For example, Highway XX originally follows Main Street through the town of Podunk. Main Street is the alignment. Taken further, a freeway bypasses Podunk and becomes the new Highway XX. The freeway bypass is now the alignment of Highway XX.


This is the generic term for paving on modern highways. The key to this form of paving is that it is composed of tiny stones bound by an agent - either cement or something petroleum based. There are two major subtypes: cement-based Portland Cement Concrete (PCC) and petroleum-based Asphaltic Concrete (AC)

Portland Cement Concrete Known since the Romans made widespread use of it,  it is the white, durable substance, made from gypsum, that is commonly known as just "concrete." The name comes, not from the city in Oregon, but one in England where this form of cement originated. Between the two, the Portland Cement is the more durable, but it is also far more expensive.

Asphaltic Concrete Otherwise known simply as "asphalt," this is the black substance based on either macadam or a petroleum based substance - it is also known as "tarmac" (tar macadam). While it is not as durable as Portland Cement, it is much cheaper initially and is more flexible in where it can be used.

Concrete Slabs

Concrete is paved in In this page I use the terms single, double and sometimes triple slab concrete. What these terms refer to is the number of parallel slabs on a section of highway that usually correspond with the lanes on a road. Concrete is divided into square or rectangular segments to accommodate its expansion and contraction due to changes in weather. This prevents cracking and the ultimate pot holes. From the driver's point of view a two lane concrete road has one line down the middle, which delineates the two slabs. Here are the specifics:

Of course there are roads with more slabs, but most are associated with the Interstate Highways or with the advent of freeways and expressways. An example is that an eight lane concrete freeway is usually made of two parallel roadways made of four ("quadruple") slab concrete, although technically it is only two because of slip form paving.


Early state highways in California were built to conform with the terrain, a construction method that both reduced the highway grade and cut costs. Unfortunately, the result was dangerous blind hairpin turns. By the early 1920s, road building technology advanced to the point where the need for such sub-standard designs was obviated, yet many older highways still had them. Daylighting was the process of taking away enough of a hillside to add visibility to sharp turns, making them safe. A very notable example of daylighting is on the old Ridge Route north of Los Angeles.

Caption reads: "State Highway, Los Angeles County, on the Ridge Route, Showing Day-Lighting on Slopes on Curves." From the California Highway Commission's 1920 Biennial Report.
Courtesy of Caltrans

Interstate Highways

This refers to highways that go through more than one state. This term is most often associated with the Interstate Highway System, but they are not the only interstate highways. An interstate (note the lower case "i") highway also refers to the US highways and state highways that maintain a route number through more than one state. The earliest interstate highways include the National Road and Lincoln Highway, both of which traversed many states. In short, interstate highways are highways that go through more than one state; the Interstate Highway System is just one set of highways that matches this definition.

This refers to the interstate system of limited access highways which is denoted by a red, white, and blue shield. This is also the largest single publics works project in history. This system was conceived to overcome the shortcomings of the US Highway System, and was first proposed in 1947 as a 40,000 mile system of highways. While most of the present highways were designated in 1947, it was not until 1956, with the passage of the Federal Aid Highway Act of 1956 that they received a funding mechanism. President Eisenhower was the championed this system; hence their title as the "Eisenhower Interstate Highway System". It is said that Eisenhower was so impressed by the German autobahns while he was the Supreme Allied Commander that he wanted a similar system of highways in the United States which would allow for better military mobilization and promote better interstate commerce.

As a testament to the national priority that was set for these highways, they received an unprecedented funding mechanism. Previously, federal highways had been funded by a 50/50 funding mechanism; the states were required to match federal spending dollar for dollar. This new mechanism called for the federal government to pay for 90% of the construction costs, with the states matching only 10%. The Federal Highway Trust Fund was another first as it was the first time that federal dollars were specifically collected and earmarked for one project.

The Interstate Highway System was originally to be a 42,000 mile system with the scheduled completion in 1972. It was not finally deemed complete until 1992, 20 years later. Even now, new Interstates are planned and it is likely that as long as the system exists, there will always be some fine tuning. In 1968, the system was modified again, with the major reallocation of funding for some routes.

In theory, the Interstate Highway System is based on a grid pattern, with a logical ordering of numbers. In reality, geography, bad planning, and boneheaded politicians (Bud Schuster) skew this. The following describes the numbering scheme behind this system.

Exceptions exist for various reasons such as I-99 (politics), I-81 (geography), or I-238 (see my I-238 Page).

Highway Types

In California, US highways took on all different forms of highway types from two lane conventional roads to modern freeways. The following is a list of terms and their definitions in the context of California. There are also some non-California terms that are applied to US highways in other states.

Lincoln Highway 

The Lincoln Highway was America's first transcontinental highway. 







Carl Fisher, the creator of the Indianapolis 500 Speedway conceived of the Lincoln Highway as the first transcontinental highway. It was a major turning point in highway history as it was the first continuous transcontinental highway. The highway was the predecessor of and the model for the later system of numbered highways.

Please see Jim Lin's Lincoln Highway Page for an excellent overview of this very historical road.

Jersey Barriers or Jersey Curbs.  

Many people recognize the "Jersey Curb" as that unattractive, drab piece of concrete that separates traffic lanes and acts as a bridge rail. It is called a "Jersey Curb" since it was actually developed in New Jersey along the Turnpike in the late 1950s and 1960s. Interestingly, it was based on an innovative concrete divider the California Division of Highways placed on Grapevine Grade in the middle of US 99 around 1943. They called this divider a "parabolic concrete barrier" since it was designed to deflect trucks who had used the previous metal and wood beam dividers to aid their braking by rubbing against them. It was two feet tall, more squat than the present incarnation and consisted of 20 foot lengths each weighing about a ton.

The value of such a barrier for preventing accidents was immediately obvious and the present form appeared in the mid-1960s. The first Jersey Curb to be used in California was placed down the middle of the Harbor Freeway (I-110) in the late 1960s. By the end of the 1970s its use was mandated for all bridge rails and for dividing urban freeways. Any questions of its effectiveness were definitively addressed when the "tank guy" in San Diego back in 1995 tried to cross one on the SR-163 Freeway to go into oncoming traffic. The sizable tank could not scale it and further caused relatively little damage. If these barriers can stop a tank, then they must be good enough to stop errant cars. Efforts have been made to soften their stark appearance with decorations ranging from a groove on the outside part of the bridge rail to elaborate designs, some of which hark back to the days of the "mission style" concrete guardrails. 

Also known as "K" Rail.

National Road or National Pike   

This was the first federally funded road built in the United States. At its peak it went from Cumberland, MD to Vandalia, IL - a distance of 591 miles. Construction had started in the late 1700s but by the early to mid-1800s interest waned in favor of the railroads. During its time it proved that people over vast distances could be united and was a model upon which later highways were based. Today it is now part of US 40 and many of its structures, especially stone bridges remain in use.

Plank Road

Plank roads were built to cross areas, such as sand dunes where a conventional highway could not be placed. The most famous example of a plank road is the one that existed over the Algodones sand dunes of the Imperial Valley in southeastern California. This road, which was in use from 1914 to 1926 allowed vehicles to cross the shifting dunes and could be moved as they became covered by sand.

For more information visit the US 80 Plank Road and Sand Hills page.

Second Story Concrete

Diagrams from California Highway Commission's 1920 Biennial Report demonstrating two methods for adding "Second Story" concrete. Click on either image for larger version.
Courtesy of Caltrans

The "second story" refers to a new layer of concrete poured over an older one. This practice was especially prevalent in the 1920s when many 15 foot wide concrete highways were widened to a more modern 20 foot standard. This combined the best of both worlds by creating a new highway, but being able to use the old highway as a base. Examples of the second story concrete exist on the old Ridge Route north of Los Angeles and on many sections of US 101.

"Suicide" Lane

In the 1920s and 1930s, many roads were built with three lanes. There was one lane for each direction of traffic and a shared middle lane that was used for passing by vehicles going in both directions. This presented the very real possibility for head-on collisions as passing vehicles from both directions often shared this lane at the same time. The concept behind this lane is similar to the dashed yellow line found on two lane highways, which permits passing. Suicide lanes were finally phased out by the 1960s, with the roadways either being widened to a full four lanes (or more), or simply having them removed. Today a "suicide" lane refers to the middle lane of a road that can be used to turn left from both directions, which is similar to the suicide lanes of yore.

Super elevation or Banking

This is where turns are raised on the outside to facilitate cornering. The best example of what it is like is on racetracks where the road can be at a greater than 45 degree angle. It is commonly used on freeway on ramps and has been in use since the very early days of 20th Century road building.

US Highway System

This refers to the system of highways that preceded the Interstate Highway system and that is marked by black and white shields. It was established in 1925 and was the world's first country-wide system of uniformly numbered highways. It has played a major role in the development of the United States in the 20th Century by making it possible to travel and transport goods over long distances. While railroads also did this, the difference is that the highways connected to many more places and had more flexibility of transportation.

Return to the Historical US Highways Page.