Glossary of Terms - Historic Roads

Road Classifications

  • County Road: A public road built or maintained by a county government rather than the state government (e.g., Mohave County Route 20, MC 20).
  • Highway (Route): A public road between two places that typically has been paved or otherwise improved to allow travel by motorized vehicles. Technically, it includes the entire area within the right-of-way.
  • Historic State Highway System: In Arizona, the entire group of numbered state highways and U.S. highways built between 1912 and 1955, when the Federal-Aid Highway Act of 1956 was signed. Each road in the system is considered a historic road or highway.
  • Interstate Highway: A limited access route that is part of the Dwight D. Eisenhower National System of Interstate and Defense Highways (Interstate System). In Arizona, Interstates 8, 10, 15, 17, 19, and 40 serve as examples. As described in the Federal Highway Administration’s Federal-aid Highway Guide, interstate highways are “routes of highest importance to the Nation, built to the uniform geometric and construction standards of 23 U.S.C. 109(h), which connect, as directly as practicable, the principal metropolitan areas, cities, and industrial centers, including important routes into, through, and around urban areas, serve the national defense and, to the greatest extent possible, connect at suitable border points with routes of continental importance in Canada and Mexico.”
  • Interstate Highway System: The system of U.S. interstate and defense highways established with the Federal-Aid Highway Act of 1956.The numbering pattern for interstate highways is the reverse of that used for U.S. highways: Odd-numbered interstate highways run generally north to south, with the lowest numbers in the west (e.g., Interstate 17), and even-numbered interstate highways run east-to-west, with lowest numbers in the south (e.g., I-10).
  • Private Road: A road built or maintained by private citizens, not municipal, state, or federal entities that may be closed to public traffic.
  • State Highway (State Route): A public road maintained by a state that carries a number assigned by that state (e.g., Arizona State Route 82, SR 82).
  • State Highway System: The entire group of numbered state highways and U.S. highways that is the primary responsibility of the state to maintain. The system includes the right-of-way, drainage facilities, bridge appurtenances, easements, and features associated with these public roads.
  • U.S. Highway (U.S. Route): A public road maintained by a state that carries a number assigned by the American Association of State Highway and Transportation Officials (AASHTO).
  • U.S. (National) Highway System: The system of U.S. highways numbered within a nation-wide grid that is maintained by state or location governments. The system was established in 1926. The numbers and locations are coordinated by the American Association of State Highway Transportation Officials (AASHTO). North-to-south highways are odd-numbered, with the lowest numbers in the eastern U.S. and highest in the western U.S. West-to-east highways are even-numbered, with the lowest numbers in the northern US and highest numbers in the southern US. Major east-west routes have numbers ending in “0” (e.g., U.S. 60), and major north-south routes have numbers ending in “1” (e.g., U.S. 91). The Interstate Highway System has largely replaced the U.S. highway System for long-distance through traffic, although many important regional connections are still made by U.S. highways.

Functional Road Types

  • Backcountry Road: A road that lacks an all-weather surface treatment, safety devices (road markings, signs or signals), and/or design features, usually located in undeveloped rural areas.
  • Byway: A road with an all-weather surface (e.g., gravel road, city-street, or a paved highway) suitable for year-round automobile travel.
  • Expressway: A type of highway that has partial limited access from adjacent and parallel roads.
  • Freeway (Motorway): A type of highway that has limited access and is grade separated.
  • Frontage Road: A local street or road located on the side of, and usually parallel to, a limited-access highway that allows access to residences and businesses from a controlled intersection of the arterial highway.
  • Parkway: A type of highway that includes limited access thoroughfares designed for recreational driving of motorized vehicles through a scenic landscape or landscaped route without at-grade intersections or traffic stops.
  • Scenic Road (or Byway): An officially designated type of roadway recognized for possessing strong characteristics associated with one or more of the following six intrinsic qualities: (1) scenic qualities (pleasing, memorable landscapes), (2) natural qualities (relatively undisturbed natural features such as geology, vegetation, or wildlife), (3) historic qualities (legacies of the human past associated with natural or man made landscapes), (4) cultural qualities ( customs of a distinct group of people, such as crafts, foods, or vernacular architecture), (5) archaeological qualities (tangible remains of past human behavior), and (6) recreational qualities (outdoor recreation directly associated with one or more other intrinsic qualities, such as fishing, hiking, and the road-experience itself). ADOT has identified five types of scenic roads in Arizona: Scenic Roads, Historic Roads, Parkways, National Scenic Byways, and All-American Roads.

Road Design Features and Structures

  • Alignment: The vertical and horizontal curvature of a road.
  • At-Grade: Roadways or intersections at the same elevation or level.
  • Bicycle Lane: The portion of a roadway that has been set aside for bicycle use, which is distinguished from the motor vehicle portion by painted stripes or curbs.
  • Bituminous: Technical name for the mixture of asphalt and aggregate placed on the surface. Normally, it has a black appearance. It is applied at a high temperature which is sometimes referred to as "hot mix." Bituminous is also called “tar,” although it is not tar.
  • Culvert: Any structure not classified as a bridge that provides an opening under the roadway, usually made of concrete or steel, in order to drain water from one side of the road to the other.
  • Divided Highway: A highway with two or more lanes (carriageways) for motorized vehicles traveling in opposite directions, separated by a median, a traffic island, or Jersey barrier.
  • Grade Separation: A crossing of two highways, or a highway and a railroad tracks, at different elevations or levels. These separations are created by underpasses and overpasses. A bridge that spans a highway or a railroad tracks is considered a grade-separation structure.
  • Gradient: The percent of vertical or longitudinal slope.
  • Guard Rail: A steel rail at the shoulder edge of a highway used to keep vehicles from veering off the road into oncoming traffic or traveling off a roadway under hazardous conditions.
  • Interchange: A system of interconnecting roadways that permits free moving of traffic between two or more roads on different levels. The three basic types of interchanges are cloverleaf, diamond, and directional.
  • Jersey Barrier: A moveable, modular, steel-reinforced, concrete barrier used to separate lanes of traffic, reroute traffic, and/or protect pedestrians during highway construction. These safety items were developed at the Stevens Institute of Technology in Hoboken, New Jersey for the New Jersey State Highway Department to deflect motor vehicle traffic.
  • Limited Access Highway: Highways that cannot be accessed directly by adjacent roads and driveways except at specific places determined by the public authority having jurisdiction over the roadway.
  • Median: The portion of a divided highway that separates the lanes used to travel in opposite directions.
  • Median Lane: A single lane within the median portion of a road.
  • Pavement: The durable surface material laid down on a road intended to carry vehicular traffic. Although cobblestones, brick, wood plank, and other types of surface treatment were used in the past, most road surfaces today are paved with asphalt or concrete and marked with uniform signs to guide traffic. In more recent years, permeable paving methods have been used on low-volume traffic roads.
  • Ramp: A connecting roadway between two intersections highways or streets.
  • Right of Way: Land acquired by purchase, gift, or eminent domain in order to construct and maintain a public road.
  • Roadway: The portion of highway, including its shoulders, used for vehicular traffic.
  • Shared Roadway: A road that has been officially designated as a road that includes a bicycle route but does not have provisions for physical separation between motor vehicles and bicycles.
  • Shoulder: A clear, level area to the side of a road available for emergency use or stopped vehicles.
  • Traffic Island: A raised feature in the center of a roadway constructed to separate or direct traffic.
  • Traffic Lane: The portion of a roadway used for movement of a single line of vehicles.
  • Traffic Markings: Lines, patterns, symbols, words, and/or colors on the pavement, curbs, or other roadway features used to control traffic.
  • Traffic Signals: A power-operated devise used to regulate, warn, and/or direct traffic (e.g., stop light).
  • Traffic Signs: A static device mounted on a fixed or portable support, which conveys a specific message by means of words and/or symbols, and regulates, warns, or directs traffic (e.g., stop sign).
  • Undivided Highway: A road with one or more traffic lanes going in one direction with only painted stripes (no median) between it and traffic moving in the opposite direction.

Surface Treatments

  • Aggregate: A processed material made from a combination of crushed rocks, and coarse and fine sands. Aggregate can also be made from recycled products such as crushed concrete or porcelain toilets.
  • Asphalt: A dark, solid or semi-solid mixture obtained from natural deposits, such as tar pits, or as a byproduct from petroleum processing. Asphalt is further refined from tar. It can be used as waterproof binder in road construction. Asphalt surfaces are usually laid on a gravel base. When mixed with concrete, asphalt pavements generate less noise, are less expensive, and presumably easier to repair. See bituminous.
  • Bituminous Surface Treatment (BST, also called chipseal, seal coat): Stone aggregate spread over a sprayed-on asphalt emulsion or thin asphalt cement, then embedded in the asphalt by rolling over it with heavy equipment. BST is easy to apply, relatively flexible, appropriate on unstable terrains that thaw and soften in the spring, and roads that do not have heavy traffic.
  • Cement: A powdered product of alumina, silica, lime, iron oxide, and magnesium oxide burned together in a kiln that reacts with water to form a rock-like substance that bonds aggregates in concrete.
  • Concrete: A construction material made of Portland cement, gravel, sand, and water that is hard and strong. It is usually applied as slurry and worked mechanically to produce a smooth, denser surface free from honeycombs. Concrete can be reinforced with steel mesh (pavement fabric) or reinforcement bars (“rebar”). Cement concrete roadways generally are stronger and more durable than asphalt roadways, but they are more expensive to build.
  • Gravel: Crushed rock applied to a dirt road with relatively low traffic volume. Such roads are most durable when they combine a sub-base, base, and a top coat, sometimes sealed with an emulsion.
  • Pavers: Precast concrete blocks laid down for aesthetic purposes or in restricted settings that require long-duration pavement, such as at port facilities.
  • Surface Treatments: Materials used to construct a roadway. There are four basic types commonly in use today: asphalt, bituminous, concrete, and gravel. In the past, cobblestones, bricks, pavers, and wood planks were used to create a solid road.


  • American Association of State Highway and Transportation Officials (AASHTO): A nonprofit, nonpartisan association representing highway and transportation departments in all 50 states that has developed design standards to foster the development, operation, and maintenance of an integrated national transportation system.
  • Arizona Department of Transportation (ADOT): The state agency responsible for planning, building, and operating a system of interstate and state highways in the state of Arizona, as well as building and maintaining bridges and providing financial assistance to public airports for airport development projects. ADOT was organized in 1974 by combining the Arizona State Highway Department (est. 1927) and the Arizona Department of Aeronautics (est. 1962) ( ADOT's publications division publishes the well known Arizona Highways magazine. A major component of the organization is the Motor Vehicle Division which provides title, registration and driver license services to the general public.
  • Arizona State Transportation Board: A seven-member state-level board that establishes policy for the Arizona Department of Transportation (ADOT). The Board is responsible for establishing a complete system of state highway routes. It has the authority to establish, open, relocate, alter, vacate, or abandon any portion of a state route or state highway, and may designate parkways and historic and scenic roadways. The Board also awards construction contracts and monitors the status of construction projects and has the exclusive authority to issue revenue bonds for financing needed transportation improvements throughout the state.
  • Federal Highway Administration (FHWA): A division of the U.S. Department of Transportation that funds highway planning and programs. FHWA provides stewardship over the U.S. Interstate System and oversees the construction of new highways, bridges, and tunnels and provides guidance on highway and bridge maintenance and preservation. It also conducts research and implements innovations to create better and safer highways; provides technical assistance to state, local, and federal partners; provides federal financial assistance to state and local governments for constructing, preserving, and improving highways through the Federal-aid Highway Program; and provides funding for public roads and highways within federally owned lands (e.g., Bureau of Land Management, National Parks, U.S. Forests, and U.S. Wildlife Refuges) and tribal lands.
  • State Historic Preservation Office and State Historic Preservation Officer (SHPO): The public office and official designated to be the advocate for historic and prehistoric (precontact) properties in each state and territory of the USA. This position was established by the National Historic Preservation Act of 1966. In Arizona, the SHPO is a division of the Arizona State Parks Department in Phoenix.
  • U.S. Department of Transportation (U.S. DOT): A federal agency whose mission is to serve the U.S. by ensuring a fast, safe, efficient, accessible and convenient transportation system that meets vital national interests and enhances the quality of life of the American people. It oversees the following administrations and bureaus: Federal Aviation Administration (FAA), Federal Highway Administration (FHWA), Federal Railroad Administration (FRA), National Highway Traffic Safety Administration (NHTSA), Federal Transit Administration (FTA), St. Lawrence Seaway Development Corporation (SLSDC), Maritime Administration (MARAD), Research and Innovative Technology Administration (RITA), Pipeline and Hazardous Materials Safety Administration (PHMSA), and the Federal Motor Carrier Safety Administration (FMCSA), and the Surface Transportation Board (STB).

Law and programs that Affect Historic Highways

  • Federal-Aid Highway Act of 1956: Popularly known as the National Interstate and Defense Highways Act, the Federal-Aid Highway Act authorization 25 billion dollars for the construction of 41,000 miles of the Interstate Highway System supposedly over a 20-year period. At the time it was signed into law, it represented the largest public works project ever created in the U.S. The money for the construction of these interstate highways was managed through a federal Highway Trust Fund that paid for 90 percent of highway construction costs; states were required to pay the remaining 10 percent.
  • National Environmental Policy Act of 1969 (NEPA): A federal law passed in 1969 that requires an analysis of environmental impacts of federal actions, including the federal funding of transportation projects.
  • National Historic Preservation Act of 1966 (NHPA): A federal law that requires state, federal, and local agencies that use federal funds, federal permits, or undertake work on federal lands to consider the potential effect of a project on a property that is registered on or eligible for the National Register of Historic Places. If effects are identified, federal and state agencies must identify ways to mitigate the harm. The process is often referred to as “Section 106 review” after that portion or section within this law that requires this consideration.
  • National Register of Historic Places (NRHP or National Register): The National Register of Historic Places is the official list of the Nation's historic places worthy of preservation. Authorized by the NHPA, the National Park Service's NRHP is part of a national program to coordinate and support public and private efforts to identify, evaluate, and protect America's historic and archeological resources.
  • National Scenic Byways Program (NSB Program): The National Scenic Byways Program was established under the Intermodal Surface Transportation Efficiency Act of 1991, and reauthorized in 1998 under the Transportation Equity Act for the 21st Century. Under the program, the U.S. Secretary of Transportation recognizes certain roads as National Scenic Byways or All-American Roads based on their archaeological, cultural, historic, natural, recreational, and scenic qualities. This program is a voluntary, grassroots program. Its mission is to provide resources to the byway community in creating a unique travel experience and enhanced local quality of life through efforts to preserve, protect, interpret, and promote the intrinsic qualities of designated byways.