Road hierarchy
The road hierarchy categorizes roads according to their functions and capacities. While sources differ on the exact nomenclature, the basic hierarchy comprises freeways, arterials, collectors, and local roads. Generally, the functional hierarchy can more or less correspond to the hierarchy of roads by their owner or administrator.
The related concept of access management aims to provide access to land development, while ensuring traffic flows freely and safely on surrounding roads.
General classification
Controlled-access highway
At the top of the hierarchy in terms of traffic flow and speed are controlled-access highways; their defining characteristic is the control of access to and from the road, meaning that the road cannot be directly accessed from properties or other roads, but only from specific connector roads. This indirection, in conjunction with high speed limits and multiple lanes, allows these roads to support fast traffic flow with high volume, in both urban and rural areas. They are at the bottom of the hierarchy in terms of access to property, allowing access to almost nothing besides other roads and rest areas.
They are also known as freeways in the U.S., South Africa, and parts of Australia; as motorways in the U.K., New Zealand, and parts of Australia; and as expressways in numerous countries.
Controlled-access highways do not have traffic signals nor at-grade crossings with other roads (intersections), railways, or pedestrian paths. When a road's path would intersect a controlled-access highway, a flyover (US: overpass or underpass) will vertically separate the two roads, preventing the need for an intersection. Pedestrian footbridges and tunnels are also grade-separated. The opposing directions of travel are separated by a central reservation (UK) or median strip (US) generally a few meters to a few dozen meters wide, which may consist of a traffic barrier or a natural barrier like grass. This separation makes controlled-access highways a subset of dual carriageways (UK) or divided highways (US). Generally, these roads prohibit pedestrians and vehicles not powered by a motor (e.g., bicycles). Parking is also generally prohibited.
Highway systems
In many countries, the controlled-access highways form an expansive system that generally connect distant cities within the country to each other, but there are often more highways that only have local courses designed to improve connections in a smaller region, such as within a metropolitan area. Controlled-access highways are often given numbers to form a national highway system, such as the Bundesautobahn in Germany or the Interstate Highway System in the United States, but note that a national highway system may also consist of other numbered highways that are not implemented as controlled-access highways. Some countries may have more than one national highway system of a lower rank, such as the U.S. Highways (not to be confused with the Interstate system), only portions of which run on controlled-access highways. Conversely, there may also be controlled-access highways not part of a numbered system. Highways are usually given icons featuring the number of the highway called highway shields or route markers. In addition to the national highway system, there may be provincial-level or state-level (US) highway systems of a lower rank, which need not consist mainly of controlled-access highways.
Slip roads (UK) or ramps (US) are special connector roads that allow traffic access to and from a controlled-access highway. Their length is normally on the order of several hundred meters in order to allow vehicles a sufficient distance to safely change speed while transferring from one road to the other road. The course of a ramp is often curved as needed in order to connect the two roads without sharp turns, which require vehicles to slow down considerably to traverse them safely. In many cases, a ramp may be subject to grade separation and use a flyover. In the US, where the standard term for a flyover is overpass (or underpass, when the road with level altitude is the higher road), the word flyover, which is less common, is reserved for those that carry ramps. Entrance ramps (or on-ramps) allow ingress traffic to enter the highway, and exit ramps (or off-ramps) allow egress traffic to exit the highway.
Ramps may be described by their orientation. A directional ramp curves toward the desired direction of travel, a semi-directional ramp exits on the side of the road opposite to the desired direction of travel, then curves back toward the desired direction, and a non-directional ramp curves away from the desired direction of travel (such as the looping ramps in a cloverleaf interchange).
The full set of ramps that connect a controlled-access highway to another road is called a grade-separated junction (UK) or interchange (US). The interchange is classified as a system interchange if traffic remains in the highway system, traveling from one controlled-access highway to another, or a service interchange if the interchange serves a local area by allowing travel between a controlled-access highway and a road without access control. The controlled-access highway is called the mainline, and the uncontrolled road is called the crossroad. More complex interchanges involving many roads may have characteristics of both types of interchanges as required.
The number of directions one can travel toward or away from the interchange on all of the roads involved is the number of "legs". When two roads meet, there are four paths to and from the interchange, and so the interchange has four legs, but if one of the roads terminates at the interchange, it will only have three legs. Interchanges with more legs require joining elements from four- and three-legged interchanges in order to permit travel from any one direction to any other direction. If all possible connections exist, it is a complete interchange providing complete access; if not, it is an incomplete interchange providing incomplete access.
Limited-access road
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A limited-access road, often called a limited-access highway (US), divided highway (US), or dual carriageway (UK), and in some situations an expressway, is similar to a controlled-access highway in that it conforms to many or most of the standards that controlled-access highways follow, but does allow some uncontrolled access to local roads. They can be viewed as a middle ground between controlled-access highways and arterial roads. The degree of isolation from local traffic varies between countries and regions, as does a precise definition of the term itself.
Controlled-access highways can also be viewed as a subset of limited-access highways that has stricter requirements.
Arterial road
An arterial road or arterial thoroughfare is a road without controlled access that can carry a large volume of local traffic at a generally high speed, being below controlled-access highways in the hierarchy. Because their primary function is to connect collector roads (below) to controlled-access highways, some are considered limited-access roads.
Intersections of arterial roads are almost always at-grade, and use traffic signals to coordinate traffic that would otherwise intersect, but traffic signals are often omitted when minor collector roads intersect, usually placing a stop sign at the collector road to prevent the traffic on the arterial road from being impeded.
Arterial roads almost always have multiple lanes to allow for high capacity. They do not allow for access to residential properties under most circumstances.
Collector road
A collector road, also known as a distributor road, is a road that allows high access to properties and has a low to moderate capacity and a generally low speed limit. They are below arterial roads in terms of speed and capacity, but higher in terms of access, as they can allow access to residential properties.
Collector roads can vary widely in appearance. Some urban collectors are wide boulevards entering communities or connecting sections. Others are residential streets, which are typically wider than local roads, although few are wider than four lanes. Small-scale commercial areas can be found on collector roads in residential areas. Key community functions such as schools, churches, and recreational facilities can often be found on collector roads.
A collector road usually consists of a mixture of signaled intersections, roundabouts, traffic circles, or stop signs, often in the form of four-way stops. Two-way stops are generally used at intersections with local streets that favour traffic movement on the collector. In North America, a collector road normally has traffic lights at intersections with arterial roads, whereas roundabouts and two-way stops are more commonly used in Europe.
Speed limits are typically 20‑35 mph (30‑60 km/h) on collector roads in built-up areas, depending on the degree of development and frequency of local access, intersections, and pedestrians, as well as the surrounding area (the speed tends to be lowest in school zones). Traffic calming is occasionally used in older areas on collector roads as well.
Local road
A local road, also called a street, is a road in a built environment that has all kinds of properties beside it which can be accessed from the road or a parking lot connected to the road. Different types of local roads include residential streets, avenues, and alleys. They have the lowest speed limits and capacities in the hierarchy, but have the highest access to property.
Local roads have at-grade intersections and have similar specifications to collector roads. Local roads may be unpaved in some cases. A common feature of local roads is driveways, which connect the road to a residential property.