Editing Road hierarchy (section)

=== Arterial roads ===

==== Controlled-access highway ====
At the top of the mobility-access continuum in terms of traffic flow and speed are [[controlled-access highway]]s. 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 mobility-access continuum 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 (though this term sometimes includes [[limited-access road]]s depending on the region).

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, the two roads will be vertically separated by an [[overpass]] (American English) or flyover (British English) or by a [[tunnel]], preventing the need for an intersection. Pedestrian [[footbridge]]s and tunnels are also grade-separated. The opposing directions of travel are separated by a [[median strip]] (American English) or central reservation (British English) 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 divided highways (American English) or [[dual carriageway]]s (British English). Generally, these roads prohibit pedestrians and vehicles not powered by a motor (e.g., [[bicycles]]). Parking is also generally prohibited.

===== Highway access =====
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Ramps (American English) or [[slip road]]s (British English) 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 [[Interchange (road)|interchange]] (American English) or grade-separated junction (British English). The interchange is classified as a [[Interchange (road)#System interchange|system interchange]] if traffic remains in the highway system, traveling from one controlled-access highway to another, or a [[Interchange (road)#Service interchange|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 [[Interchange (road)#Four-legged interchanges|four legs]], but if one of the roads terminates at the interchange, it will only have [[Interchange (road)#Three-legged interchanges|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 ====
[[File:Route nationale 137 à La Rochelle · Périgny (17, France).JPG|thumb|Dual carriageway near [[La Rochelle]], France]]
A [[limited-access road]], also known as ''limited-access highway'' in the U.S. and as ''expressway'' in numerous countries (including parts of the U.S.), 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 minor 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.

==== Minor arterial roads ====
[[File:PageMillRoad.png|thumb|Minor arterial road with [[bike lane]] in [[Palo Alto]], California]]
A minor [[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 mobility-access continuum. They primarily service trips of moderate length and connect collectors to higher types of arterial roads.

Intersections of minor 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.