Editing Road hierarchy (section)

===== 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''.