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