An overpass (called a flyover in the UK and most Commonwealth countries) is a bridge, road, railway or similar structure that crosses over another road or railway. An overpass structure is one that carries a higher capacity road above a lower capacity road, whereas a structure that permits a lower capacity road to travel above a larger capacity road is an underpass. Capacity is determined as either the number of lanes of travel provided or measured traffic count. In instances of actual or perceived equality between the traffic flows, the term structure can be used.
In North America, a flyover is a high-level overpass, built above main overpass lanes, or a bridge built over what had been an at-grade intersection. Traffic engineers usually refer to the latter as a grade separation. A flyover may also be an extra ramp added to an existing interchange, either replacing an existing cloverleaf loop (or being built in place of one) with a higher, faster ramp that bears left. Such a ramp may be built as a right or left exit.