A passing lane is the lane on a multi-lane highway or a motorway closest to the center of the road (the central reservation). In British/Irish terminology, the passing lane is termed an "outer lane" or "outside lane", while a normal lane nearer the hard shoulder is termed an "inner (or inside) lane".

A passing lane is often misnamed a "fast lane", causing drivers to use it for extended periods of time. In theory, a passing lane should only be used for passing, thus allowing, even on a road with only two lanes in each direction, motorists to travel at their own pace. Good driving practice is to slip out of the passing lane once slower cars have been passed.

A possible problem arising from misuse of the "fast lane", is that it may encourage other motorists to keep up with the faster driver (who may indeed be exceeding the speed limit). Alternatively the driver in the outer lane may be driving too slowly. As a result, sometimes a motorist will attempt to overtake the outer car on the inner lane, endangering slower drivers on the inner lane.

A driver hoping to pass a "fast lane" motorist is stuck in an awkward situation. One strategy is to signal a lane change toward the center median. Another is to flash headlights. A third, which again sacrifices safety, is to drive very close to the "fast lane" driver's bumper (this is known as tailgating).

In hilly country, some standard highways (not dual carriageway) are built with three lanes. Two lanes are used for traffic heading in the uphill direction, with one lane being a passing lane, and one lane is used for downhill traffic.