While the terms ‘bunch’ and ‘platoon’, referring to a group of vehicles in a traffic stream, often are used interchangeably, their most common usage in Australasia (and that employed in this Guide) distinguishes between them as follows.
A bunch is a single-lane group of vehicles, associated with uninterrupted flow conditions and arising because of different desired travel speeds of different drivers and limitations on overtaking opportunities. Generally, a bunch consists of a lead vehicle, travelling at its own desired speed, closely followed by none, one or more other vehicles with equal or higher desired speeds. Under this definition, a bunch may consist of a single vehicle. The formation of a bunch of two or more vehicles is due to causes entirely internal to the traffic stream. The most common manifestation of traffic bunches is in traffic flow on two-lane two-way rural highways.
A platoon, on the other hand, may be a single-lane or multi-lane group of vehicles and is associated with interrupted flow conditions. A platoon is formed when traffic is stopped by an element external to the traffic stream, such as a red aspect at a signalised intersection. When the traffic is able to again proceed (the signal turns green) it moves away as a group or cluster of vehicles that is called a platoon.
This section is concerned with traffic bunches and the closely associated activity of overtaking, particularly where the overtaking manoeuvre makes use of the lane in which the opposing direction of traffic has priority. Traffic platoons are addressed in Section 7.4.