A bi-articulated bus or double-articulated bus is a higher-capacity type of articulated bus. It is an extension of a conventional or single-articulated bus, in that it has three passenger compartment sections instead of two. This also involves the addition of an extra axle and a second articulation joint. Due to the extended length, bi-articulated buses tend to be used on high-frequency core routes or bus rapid transit schemes rather than conventional bus routes.
One of their main advantages is that they reduce the number of drivers needed to run a service for a specific number of people — i.e., it is usually much more cost-efficient to run a bi-articulated bus with one driver, than, for example, to run two smaller ridgit buses providing the same total number of seats.
Disadvantages include some difficulties in traffic, the need to have bus stops catering to the extended length, and the fact that two buses with the same capacity can be used more flexibly, such as having one bus arrive every five minutes, instead of one of the larger articulated buses every ten minutes (as an example providing the same service capacity, but different frequencies).