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In project management, the bus number is the number of team members whose loss would endanger a project. For example, if there are two people on a team that are essential to a project's success, the project's bus number is two -- if those two team members were hit by a bus, the project would be in trouble.
One is the least desirable bus number because it means that one or more team members possess knowledge that nobody else has. If any team member with unshared knowledge is moved to another team, quits or becomes otherwise incapacitated, the project will be in jeopardy.
In a whole-team approach to project management, the goal is to make a project's bus number as close to the actual number of team members as possible. For instance, if there are ten people on a team using the whole-team approach, the team's bus number should be nine. That means that if nine members of the team were run over by a bus, the surviving member could bring replacement team members up to speed quickly.