The bus factor is a measure of how many project members could die in a bus accident before the project becomes irreparably endangered. Since an exact calculation is often not possible, the bus factor for each project should be determined through logical reasoning.
A bus factor of 1 – meaning a single/specific person would have to leave the project for it to fail – must be avoided at all costs!
How many people would have to be run over by a truck (or leave the project) before the project is crippled?
The most effective method for mitigating the bus factor is documentation. If all business processes are stored in a wiki or another documentation system, they can simply be looked up when needed.
Of course, access rules should also be adjusted so that potential stand-ins for the person "hit by the bus" have direct access to the necessary rooms, files, or computer systems.