Bus factor

Bus factor (also called truck factor or lottery factor) is the minimum number of people on a project whose sudden departure would cause the project to stall. The phrase comes from the morbid thought experiment: "if this person got hit by a bus tomorrow, could the project continue?" Bus factor 1 means no — the project is one person away from dead. Bus factor 5 means yes, until four more people also leave.

The original framing was for corporate teams, but the term is now used most often in open source. In OSS, bus factor 1 is the default state, not the exception. A 2024 study by Harvard and the Linux Foundation analyzed the top 500 most-depended-upon open source packages and found a median bus factor of exactly 1 for the top 100. The reason is structural: 60% of OSS maintainers are unpaid (Tidelift 2024), 44% report burnout (Sonar 2024), and most projects are started by a single person who keeps them going as long as they have time and energy. When that runs out, the project doesn't get a graceful handoff — it gets a frozen repository with a build-up of unanswered issues.

The fix is structural: sponsorship that pays the maintainer to keep going, contributors who become co-maintainers, and handoff mechanisms that keep the project alive when the original maintainer steps back. The bus factor of a project is the single most useful metric for predicting whether it will still be shipping in two years.