This so called "bus factor" has always bugged me. Documentation is good, but talk of untimely death by bus is silly.
The reference to untimely death by bus strike is an example of macabre geek humour; not a serious suggestion that the greatest threat to developers is posed by motorized mass transportation.
Most often developers leave projects -- and especially open source projects -- for far more banal reasons: They move to a different position and don't have the time or inclination to continue maintaining their old code.
Can someone else get access to all the company-owned code the developer was working on? Is someone insisting that the developer doesn't keep three months of work on a laptop? Who else can get access to the passwords and keys for company-owned servers and services maintained by the developer?
Not really. "Key developer gets a new job" means they are still available for you to ask questions to, get help locating passwords/keys from, and so forth. In the "key developer gets hit by bus" scenario, you don't have that luxury.
The idea isn't so much about being hit by a bus as "what would we do if a key contributor were to vanish off the face of the Earth with no notice or warning?" If the specific idea of the Death Bus seems too grim, feel free to replace it with another deus ex machina that would have the same effect (alien abduction, Christian rapture, surprise release of a new season of Firefly, etc.).
1) A new key developer would be hired to maintain the project, and things would be a little rough for a couple months but the company would survive.
2) Every decent person would care more about the key developers life than the "major project".
This so called "bus factor" has always bugged me. Documentation is good, but talk of untimely death by bus is silly.