Clearly "hit by a truck" is an extreme thought however you could easily substitute "take vacation at the same time" to get the same idea. If any part of your project has a truck factor of one then you are in a particularly fragile situation. If that one person leaves or is unable to work on the project, you will suffer the consequences.

Over time, anyone can be replaced. Truck factor is an indication of how expensive it will be to replace specific people.

In an ideal situation, everyone on the team will know all parts of the system so that the loss of any one person would have minimal impact. In reality, many projects rely on one or more "heros" who are the only one who understand certain critical parts of the system. When these heros leave (and you should assume they will), you must be prepared to recover.

If you have a hero on your team, the best thing you can do is reassign that person to a different part of the system. This will allow the replacement to ramp up while the hero is still available for support. If you wait until the hero has left then the ramp-up will be significantly more expensive.

An added benefit to reassigning the hero is that this person will now have the opportunity to work on something different. Since the hero's tend to be the most technically competent members of the team, this will usually mean that the new area will improve once the hero has worked on it for a while.

Truck factor is a quick metric that will highlight potential problems in your project. Having hero's on your team can be very beneficial but only if you don't become dependant on them. Truck factor is one metric that will highlight your dependencies.

Cross posted from Mike Bowler's Weblog