Breakdowns

From OpenTTD

Vehicle Breakdowns occur when a vehicle is in transit. It usually cause a vehicle to stop for a while, causing traffic buildup, but planes will only get a speed reduction.

The amount of breakdowns that occur to a vehicle is directly relevant to it's reliability rating and age. The number can be reduced by regular Servicing to a vehicle via a depot or hangar. In the case of road vehicles, a breakdown can occur to trigger the Small UFO Disaster, where a UFO will crash into a broken down road vehicle. Breakdowns can be turned on or off via the Difficulty window.