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 which lasts until its speed drops below 6 mph/9 kmh.
How often a vehicle breaks down is directly related to its reliability rating and age. It can be reduced by regular servicing of vehicles at a depot or hangar.
In case of road vehicles, a breakdown can 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.