The Stop Short description for vehicles says a vehicle that was racing continues to move after you try to stop it. You need to use a move action to stop and can make a Piloting check to reduce the distance it moves. How far will the vehicle continue to move if the PC chooses not to make the Piloting check or rolls poorly (1 or 2 on d20)? RAW does not address?
It keeps moving forward but slows down gradually
This behavior is described on the Race action:
After taking a race action, a vehicle doesn’t slow down immediately. On your next turn, you have four options: you can use another full action to continue to race at full speed, use a move action to drive at the vehicle’s drive speed, use a move action to stop short, or relinquish control of the vehicle as a swift action. If you take a swift or move action, you can also take a standard action during that turn.
After you race, you have only four options:
- Keep racing as a full-round action;
- Drive at the vehicle drive speed using a move action;
- Try to force stop using a move action;
- Relinquish control as a swift action;
There are no other options, even if you refuse to spend your actions on one of those four.
This means that you raced for a turn, moving the vehicle at full speed, then tried to stop using a move action the next turn. If this check failed, you can either try downgrading your standard action into another move action, or be forced to relinquish control as a swift action, which explains exactly what happens:
If you are knocked out or cease actively piloting, your vehicle becomes uncontrolled. If you delay your action, the vehicle becomes uncontrolled and continues to act on the same initiative count as it did before. This separates your initiative count from that of the vehicle, and the vehicle continues to move (see below) at your previous initiative count until a pilot takes control of it or it crashes or otherwise is brought to a definitive halt.
Unless otherwise specified, an uncontrolled vehicle moves straight ahead at its most recent heading as if taking two drive actions on its turn. It slows down incrementally with each action taken (usually to three-quarters the speed of its last action) until it comes to a stop or crashes. At the GM’s discretion, it could slow down more if it’s on uneven terrain or an upward slope, or it could stay at the same speed or even accelerate if it’s in a zero-g environment or on a downward slope.
As for the Force Stop action:
Stopping a vehicle after a race action (see Race below) requires a move action (stopping after a drive action doesn’t require an action; see Not an Action). Normally, a vehicle continues to move following a race action. You can attempt a Piloting check (see Pilot a Vehicle) to reduce the distance your vehicle moves before stopping after a race action by the result of your check, rounded down to the next 5-foot increment. For example, with a result of 17 you would reduce the distance moved by 15 feet (3 squares).
By the action description, it stops immediately on a successful check. But as far as not taking the action or failing the check, as I mentioned previously, the vehicle continues to move.
The difference between using the Force Stop action or Drive then Stop, is that the vehicle will move further.
If it's Drive speed is 20 ft, for example, it will move another 20 feet in the direction you choose, while force stopping it wouldn't move this extra distance at all. If you tried to force stop after a Race action and failed (on both checks, if taking two actions to do that), assuming it has a 200 ft Full speed, this vehicle will move another 150 feet (3/4 of 200) this round in the same direction as previously.