Signal passed at danger

A signal passed at danger (SPAD), known in the United States as a stop signal overrun (SSO) and in Canada as passing a stop signal,^: 75 is an event on a railway where a train passes a stop signal without authority. This is also known as running a red.

The name derives from red colour light signals and horizontal semaphore signals in the United Kingdom, which are said to be at danger when they indicate that trains must stop (also known as the signal being on). This terminology is not used in North America where not all red signals indicate stop.^: 72 In the UK, a signal passed at red (SPAR) is used where a signal changes to red directly in front of a train, due to a fault or emergency, meaning it is impossible to stop before the signal.