A signal passed at danger (SPAD), known in the United States as a stop signal overrun and in Canada as passing a stop signal, is an event on a railway where a train passes a stop signal without authority. In the United States and Canada, this may be known colloquially as running a red, though this idiom principally refers to automobiles passing red traffic signals. 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. In the UK, the alternative description signal passed at red (S.P.A.R.) is used where a signal changes to red in front of a train due to either a technical fault or in an emergency, such that the train is unable to stop before passing the signal despite being driven correctly. Causes Because of its high inertia, it takes a considerable distance to stop a train, and incidents of this type often involve a slight or very slight overrun of the signal, at low speed, because the driver has braked too late, often after sighting the signal too late. In some situations, however, the driver is unaware that they have passed a signal at danger and so continues until a collision occurs, as in the Ladbroke Grove rail crash. In such cases it is up to the safety system (where fitted) to apply the brakes, or for the signaller to alert the driver. Some of the causes are: Misjudgement Inattention Distraction Fatigue Misreading of an adjacent signal due to line curvature, or sighting on one beyond Misunderstanding Miscommunication Incomplete or lapsed route knowledge Acute medical condition (medical emergency), such as a heart attack or stroke Chronic medical condition, such as sleep apnea causing microsleep Prevention Automatic train protection Automatic train protection (ATP) is a much more advanced form of train stop which can regulate the speed of trains in many more situations other than at a stop signal. ATP supervises speed restrictions and distance to danger points. An ATP will also take into account the individual train characteristics such as brake performance. Thus, the ATP determines when brakes should be applied in order to stop the train before getting to the danger point. In