signal was replaced to danger in front of the train by the signaller in accordance with the rules and regulations and the train was unable to stop before passing the signal. A Runaway SPAR (formerly Category D SPAD) is where an unattended train or vehicles not attached to a traction unit run away past a signal at danger. Note that where this was the fault of the driver, this will be classed as a SPAD. Some SPADs are defined as a; SAS SPAD – "Starting against signal" SPAD, where the train was standing at a danger signal and the driver moved past it. SOY SPAD – "Starting on yellow" SPAD, where the train started on a caution signal and the driver did not appreciate that the next signal would be at danger. Passing signals at danger – with authority Signals form part of a complex system, and it is inevitable that faults may occur. They are designed to fail safe, so that when problems occur, the affected signal indicates danger (an example where this did not happen, known as a wrong-side failure, was the Clapham Junction rail crash due primarily to faulty wiring). To keep the network running, safety rules enable trains to pass signals that cannot be cleared to a proceed aspect. Provided that authority for the movement is obtained, a SPAD does not occur. There are two methods of obtaining that authority: Driver obtains signaller's authority to pass a signal at danger Once the train has been brought to a stand at a signal which is at danger, the driver should attempt to contact the signaller. If the signal cannot be cleared then the driver must obtain the signaller's authority to pass it at danger. Methods for contacting the signaller may include GSM-R cab radio, signal post telephone or mobile phone. The signaller can authorise a driver to pass a signal at danger when: The signal is defective or disconnected The signal cannot be cleared because signalling or level crossing equipment has failed The signal is to be passed at danger for shunting purposes The signal cannot be cleared because a train or movement which has reversed is then required to start from beyond that signal An electric train is to pass the signal protecting an isolated section and proceed towards the limiting point A train has been accepted using restricted acceptance because the line is clear only