Central London traffic ground to a halt during Wednesday morning's rush hour, due to a computer system that failed while engineers attempted to alter its software.
Thousands of drivers were trapped in their cars between the north and south circulars, according to Transport for London (TfL), with the worst-hit areas reported to be around Trafalgar Square.
The problem occurred at 6.15 a.m. as engineers attempted to upload a new set of rules for how the lights are phased across London, TfL said. The complex rules crashed part of the computer system, causing about 800 traffic lights to crash out of the coordination system.
The computer problem was fixed by 8.30 a.m., but by mid-morning traffic had still barely begun moving. This was partly due to a broken-down bus in Trafalgar Square, which exacerbated the problem.
TfL described the changes to the phasing system as a routine variation, and said the problems were caused by a particularly complicated new set of rules which contained conflicts. North and south areas continued functioning while, in the central area, traffic lights reverted to local patterns. The lights did not jam, but operated in a fixed pattern that was not coordinated with other lights in the area.
London uses a traffic-light control system called Split Cycle and Offset Optimisation Technique (SCOOT), first trialled in Glasgow in the early 1970s, which automatically responds to fluctuations in traffic patterns. The phasing cycles are varied periodically to improve traffic flow. TfL is responsible for about 4,500 traffic lights across the city, but not all are SCOOT-controlled.