Last week's collision between two trains on the world's tallest rollercoaster followed mandatory system improvements designed to prevent such an accident.
After a collision in 1994, in which 26 people were injured, safety inspectors required the owners of the computer-controlled Pepsi Max Big One at Blackpool Pleasure Beach to install a back-up system and a hard-wired emergency control to ensure that staff could stop a car should the computer systems fail.
Inspectors also found that, prior to the 1994 accident, the owners may not have fully adhered to guidelines that related to the introduction of programmable electronic systems. They recommended that the fairground's owners commission an independent risk assessment and a safety requirement specification.
This safety assessment was carried out by AEA Technology and its recommendations are said to have been implemented.
In July 1994, the Health and Safety Executive also issued a prohibition notice to Blackpool Pleasure Beach which said the operation of the Big One rollercoaster involved a "risk of serious personal injury" and that it was "unsafe for the carriage of persons because of deficiencies in the braking and control systems".
The notice also said the Pleasure Beach had "not taken all reasonably practicable measures to ensure the safety of passengers". As a result, the owners made a series of improvements to ensure that carriages were never allowed to come dangerously close to each other.
But in last week's accident, as in 1994, a carriage failed to stop and hit a stationary carriage. Fourteen people were injured, two seriously.
David Cam, director of the amusement park, told BBC News: "We know that one car was sitting in the brakes outside the station, we don't know why. We do have a completely automated, computer-controlled system, which gives us the advantage of being able to download details of the braking system so we can find out precisely what happened in those seconds before the accident."
A safety-critical software system, developed by contractors Allen-Bradley, keeps the ride's three trains at a safe distance from each other. It monitors the location of the trains and controls the braking, ensuring that the speed of the carriages is within safety limits, yet fast enough to thrill passengers. The deployment of three separate trains is an economical use of the track, allowing dozens of passengers to be on the ride while others are getting on and off.
Limitations imposed by the 1994 Prohibition Notice were lifted in 1995 once largely paper-based evidence was presented to the Health and Safety Executive that technological and other improvements had been made.
However, after the 1994 accident, safety inspectors pointed out that, even with the required improvements, there were always risks with three-train automated rollercoasters. This is despite a comment by one safety inspector that the programmable logic controller was one of the most reliable he had encountered.
A spokeswoman for Blackpool Pleasure Beach said: "We are looking at whether something went wrong with the computer system."