At its most simple the strategy, which is intended to produce chaos and disruption with little risk of loss of life, can involve one terrorist telephoning a number of targets from anywhere in the world.
The IRA know that as long as the calls include a known codeword, the emergency authorities are certain to respond.
As Dr Michael Page, of Bradford University's Department of Peace Studies, explained: "It's very difficult to protect transport infrastructure. You cannot have every signal box and motorway bridge manned or monitored. The system is virtually indefensible.
"We are a free, open, democratic country, so we don't have vast numbers of police that can be positioned everywhere - its virtually impossible to deal with this."
However, the terrorist tactics do have weaknesses that can be exploited. To ensure their actions obtain maximum publicity and to maintain credibility, the terrorists need to plant some bombs, such as the device that went off in Leeds last Friday. It is while they are being placed or during recognisance that they risk being identified.
The use of closed circuit television cameras and enhanced public awareness of suspicious behaviour are considered by the police as two of the most important anti-terrorism tools available. But probably more important is the role of the intelligence services, particularly MI5 and Scotland Yard's Anti Terrorist Branch. IRA members are unlikely to be caught in the act of making a bomb threat, but via careful surveillance and the use of informants, active service units can be tracked down.
Dr Page believes the IRA might want to change tactics and target different forms of transport such as bridges or tunnels. He also speculated that it might want another "spectacular" hit similar to last year's Docklands bomb, although this is more likely to result in death, which would seriously damage Sinn Fein's hopes of entering talks with the new government.
He said: "The IRA likes to change its tactics and to be unpredictable."
David Veness, the Metropolitan Police's assistant commissioner with responsibility for specialist operations, emphasised that "the security assumption has got to be that there's a threat to human life". Evacuation was not automatic, he said, but employed only after sophis-ticated assessment of the threat.
Dr Richard Clutterbuck, lecturer in security at Exeter University, said people were ready to take a greater degree of responsibility for their own safety, adding:"I would like to see the police tell us the dangers and let us decide on whether we want to take the risk or not..."
Traffic disruption such as that caused by the IRA in England yesterday was once commonplace in the early 1990s in Belfast, but has not been seen on a large scale in recent years.Reuse content