Militants are increasingly turning to vehicle-ramming attacks, like the one staged near Britain's parliament on Wednesday, because they are cheap, easy to organise and hard to prevent.
Experts say the tactic of mowing people down avoids the need to obtain any explosives or weapons and can be carried out by a "lone wolf" attacker without using a network of fellow militants – all lessening the risk of alerting security agencies.
"This kind of attack doesn't need special preparation, it is very low-cost, within anybody's reach," said Sebastien Pietrasanta, a French Socialist lawmaker and terrorism expert.
"It is often a case of individual action," he told Reuters. "They can be quite spontaneous."
Five people were killed and at least 40 injured in London after a car ploughed into pedestrians and an attacker stabbed a policeman close to parliament in what police called a "marauding terrorist attack". The attacker was shot dead.
Trucks were used to devastating effect last year against crowds in Berlin and Nice, in contrast to more organised attacks that have already hit Paris and Madrid – as well as London in 2005 – using teams of bombers or gunmen.
Islamic State claimed responsibility for both the Nice attack last July, when a truck killed 86 people celebrating Bastille Day, and for the Berlin attack in December, when a truck smashed through a Christmas market, killing 12 people.
While no group has yet claimed responsibility for Wednesday's attacks, the Islamic State is under intense pressure in Syria and in Iraq, where one of its last strongholds, Mosul, is under assault from Iraqi forces backed by a coalition that Britain is part of.
Islamic State encouraged readers of its online magazine Rumiyah in 2016 to use vehicles to kill and maim.
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Israel Attacks
Vehicle attacks are not a new tactic in the Middle East.
In 2008, a Palestinian rammed a bulldozer into vehicles on a Jerusalem street before a visit by then US presidential candidate Barack Obama, wounding at least 16 people.
Another Palestinian drove his truck into a group of Israeli soldiers in Jerusalem in January this year, killing four of them in an attack that Prime Minister Benjamin Netanyahu said was likely to have been inspired by Islamic State.
Former senior CIA analyst Paul Pillar said that while concern has long focused on "sophisticated or high-tech methods of terrorist attack, the most readily available methods for killing a lot of innocent people have always been simple and require no sophistication or training.” He added:
This includes mowing people down with a vehicle on any crowded city street. Locations might be chosen that have some other political or religious significance –such as a Christmas market, or the vicinity of a national parliament – but there always are vulnerable public places with lots of people.
Jean-Charles Brisard, president of the Centre for the Analysis of Terrorism, a European thinktank, said Wednesday's attack seemed to be "rudimentary in its conception".
Using a car as a battering-ram was a tactic that was highly rated by militants because it was lethal, he said. "With a vehicle, they cause a lot more deaths than with a knife or a machete."
"Attacks today are increasingly unpredictable, with rudimentary weapons, handguns, knives, vehicles," he said.
Anne Giudicelli, head of security consultancy Terr(o)RISC in Paris, said the extra vigilance over large cities had helped to spawn a change in the militants' approach.
Every time you put in place a new measure after an attack or a thwarted attack, the assailants adapt to get around the measures in place and find the gaps.
(This article has been edited for length.)
(Published in an arrangement with Reuters.)
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