By Becky Branford
The car bomb has become one of the tools most frequently used by insurgents in Iraq to attack US-led forces and Iraqi authorities, and their perceived collaborators.
Car bombs have become almost a daily feature of life in Iraq
On Wednesday, a string of car bombs hit Baghdad - four within just 90 minutes. Many people died.
The use of car bombing as a tactic has increased steadily since the US-led invasion of Iraq, say analysts, as insurgents become clearer about their strategy and goals.
Car bombs are mobile weapons that can both precisely target individuals seen as key to the occupation, and cause huge damage to infrastructure.
Large, yet inconspicuous
Using cars as weapons also has a number of other advantages, says Paul Beaver, defence and counter-terrorism analyst.
"They're easily available, they can do a huge amount of damage - you can pack up to half a ton of explosives into them - and they are very difficult to defend against," he says.
"Cars are so common - very few countries don't have cars. In places like Baghdad, there are lots of old cars going around, looking heavy, looking overloaded."
The most complex task in assembling a car bomb is constructing a detonation system.
But the insurgency in Iraq "includes many individuals with the requisite skills: former members of the dissolved security services, foreign jihadists, and a technically proficient population in general," Dr Magnus Ranstorp, director of the centre for study of terrorism and political violence at St Andrew's University, told the BBC News website.
Car bombing is not new. The IRA in Northern Ireland and the Basque separatist group Eta have both used car bombs.
But the use of car-bombing in Iraq nonetheless poses new challenges for the security forces there, say analysts.
Not all car bombings in Iraq are suicide bombings - and insurgent cells are loathe to use this method, as it means the loss of a valuable operative, says Dr Ranstorp.
But some insurgents' willingness to commit suicide in the attack instantly gives them an advantage over troops unwilling to lose their lives.
"It requires very small material and financial costs," says Dr Ranstorp, "and needs no escape routes. It also avoids the risk of the operative revealing an operation if he is captured and interrogated, which protects the cell."
Car bombs can be used both to target individuals and infrastructure
There are additional factors which make car bombings in Iraq harder to defend against.
In the UK, for instance, hi-tech means of almost instantaneously scanning and identifying a car license plates have been developed - systems impossible to install at present in Iraq, with no functioning vehicle registration service.
The targets sought by insurgents are numerous and often mobile - such as military convoys. These are much harder to protect than stationary targets, such as US embassies, which can be heavily fortified and subject to permanent risk reduction techniques.
And, as Mr Beaver points out, the best defences against improvised bombings, including car bombings, are good intelligence and vigilance, both of which are dependent on public co-operation and support - likely to be lacking in Iraq.
In the run-up to the 30 January elections in Iraq, election officials have announced three measures to reduce the risk of violence:
Foreign borders will be closed from 29-31 January to prevent foreign fighters entering Iraq to join the insurgency
Only vehicles with official permits will be allowed on the roads over the same period
The location of polling stations will be kept secret until the last minute.
While analysts agree these measures are likely to make attacks on election day more difficult, they differ as to what extent.
Heavy security around polling stations could leave voters vulnerable
For Mr Beaver, the measures "do sound effective, I think they're making a huge attempt and they're looking pretty good. But it's going to mean disruption, and nothing's foolproof."
But for Dr Ranstorp, these measures, in addition to tight security around polling stations, "may merely displace the threat. It's more likely there will be human mules that will at short notice go in [to polling stations] on foot, concealing explosives under a suit, on the body."
US and Iraqi security services "will have to worry about the voters themselves being operatives. They'll have to check individuals, which of course will slow down the electoral process.
"Every single person at a checkpoint will be a potential suspect. And of course this has a tremendous psychological impact.
"Managing this process will be a nightmare," he says. "It's the ultimate nightmare scenario."