The letter bomber is likely to be a frustrated motorist who has "snapped", and is driven by the desire for revenge, to gain notoriety, and to cause mass panic, according to psychologists and police experts.
The culprit who sent the three most recent devices is not thought to have a single aim, instead he - for it is almost certainly a man - is lashing out at institutions he feels have wronged him.
So far the homemade bombs have caused only minor injuries, but the fear is that the bomber will escalate the campaign by using more deadly packages to gain publicity. There is also a risk of "copycat" attacks.
Dr Keith Ashcroft, a forensic psychologist, said the fact that the packages were being delivered to a wide range of organisations involved in dealing with motoring offences suggested "a frustration response that they can't express their anger and they want to create mass panic".
He added: "It is possible that someone who is frustrated with other aspects of their life is channelling their hate, rather than a specific vendetta.
"They have a troubled background and they are picking on a case to make them notorious."
The incidents appeared to be more of a criminal action than a political one, he said, and not something intended to cause mass killing like a terrorist attack.
Dr Ashcroft said: "This is not someone who thinks they're representing all motorists."
Dr John Potter, a consultant psychologist, added: "We're probably going to be dealing with something like a frustrated motorist who's gone past the limit that they can cope with."
He described such actions as "cry for attention. What they don't realise is the human side, the other person who opens the letter."
"They will think they are invincible and start to take chances," he said.
Dr Potter said the culprit would feel increasingly tempted to tell someone about their actions.
Forensic specialists say that the bomber is likely to be using a simple explosive device, possibly using gunpowder from a firework, fitted inside a padded envelope that is detonated by the opening of the package.
Obtaining the materials and constructing the bombs is relatively simple, with instructions available on the internet, but it takes practice to build a device that will detonate on opening.Reuse content