Police officers can do it. Ambulance drivers can do it, usually. So can firefighters. Even David Beckham has done it. So when can people get away with breaking the speed limit?
When can you go over the line?
"Robert ran out at the pedestrian crossing to catch his bus. He either didn't hear the police siren, or couldn't tell which direction it was coming from. The car was on him before it saw him," says Jeremy Scutts, whose son was knocked down by a speeding police car in 1995.
Robert, then 17, was badly injured and spent five days in a coma. He continues to suffer debilitating problems with his memory and balance. "He was going to university and then join the air force. As it was, he didn't even finish his A-levels," says Mr Scutts.
A need for speed?
The patrol car which struck Robert was answering a call to chase robbers who had fled the scene of a burglary on foot.
"I really think police cars should take into account the level of the incident they are responding to," says Mr Scutts. "Even when the police drove us to the hospital to see Robert, the officer was going 80mph. I kept thinking what might happen if another car pulled out."
Only a tiny proportion of the UK's 3,500 annual road deaths result from accidents involving vehicles on 999 calls, but the laws allowing panda cars, ambulances and fire engines to break the rules of the road remains controversial.
The Road Traffic Regulation Act 1984 - intended to keep ordinary motorists on the straight and narrow - gives these vehicles leave to break the speed limit "if the observance of that provision would be likely to hinder the use of the vehicle for the purpose for which it is being used on that occasion".
Ambulance man Mike Ferguson faces a speeding charge
But as ambulance driver Mike Ferguson has found to his cost, turning on the sirens and flashing lights on his ambulance service car is still no licence to putting the pedal to the metal.
Mr Ferguson is facing a court appearance, and potentially a career-ending ban, for allegedly touching 104mph as he rushed a liver to a transplant patient - a task which he, if not Lincolnshire police, thinks should fall within the terms of the exemption.
This prosecution has prompted anger and incredulity from many quarters. However, in a 2002 report, the Association of Chief Police Officers (ACPO) said such cases had to be pursued to ensure that emergency workers do not abuse their status.
"Each and every recorded event must be capable of justification to preserve the integrity of the ... police service," said the ACPO code of practice.
Every day emergency vehicles exceed the speed limits intended to keep road users safe. In 1997, police vehicles alone were involved in 27,721 accidents.
Fatalities from such accidents also crept up during the 1990s. The first five years of the decade saw a total of 100 deaths, while some 25 people were in incidents involving police drivers in the year 1999/2000 alone.
Ambulances and fire engines too have claimed lives. In one heartbreaking tragedy, a 37-year-old man was killed by an ambulance rushing a child victim of the Omagh bombing to hospital. The ambulance driver pleaded guilty to dangerous driving, despite the dead man's family opposing a prosecution.
Police vehicles were involved in 27,721 accidents in 1997
While all the emergency services must weigh the risks of dawdling on the way to potentially life-and-death emergency against the risk to other road users, the police are in a particularly tricky spot given their duty to uphold traffic laws.
Just last month, a Scottish police driver marred Strathclyde's No Speeding Day by hitting another car as he rushed to a prior traffic accident.
Driving the point home
"This is not the kind of example police should be setting to the public - we want people to put the brakes on irresponsible driving," a traffic officer told a Scottish newspaper.
Indeed, some of those ordinary motorists fined, banned or penalised in the more than 1 million speeding offences spotted by the authorities each year should seek their own exemptions, says motoring law solicitor Geoffrey Miller.
The Highway Code may say that drivers should never exceed the speed limit, but says Mr Miller, "the rules are not set in stone".
"If an emergency is involved, a court might accept that there was a special reason for speeding."
Footballer David Beckham escaped losing his licence when he convinced a court he was only speeding to escape a pursuing photographer who was driving dangerously.
Rushing to the toilet can excuse driving offences
Such "special reasons" for breaking traffic laws can include even the most unlikely emergencies. Beckham's boss at Manchester United, Sir Alex Ferguson, was cleared of a traffic offence by explaining that severe diarrhoea and the need to reach a toilet prompted his driving on a motorway hard shoulder.
"Diarrhoea is a good example of what a court might accept as a special reason for speeding, though not if you were going at 110mph," say Mr Miller.
Given such interpretations of the law, Mr Miller is convinced that "rushing to hospital with an organ" is a very sound excuse for speeding.