What the law says

The law is very clear on the use of mobile phones while driving: you cannot do it. With anything else - whether it is eating an apple, drinking a cup of coffee or tuning the radio - it is up to the discretion of the police officer and the interpretation of the law by the court.

The relevant laws are the Road Vehicles (Construction and Use) Regulations, 1986, which say that the driver has to be in proper control of the vehicle. These outlaw anything which a police officer considers to have a detrimental effect on the motorist's ability to concentrate on the road.

As well as Sarah McCaffery and her apple, that could include reading a newspaper, consulting a map, lighting a cigarette or even talking to or looking at a passenger.

Being stationary in traffic - at a red light, for instance - is not necessarily a defence. Paul Watters, head of roads policy at the AA Motoring Trust, said: "The car is live on the highway and you as the driver are responsible. You don't know when the light is going to turn green, or when a fire engine is going to come up behind you and require you to move suddenly.

"It is very case-specific. Usually the police will not bring a prosecution unless there has been very bad driving."