An interesting piece of research from Perth, they did a study of people who had required hospital treatment due to a car accident and found that using hand-held mobile phone gave you almost a 5x chance of having an accident in the next 10 minutes, whilst using a hands-free system meant almost a 4x chance of an accident.
This study is bolstered by a study done in Japan which appears to have found at least one reason why trying to use a mobile phone whilst driving is so dangerous. It’s all down to the audio quality as the signal drops out as you move from cell to cell or from obstructions.
To quote from the New Scientist article on the report:
Hamada’s team measured the sound quality of mobile phone calls in parked cars and in cars travelling at 65 kilometres per hour. A comparison of the two types of voice signal revealed silent periods of about 300 milliseconds interrupting the signal roughly six times a minute. They also discovered a time lag of about 300 milliseconds for a phone in a moving car, while for 5 per cent of the time, the frequency range becomes distorted.
Then researchers then played 11 volunteers an audio recording of a story that included similar interruptions. As the volunteers struggled to hear the distorted parts of the recording, their right parietal cortex, the part of the brain that perceives sound, became more active (Transport Research Part F, DOI: 10.1016/j.trf.2005.04.016).