In 2003 it became an offence in the UK to use a handheld phone while driving. It is widely believed that the use of handheld phones is substantially more dangerous than the use of hands-free phones and that this was a sensible precaution. This belief is, however, bollocks.
The major increase in risk is caused by the cognitive workload involved in holding a non-colocated conversation, not the use of hands as concluded by case-crossover studies, epidemiological studies, simulation (PDF) and meta-analysis (PDF)
The Cognitive Tasks
These tasks are cerebral in nature and include activities such as remembering a telephone number and engaging in conversation. These tasks can disrupt driver performance by diverting attention to an engaging cognitive context other than the one associated with driving.
Driving and Talking
Driving relies on visual attention while tasks involved in holding a conversation require non-visual attention. These tasks occur in two separate modalities: the visual-spatial and the verbal-auditory. Multiple-resource theory suggests that it is most difficult to divide attention within a single modality, but that there is a less, and perhaps still significant, interference across modalities. This implies that as driving and using a phone occur in different modalities it should pose less of a distraction than if they occurred in the same modality. However, there is still a risk in dividing attention between the two risks.
It is reasoned that if it is the attention switching that causes the most distraction, and not the physical act of holding the phone, then it raises the question as to whether we should stop the driver from engaging in conversations with other passengers or listening to a radio. The comparison of the two types of conversation reveals that because the passenger and the driver share the same environment they can both recognise road dangers and employ subtle communication which allows them to alter their conversation accordingly. Also, attending to verbal material, such as listening to a book on tape, does not produce the same detrimental effect that phone conversation does in producing dual-task interference. It is suggested that a passive activity such as this can be ignored when appropriate and that full attention can be directed towards the primary task of driving.
There is another, more subtle, cognitive effect at work that may influence the attention of a driver. The very act of speaking to somebody who is not present seems to cause a distinct type of distraction as the speaker focuses his attention in a notional place that represents the person with whom he is speaking.
A Driver perspective
The benefits to allowing the use of a phone in a car include personal security and the ability to report accidents and crimes quickly. In one study, 39% of drivers who had a motor collision while using a mobile phone then used the phone to call the emergency services.In experiments measuring headway, it was shown that drivers can compensate for the increased risk of using a phone while driving by increasing their distance between their car and the one in front. This effect can often be nullified as they may be unaware of the impact of the phone on their ability to react and thus not make the adjustment. It may also be because drivers tend to overestimate their driving ability.An interesting finding from experiments in dual-task driving studies is that subjects can improve with practice. This raises interesting training and testing alternatives to banning mobile phones outright.
It would now be very difficult to enact a total ban on phones in cars relying on evidence that was available during the first law. Similarly, those that did purchase a car kit would have a legitimate reason for complaint.
Texting. Hmm, let’s leave that for another day but remember phones don’t crash cars – people do.
EDIT May 1 2014
Some people contacted me saying that it is obviously more dangerous to be trying to change gears, steer around a corner with a phone in your hand. That is true, however most crashes are due to a lack of attention followed by decreased reaction times resulting in rear-ending the car in front or leaving the road which happens when using hands-free phones as well as hand-held.