Regulating conversation during driving: a problem for mobile telephones?

Why are hands-free mobile telephones linked to driver distraction and increased involvement in accidents? We suggest that during normal in-car conversation, both the driver and passenger will suppress conversation when the demands of the road become too great. However, a remote speaker on a mobile telephone has no access to the same visual input as the driver, and will be less likely to pace the conversation according to roadway demands. To test this hypothesis pairs of naïve participants drove a circuit of roads including dual carriageways, rural, urban and suburban roads in Nottinghamshire, UK. One of the participants in each pair was the driver, while the other was the conversational partner. Across three laps of the circuit the partner engaged in a verbal task with the driver while sat in the same car (with or without a blindfold), or via a hands-free mobile (cellular) telephone. The number of utterances, words, and questions were analysed for both drivers and passengers across the different types of road. The results demonstrated that the normal in-car conversations were suppressed during the most demanding urban roads. The mobile telephone condition prevented suppression from taking place in the passengers' conversations, and even encouraged drivers to make more utterances that they would normally do with a normal in-car conversation. The results demonstrate a potential problem when using hands-free mobile telephones while driving.