Moreover, they cause almost twice as many errors as drivers driving without the distraction of a mobile phone conversation.
In the study led by Dr Melina Kunar, from the University of Warwick's Department of Psychology, and Dr Todd Horowitz, from Harvard Medical School, the participants were asked to pay attention and respond (by pressing one of two keys on a keyboard) to a series of discs moving around a computer screen.
Some of the participants carried out the task with no distraction. Others carried out the task while also using speaker phones to simultaneously engage in a normal phone conversation, discussing things such as their hobbies and interests.
The researchers found that on average the reaction times of those engaging in the hands free telephone conversation were 212 milliseconds slower than those who undertook the task without the simultaneous telephone conversation.
A car travelling at 60 miles an hour would travel 5.7 metres (18.7 feet) in that time so the distracting conversation would obviously increase any braking distance at that speed by the same amount.
The participants distracted by a phone conversation also made 83pct more errors in the task than those not in phone conversations.
"Our research shows that simply using phones hands free is not enough to eliminate significant impacts on a driver's visual attention," said Kunar.
"Generating responses for a conversation competes for the brain's resources with other activities which simply cannot run in parallel.
"This leads to a cognitive "bottleneck" developing in the brain, particularly with the more complicated task of word generation," he added.
The study suggests that hands free telephone conversations which require people to carefully consider the information they hear and then to make complex cognitive choices based on that information (a business decision for instance) have a particularly significant negative impact on a driver's ability to process and act on the visual information that is critical to their driving performance.
The study has been published in the Psychonomic Bulletin and Review 2008.