A new study says that driving while talking on a hands-free phone can be just as distracting as talking with a hand-held phone.

The study, conducted by the University of Sussex, compared drivers using hands-free devices and drivers without any distractions and measured how many hazards they recognized on the roads.

According to the study, conversations which use visual imagination create competition for the brain’s processing capacity, making the brain choose between imagining the conversation and processing the road.

“The person at the other end of the phone might ask, ‘Where did you leave the blue file?’ causing the driver to mentally search a remembered room,” said Graham Hole, a senior psychology lecturer at the university.

The study found that drivers using a hands-free device focused on smaller areas of the road, sometimes even looking directly at a hazard but failing to recognize it. They also found that conversations may use more of the brain’s processing resources than previously thought.

“The use of hand-held phones was made illegal primarily because they interfere with vehicle control,” said Hole. “But a mounting body of research is showing that both hand-held and hands-free phones are dangerously distracting.”

The study ran two experiments. The first one had participants either undistracted or distracted by listening to sentences and deciding whether they were true or false. Some of the sentences evoked visual imagery, like, “A five pound note is the same size as a 10 pound note.” The other half of the statements did not -- for example, “Leap years have 366 days.”

All of the distracted participants were significantly slower in responding to hazards that they saw, and missed more hazards than the undistracted participants. According to the study, they would also look at the hazard but fail to recognize it.

The second experiment involved some of the participants mentally moving around an imaginary grid by following verbal instructions, compared to other participants who were not. The distracted participants were more likely to miss a hazard in their periphery due to visual tunnelling or only focussing on a small, central area of the road.

However, the study says that the same does not necessarily apply to a chatty car passenger because the driver can better moderate the conversation based on the driving situation. The person on the phone is oblivious to the road.

“Conversations are more visual than we might expect, leading drivers to ignore parts of the outside world in favour of their inner ‘visual world,’” said Hole.

According to Hole, the only safe phone in a car is one that’s turned off.