Don't - seriously, don't - ask your lover over the mobile what they're wearing while you're driving, even if you're hands-free.

The latest study on drivers' use of mobile phones concludes talking on a hands-free phone while driving is just as hazardous as holding the phone in your hands. And the most dangerous kind of conversation is one which sparks your visual imagination. Participants in the study who were distracted by visual imagery reacted half as fast to hazards as those who were not distracted.

It needn't even be something as exciting as a vision of your beloved in lycra. Simply imagining the facial expression of the person you're talking to is enough, said senior psychology lecturer Dr Graham Hole of the University of Sussex, because "the visual imagery competes for processing resources with what the driver sees in front of them on the road."

The experiment required 20 males and 40 females to sit in a car seat behind a steering wheel with pedals representing a brake and an accelerator. They were shown seven minute videos simulating road driving in which they had to respond to unexpected hazards by hitting the brake. One group was not distracted, the other group was distracted by a male voice on a loudspeaker making statements they had to identify as true or false.

All the distracted drivers had slower response times than those who weren't distracted, but those whose answers required visual thinking - for example, "a ten pound note is bigger than a five pound note, true or false" - had the worst responses. Sentences such as "Leap years have 366 days" were less distracting.

"Our study adds to a mounting body of research showing that both hand-held and hands-free phones are dangerously distracting for drivers", Dr Hole said. "The only 'safe' phone in a car is one that's switched off".

He said chatty passengers tend to pose less of a risk than mobile phone conversations because they usually moderate the conversation when hazards arise, but someone on the end of the phone is oblivious to other demands on the driver and to non-verbal cues that might otherwise stem the flow of chat.

Distracted drivers also suffered from "visual tunnelling", the study found. They focussed their eyes on a small central region directly ahead of them that was four times smaller than the area looked at by people who weren't distracted.

In NSW it is an offence to use a mobile phone while driving unless it is in a cradle fixed to the vehicle, is operated hands free and doesn't obscure your view of the road.

Even then it can only be used to make or answer calls, to play audio or as an aid to driving such as with the use of navigation apps. Learners, provisional P1 licence holders and motorcyclists are not permitted to use mobile phones at all while driving.

In the first ten months of this financial year NSW police issued notices for 32,175 driving offences involving mobile phones with fines totalling $10.3 million.

A 2015 Deloitte survey asking 1600 Australian smartphone owners about their mobile phone usage habits found 42 per cent of those aged 18-75 had used their phones while driving. The NSW Centre for Road Safety warned even short lapses of concentration when driving could have "significant and devastating" consequences.