Researchers at the University of Utah's Applied Cognition Laboratory (PDF) have found people talking on a phone while driving along a simulated multilane freeway are less likely to change lane when they catch up with a slower-moving vehicle, resulting in a lower mean speed and a negative impact on traffic flow.

That said, changing lanes in such circumstances can actually impede traffic flow if not done correctly. Have you ever encountered a snarl-up on a freeway, only to see it clear a few kilometres along the road with no indication of an accident or breakdown?

Chances are that it was caused by someone changing lane into a too-small gap, causing the vehicle coming up behind to brake. That deceleration propagates back along the road, with heavier breaking needed by drivers who fail to leave sufficient separation, and that results in traffic coming to a near standstill. Consequently, drivers in that lane try to move left or right, triggering similar 'shock waves' in those lanes.

And don't think you're OK if you use a hands-free kit or Bluetooth headset. Other research carried out by the Applied Cognition Laboratory has shown that once the call is established, there's no difference in levels of distraction between holding the phone and hands-free operation, and talking on the phone is a much greater distraction than conversing with a passenger.

So switch your phone off before you drive away. I'd suggest making it a New Year Resolution, but they're not very successful.