Motorists who talk on mobile phones drive more slowly, pass sluggish vehicles less often and take longer to complete their trips, according to new research.

"The average person's commute is longer because of that person on the mobile phone right in front of them," said Dave Strayer, a psychology professor at the University of Utah.

Joel Cooper, a doctoral student in psychology at Utah, added: "If you talk on the phone while driving, it is going to take you longer to get from A to B, and it is going to slow down everybody else on the road."

The researchers conducted the study with Ivana Vladisavljevic, a doctoral student in civil and environmental engineering, and Peter Martin, an associate professor of civil and environmental engineering and director of the Utah Traffic Lab.

The new study shows that using mobile phones while driving is dangerous and causes delays.

Previous research on mobile phones and driving from Professor Strayer's research group indicates that hands-free mobile phones are no less dangerous while driving than handheld devices because the major distraction is the conversation itself.