What Garly says about the cell phones seems to be right.
There was, as always, a study done on it.

The one I saw was compared telephone conversation to a chatty passenger. The cell phone was far worse because even a chatty passenger shuts up and lets you drive when you are getting near corners and intersections.
Being on the phone was as bad as having kids fighting in the back seat, and whether it was a hands on phone or not seemed to make very little difference
The part that got me was they investigated the effect of having a coffee. It seems while a person is drinking a cup of coffee they are as bad in reaction time as a person who had two or three beers

-----
Here is another one that compared cell phone users to drubk drivers.

A Comparison of the Cell Phone Driver and the Drunk Driver

David L. Strayer
University of Utah

Dennis J. Crouch
University of Utah

Frank A. Drews
University of Utah


July 2004

AEI-Brookings Joint Center Working Paper No. 04-13


Abstract:
We used a high-fidelity driving simulator to compare the performance of cell-phone drivers with drivers who were legally intoxicated from ethanol. When drivers were conversing on either a hand-held or hands-free cell-phone, their braking reactions were delayed and they were involved in more traffic accidents than when they were not conversing on the cell phone. By contrast, when drivers were legally intoxicated they exhibited a more aggressive driving style, following closer to the vehicle immediately in front of them and applying more force while braking. When controlling for driving conditions and time on task, cell-phone drivers exhibited greater impairment than intoxicated drivers. The results have implications for legislation addressing driver distraction caused by cell phone conversations.

Keywords: cell phones, drunk drivers, performance, legislation