Depending on your point of view you can draw one of two conclusions from this paper: A Comparison of the Cell Phone Driver and the Drunk Driver. Here is the 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.
The abstract is truthful, but as you can see from this quote further on in the paper, the authors spun the lede in the more politically correct direction.
When drivers were conversing on a cell-phone, they were involved in more rear-end collisions, their initial reaction to vehicles braking in front of them was slowed by 8.8%, and the variability in following distance increased by 24.5%, relative to baseline. In addition, compared to baseline it took participants who were talking on the cell phone 14.8% longer to recover the speed that was lost during braking.
By contrast, when participants were legally intoxicated, neither accident rates, nor reaction time to vehicles braking in front of the particpant, nor recovery of lost speed following braking differed significantly from baseline.