The primary responsibility of the driver is to operate a motor vehicle safely. The task of driving requires full attention and focus, and cell phone use can distract drivers from this task. Therefore, the safest course of action is to refrain from using a cell phone while driving.
Research shows that driving while using a cell phone can pose a serious cognitive distraction and degrade driver performance. The data are insufficient to quantify crashes caused by cell phone use specifically, but NHTSA (the National Highway and Transport Safety Administration, in the US) estimates that driver distraction from all sources contributes to 25 percent of all police-reported traffic crashes.
Recent research indicates that whether it is a hands-free or hand-held cell phone, the distraction is significant enough to impact a driver's performance.
While drivers should make every effort to move to a safe place off of the road before using a cell phone, in emergency situations every driver should use their judgment regarding the urgency of the situation
Some say that talking on a cell phone is no worse than having a conversation with someone in the car, and some research shows each to be equally risky. A big difference between the two is that a passenger is observing the driving situation and can pause for, or alert the driver to, potential hazards, while the person on the other end of the cellphone is unaware of the roadway situation.
In a controlled study, which compared eating while operating a voice-activated cell phone to continuously operating a CD player, it was found that the CD player operation (and it would likely be the same result with an MP3 player) was more distracting than the other activities. In another study, results also showed that manual dialing was about as distracting as grooming or eating while driving, but less distracting than reading or changing CDs. The other tasks, though, are typically done less frequently and for a shorter duration in any given trip.