Because legislation has been proposed in a number of countries and U.S. localities that hands-on cell phones be banned, it is easy to reach the concluding that hands-free cell phones are less risky than hands-on cell phones.
But a number of studies have strongly indicated that the most distracting part of a cell phone call is not dialing the call, but the cognitive load of talking on the phone.
Typically, dialing a cell phone number takes about ten seconds. The typical cell phone call is two minutes–that is, the conversation lasts, on average, twelve times as long as the dialing. At least one “anchor task” study indicates that dialing the cell phone is roughly as distracting as radio dialing.
Talking on the phone is very distracting for at least three reasons. First, conversation requires a lot of cognition. My favorite quotation on this subject is
Anyone of driving age has made numerous phone calls, using land lines, during their lifetimes. How do we respond to someone who is standing in front of us trying to capture our attention while we are on the phone? Often we wave them away, or interrupt our conversation on the phone to address the other person. Even after years of talking on the land line phone, our ability to concentrate on more than one activity doesn’t seem to improve. (Frances Bents, from the Ask the Experts forum described above).
Secondly, because the person we are talking to cannot see the other potential distractions, and so is not aware when he or she should be quiet or wait for an answer.
Finally, cell phone use while driving is dangerous because people overrate their ability to judge the consequences of small probability/high cost risks. The chance of an accident while driving and talking on a cell phone may be small, although it must be said that what society might consider “small” and what the actual number of accidents attributable to cell-phone use is not currently known. But even if it is small–again, this is not clear–people probably underestimate the risks involved.