In "Oh, no" news, UC San Diego researchers say that
"'Hands Free' Isn't Mind Free: Performing Even Easy Tasks Impairs Driving", where they demonstrate that in their test group, use of a hands-free telephony device still results in 174 milliseconds delay in response time. "
Participants were 174 milliseconds slower at braking when the two tasks occurred at the same time than when they were presented 350 milliseconds apart. While 174 milliseconds may sound tiny, it translates to 16 feet in a car going 65 mph." Now, I'm going to have to say that I'm not liking the way that these results are reported on the ScienceDaily site.
Response time (RT) studies have always shown a high degree of variability in response times both within the same subject and between subjects, and I suspect that this study is no exception. But we got nothing to indicate just how large the standard deviation was in the response time difference in the study group, just the impossibly precise figure of an additional 174 ms added to normal response time.
What does that mean? If this study is like other RT studies, it means that it is likely that the range of RTs with the treatment substantially overlapped the RTs when stimuli were more than 350ms apart. And on the other end, that people will quite likely experience the occasional RT that is much longer than the 174 ms delay average. Deciding to drop back an additional 16 feet at 65 mph, based upon the news report of the research, to compensate for using a hands-free device should not be thought of as a sure thing: the RT in some particular case could be longer than the average figure, leading to very bad outcomes. (
Source)