(Lissy et al p. 67)
A study has been published in the Journal of Experimental Psychology and this study lends credence to this position. It showed that a subject engaged simultaneously in driving and a verbal task (repeating the words of the experimenter) visually scanned a much smaller area outside of the vehicle than when not engaged in such a secondary task (Recarte & Nunes p. 31-42). Performing simple spatial imagery tasks while driving (e.g., mental rotation of letters) caused the scanned area to shrink even more. Critics cite this study (among many others) to buttress the position that any task which significantly occupies a driver's mental resources (such as talking on a cellular phone) may have a negative impact on safety (by making the driver less likely to notice unexpected events) and, thus, should be addressed by legislation.
Driver distraction is a definite problem in terms of its impact on safety. National Highways Traffic Safety Administration (NHTSA) estimates that 25 percent of traffic accidents involve at least some degree of distraction on the operator's part, although only a small fraction of these involve the use of cellular phones. (Dreyer et al p. 1814)
Driver distraction is a long-standing concern, one that has been debated for more than 90 years.