In early June, 2012, a Massachusetts man was convicted of homicide in a car crash that happened when he was texting while driving. His testimony was that he was distracted and looked away from the road for "one quick second." 
Several years ago, the Virginia Tech Transportation Institute conducted a study of distracted driving which indicated that even minor distractions, such as reaching for a cell phone, measurably increases the chances of a driver being involved in a crash or near-crash event.  More significant distractions, such as texting while operating a heavy truck, increase the risks of a crash by more than 23 times. Many states, including Virginia, have reacted to this growing problem by enacting statutes that limit or prohibit activities likely to distract drivers' attention from the road.
Earlier this year, the Federal Department of Transportation's National Highway Traffic Safety Administration (NHTSA) published proposed guidelinesthat would classify some activities, such as watching video footage unrelated to driving, as inherently incompatible with safe driving. Other activities would be measured to determine the degree to which they distract drivers. While the proposed guidelines are currently in their infancy, the effect of the guidelines is potentially far-reaching. In the case of all inherently incompatible activities and those which are determined to be distracting to an unacceptable degree, the guidelines recommend that devices (such as cell phones) be redesigned to prevent performance of an activity while driving.
In studying this issue, the NHTSA determined that 17% of crashes in 2010 involved some form of driver distraction. While the Virginia Tech study included data for both passenger and commercial vehicles, the data the NHTSA relied on appears to relate only to passenger vehicles. Virginia Tech's study indicates that the risk of a crash when performing many activities in a commercial vehicle is correspondingly greater than while performing the same task in a passenger vehicle. For example, compared to a non-distracted driver, the risk of a crash when dialing a phone in a passenger vehicle is 2.8 times greater but 5.9 times greater in a commercial vehicle. Similarly, compared to a non-distracted driver, the risk of a crash when reaching for a cell phone in a passenger vehicle is 1.4 times greater but 6.7 times greater in a commercial vehicle. As a result, the NHTSA data may not fully describe the magnitude of this problem.
Distracted driving is an increasingly significant cause of motor vehicle collisions, and cell phone (or smart phone) use appears to be a growing cause of distracted driving. When you are driving, limit cell phone use as much as possible and drive defensively; another driver may be distracted so be alert for sudden vehicle movements.