Traffic Safety: Here is a passage from a study published by the National Highway Traffic Safety Administration in 2007."The National Occupant Protection Use Survey (NOPUS) is the only probability-based survey of driver cell phone use in the United States. The survey observes usage as it actually occurs at a random selection of roadway sites, and so provides the best tracking of the extent to which people in this country are using cell phones while driving."NOPUS observed 43,000 drivers in this study.Which question is this observational study designed to answer?Which type of cell phone use is safer to use while driving: hand-held or head set?What proportion of U.S. drivers talk on a cell phone while driving?Does use of a cell phone while driving increase the risk of an accident?