Objective: Cell phone use while driving is hazardous; it quadruples the risk of a collision and multiplies the risk of a fatality nine-fold. The National Highway Traffic Safety Administration estimates that 8% of young drivers and 5% of all drivers use cell phones while driving. Participants and Methods: The authors trained graduate student volunteers to observe the daytime cell phone use of 3,650 drivers leaving the student exits of college parking structures at a large university. Results: The student observers recorded a cell phone usage rate of 11.1%, which was significantly higher than that seen in the National Occupant Protection Use Survey (p less than 0.00001). Female drivers were 1.51 times more likely to be using a cell phone while driving than were men (12.9% vs 8.6%, p less than 0.001), and drivers with passengers were 0.15 times more likely to drive while telephoning than were solo drivers (1.8% vs 12.1%, p less than 0.001). Conclusions: The authors offer suggestions for possible interventions to reduce this hazardous behavior. (Contains 3 tables.)