Answer: Drivers in the United States who use cell phones while driving.
Explanation:
Population is defined as the whole group which an experimenter or researcher wants to draw conclusion from.
From the question, we are told that 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.
Therefore, based on the scenario above, the population will be drivers in the United States who use cell phones while driving.