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It is clear that the charts though are meant to be misleading. They acknowledge in the study that the reported times of the crashes are not precise with many that occur a few minutes before or after the hour are reported on the hour. They do group the collisions into 10 minute bins as a result but they don't do the same for the call times in the chart. If they tried harder to ensure both charts reflected the same time periods, graphically, the charts would not appear to as strongly support the conclusion as the cell calls drop dramatically in the few minutes before 9 and rise just after. The average increase in usage would be no where near as dramatic.