A recent study shows that those highways that carry the most traffic, and thus tend to be the most congested, have the lowest rate of fatal traffic accidents. Which one of the following, if true, most helps to explain the phenomenon described above? ['Most of the motorists on very heavily traveled highways tend to be commuting to or from work.', 'Most serious accidents occur when vehicles are moving at a high rate of speed.', 'Drivers have more accidents when they become distracted.', 'The highways that have the highest rate of fatal accidents have moderate volumes of traffic.']