After the rush-hour speed limit on the British M25 motorway was lowered from 70 miles per hour (115 kilometers per hour) to 50 miles per hour (80 kilometers per hour), rush-hour travel times decreased by approximately 15 percent. Which one of the following, if true, most helps to explain the decrease in travel times described above? ['Before the rush-hour speed limit was lowered, rush-hour accidents that caused lengthy delays were common, and most of these accidents were caused by high-speed driving.', 'Travel times during periods other than rush hours were essentially unchanged after the rush-hour speed limit was lowered.', 'The number of people who drive on the M25 during rush hours did not increase after the rush-hour speed limit was lowered.', 'After the decrease in the rush-hour speed limit, the average speed on the M25 was significantly lower during rush hours than at other times of the day.']