Mark Hill is a traffic police officer in Watford, near London. He works on some of the busiest motorways in Britain, the M1 and the M25. There are traffic police on duty 24 hours a day. There are three shifts, and each shift is eight hours. On average they have to deal with three to four accidents each shift. Here is Mark Hill talking about his job: "We deal with anyone in the accidents who is injured. That's the first thing. Then we have to clear the road and get the traffic moving again. "Most accidents happen because people drive too fast—especially when the roads are wet. Sometimes we get accidents that occur because drivers don't follow the rules. For example, I've seen a number of cases of drivers overtaking on the left. This is illegal in Britain. If you want to overtake, you have to go into the fast lane on the right. "Not everyone can use a motorway in Britain. People that ride a bicycle or a small motorbike are not allowed to use it. "I like my job. I have a varied working day—I never know what I'm going to do from one day to the next. And I meet all kinds of people. I don't think there's any other job that can give you that experience./
单选题
According to the passage, accidents sometimes happen because ______.