You have heard the expression "horse sense". As you may imagine, it came from the time when the majority of people traveled in vehicles drawn by horses. A horse does have a lot of common sense, and there are many true stories about horses having saved their drivers from danger of death. Today for the most part, people travel by train, ship, airplane, and car, but very little by the use of horses. Modern vehicles are _ by engines, and an engine does not have any sense. In case of danger, many horses could be depended on to help. Not so with the engine; the driver must do all the things. In recent years, the speed of travel has increased greatly. With the increased speed comes greatly increased danger. The driver of a powerful engine needs to be careful. A moment of being careless may cost a life. In order to drive today, a person must be able to see clearly, and he must learn and follow carefully the rules for safety on the road. Greater attention is the price we pay for using a machine instead of a horse. The passage mainly talks about   _  .
engines safe driving using horses modern machines