The main purpose of the construction of expressways is to make it possible to travel more quickly. There is a minimum speed limit of 60 kilometers per hour for the expressway, to prevent the vehicle from driving too slowly and cause a rear-end collision accident, and the highway in our country generally stipulates that the maximum speed is 120 kilometers per hour simultaneously, so as to prevent the vehicle from being too fast and causing the driver to not react in time and thus cause more serious traffic accidents. 
Add ** note, no more than 140 words (optional) According to the "Regulations for the Implementation of the Road Traffic Safety Law", it is clearly stipulated that the driving speed of the highway shall be marked, and the maximum speed shall not exceed 120 kilometers per hour, and the minimum speed shall not be less than 60 kilometers per hour. The maximum speed of a small passenger car driving on a highway must not exceed 120 kilometers per hour, 100 kilometers for other motor vehicles and 80 kilometers per hour for motorcycles. 
Add ** comment, no more than 140 words (optional) If the highway is three lanes in the same direction, the speed of the first lane on the left is usually 110-120 km/h, the second lane is 90-120 km/h, and the third lane is 60-120 km/h. The standard of 120 km/h for highway design speed was specified in the "Technical Standards for Highway Engineering" in 1981, and has been used for 42 years. Traced back to China's earliest built Hujia Expressway, the construction and opening time was in 1988, just opened to traffic set the speed of vehicles shall not exceed 120 kilometers per hour. The development of highways in China is based on the speed limit standard designed by the United States, and the maximum speed limit standard of 120 kilometers per hour has been set with the limit value of 2000 miles per hour, which has not changed until now. In fact, the establishment of this standard does not only take into account the problem of traffic flow, of course, there are also various factors such as the friction coefficient between the tire and the road surface, the slope of the lane, the protective strength of the guardrail, and the local weather conditions. The slope is easy to understand, the uphill situation is better, but in the event of a long downhill or downhill process, the excessive speed is likely to cause an accident, due to the fact that people habitually turn the steering wheel in an emergency when driving at high speed, it is very easy to contact with the roadside guardrail, if the speed is too fast to exceed the bearing limit of the guardrail, flying out of the roadbed part may cause a very serious traffic accident. China is a vast country, and many areas have bad weather such as rain, snow, and fog all year round, and in this case, driving at high speeds, the tires of the vehicle are very easy to slip and lose grip, resulting in the difficulty of controlling the vehicle. According to the statistics of the transportation department, the speed limit is set at 120 kilometers per hour, and the probability of a car accident at this speed is much lower than the probability of a traffic accident above this speed, which is why the speed of 120 kilometers per hour has been used to this day.