In Taiyuan, during regular business day (Monday to Friday), the city has vehicle restriction policies. On each day, certain vehicles are not allowed to go on the main streets. However, on the other hand, some service types of vehicles has no restriction no matter which day it is, i.e. taxi, police cars, firetrucks, cars using electricity, etc. Why does the city do this? Please analysis from both sides of view, why do they have restrictions? And why do some vehicles have no restriction?