Because the population of the city itself is not very frequently, but will probably stick in some time after school or work. But then the overall Festival traffic jams, car is not confined to normal.
The population within the city no more cars are rarely infected but may be sometimes after school or at a festival organizer but overall, no traffic jams, the car can travel through as usual.
Because the population is not much within the city car will not stick but might be occasionally in the time after school or have held the festival. But overall, the traffic jam, cars can travel through normal.