Traffic is controlled by a light. Cars facing a green light may turn either left or right or go straight. Right turns on red are not permitted, and everyone obeys the traffic lights. This is not a simulation of Boston traffic. Evidently, there are four queues to maintain.
Cars arrive in each of the queues on the average once every five seconds.
The light changes every 30 seconds, although you will be allowed to adjust this. Let us say that the North-South street has a green light. If the cars in both the northbound and southbound directions want to turn right or go straight, or if they both want to turn left, then both cars will proceed through the intersection. This takes 4 seconds. The simulation should proceed second by second for twenty minutes.