An ice cream parlor is staffed with two workers. Each worker can serve an arriving customer at the same rate of $20$ customers per hour. Their service times, including taking an order, serving the order, and taking the payment, have a standard deviation of $1$ minute. Customers arrive at the parlor every $4$ minutes on average, with a standard deviation of the interarrival times of $4$ minutes. Customers are asked to form a single line, and the customer in front of the line will go to either one of the two workers who is available first. What is the average system utilization? Please write your answer in the format of $0.$xxx$.$