At a coffee shop drive-thru, customers arrive at an average rate of $15$ cars per hour, and the service rate is $20$ cars per hour. Under the M/M/1 queuing model, the average number of cars in the system is $L = \frac{\lambda}{\mu - \lambda}$. What is the average number of cars in the queue waiting (excluding the car being served), given by $L_q = L - \frac{\lambda}{\mu}$?