The adequacy of a parking facility can be measured by calculating the probability that an entering vehicle will not be able to find a parking space. A high probability of rejection (not finding a space) may indicate that expansion of the parking facility is warranted.
The probability of rejection can be calculated by comparing the traffic load to the number of parking stalls as shown below.
First, the traffic load is estimated using:
A = Q*T
Make sure that your units of time cancel each other. If you give Q in vehicles per hour, then use T in units of hours.
Next, calculate the probability of rejection using the following formula:
If the probability of rejection is high, you may want to consider adding more parking stalls to the parking facility.