The constraint is on a "Use Vehicle" use case. I won't burden you with the details of how I derived the formula, but suffice it to say it involved balancing the interests of the customer with practical realities. It also assumes certain population, demographic, and psychographic profiles described elsewhere in the market study I provided.
A driver should be able to plan (in advance, by making a reservation) with a certain degree of confidence that a vehicle will be available when she needs it. Therefore, for advance reservations, it is desirable to maximize the probability that a vehicle will be available for reservation at the time the driver wishes to make the reservation.
For a given number of hours of advance notice x, the following formula yields the minimum percentage probability y that a vehicle should be available for reservation:
y = 100 - 1 / (.048 * 1.039x)
I described yesterday what a constraint is. I also promised to give an example of a nonfunctional requirement. Below is an example from a recent market requirements document I composed for a client. I have edited it slightly to avoid disclosing confidential information.