Here is what I have done so far:
My goal was to prevent users from being able to select a given vehicle at a given date / hour if it had already been selected. So, I created a Vehicle Table, a Date Table, and an Hour table. I then created tables that contain the unique Dates&Vehicles and the Dates&Vehicles&Hours.
Now I am creating a Sign-out table that handles two situations.
1. An employee needs a vehicle for a number of days. In this case, the Employee selects a Date, and available vehicles for that Date are pulled in from the Date&Vehicle table. (This is managed by creating a summary field in Date&Vehicle table which counts that number of occurrences for each vehicle date in the sign-out field. If this number is greater than one, then the vehicle is not available on that date. The Date Vehicle Table is also connected to the Date&Vehicle&Hour table, and if the Date&Vehicle is marked unavailable, than all of the child hours are also marked unavailable.)
2. An employee needs a vehicle for a number of hours. This is similar to the daily selection, but it pulls the information from the D&V&H table instead of the D&V table.
There are some problems with this solution, and it is a lot of work, but it works.
Any thoughts?