Schema for a reservation system
What I would suggest to you before putting a lot of work into designing a system, is to take a leaf from the book of the great Issac Newton - "If I have seen further it is by standing on the shoulders of giants". 

By this I mean take a long hard look at what's been done already - there are systems that could be of interest to you in this domain - take a look at this for example. Of course, in one sense, a taxi is just a resource to be scheduled - so you could look further afield at open source software for scheduling generally for ideas and inspiration.

That's not to say that you can't have your own ideas, but just take a look at some of the contenders in the open source world out there - borrow what you want and need from them and disregard the rest.

With respect to your particular question, I would go to the trouble of maintaining the details of day_off, booked, ill, unavailable... that way you can reward drivers who are never ill, rarely unavailable (i.e. when not actually booked) &amp;c. The more information you can put into the database, the more you can do with it later. Just IMHO.