I would say that your sentence was fine to start the request. However, I would probably add the destination too. For example: Are you free to take me to the airport? or Can you please take me to the airport? or Could you please take me to the airport? You could also ask the taxi driver if (s)he is available or currently on another job, but usually it's enough to ask one of the questions above. Your sentence was fine and definitely shouldn't cause any surprise.