Travel time

I perform a lot of local travel for a client, but it looks like I may be asked to do 2-3 day bughunts and evaluations at more remote locations in the future. I can’t help but feel that the many hours I will spend in airports and rental cars are worth something. How do I bill for this time or is it customary to eat this time and possibly charge a higher hourly rate onsite? (Actual travel expenses will be passed on to the client directly).