That's an intersting question. Certainly something we don't talk about much, because there are relatively so few jobs on Upwork that involve travel.
If I was hired to design a database for a client in London, and I drive to an office Iike to use a few miles from my home in Arizona, where I do the work... Although sometimes I just work from home...
Then of course I am NOT going to bill for mileage or time.
If I was hired (through Upwork) to work on-site for a local company... Then I am not going to bill for mileage or time.
What you are talking about is completely different, and it is a very rare thing indeed among Upwork jobs: You are talking about a job that entails driving and travelling AS PART OF THE JOB.
So you would certainly bill for both mileage and time, in my opinion.