If you have to pick them up and drop them off, I don't think that you should have to pay them beyond the jobsite. Legally, this may be a different story that you will likely only find out by paying a lawyer.
Are you having them drive from the shop to jobs to the shop? Between jobs? Or just to get to work?
I think that this is something that you will should explain at the beginning of their employment.
I usually have my employee meet me at the shop and pay him from shop arrival to shop departure.
Sometimes, due to needing to do other things before or after a job, such as bidding, I will have him/ them meet me at the jobsite. I consider it commuting. They are not doing anything work for me.
If it is an unusually far away job, I have them meet me at the shop. I don't expect them to drive their own vehicle 40 miles one-way, as when they are hired, they are told that they need to have their own reliable transportation to jobs, if we are not meeting at the shop.
When an employee is meeting me at the shop (at my house) I use them to help me around there. If I am not having them help me to do any work, and it is just "carpooling" to save them the expense of driving, we start and end the workday at the job (my current and previous employee live 5 minutes bike ride from my house). I make sure not to ask things of them, if this is the case. They are free to talk on the phone, text, headphones, or chitchat.
If I'm expecting them to help navigate, watch traffic, make work phone calls, etc, its on the clock.
This is my take, but, again, legally speaking, and in the event of an injury accident in regards to WC, it will be a matter for lawyers.
The carpooling thing works out when I know that I'm not going to do any bids/ errands before/ after work, such as on a long day when I'm driving the chip truck/ chipper. I let them know that I might have to go talk to a neighbor after a job is done, and they might have a little bit of downtime if they are carpooling to save themselves the expense. This being said, I try to work in the neighbor while finishing clean-up.