Muddying the waters further:
IIRC the IRS says that between home and usual work-site is your "normal commute" and isn't deductible mileage, and I will infer from that that it isn't paid time, either. Never was for me. Once at the work site, which might be the shop, everything is on the clock except for customary unpaid breaks (ie.lunch) until you return to that place or
head for home. That's one of the reasons to stop at the office/shop on the way
. This also means that stopping at the distributor to pick up parts puts you on the clock, unless you actually work out of that distributor site. This is one of the reasons that companies like to send their workers home in a company vehicle; they can go straight to the job site off the clock.
We usually payed 8 hours per calendar travel day, regardless of how many hours you were awake, but IIRC billed cost+maybe 20% to the client since no work was being done. Basically the money was a wash, no cost or profit for our company. It was all up front in the contracts. So a cross-country flight, up at 4am, land at 4pm, head to motel, all for the company- still payed 8 hours. The only time it got messed around were things like drive 4 hours to the client, 6 hours work, 4 hours back. We payed the entire thing as one travel day and 6 hours work.
There is no one-size-fits-all.