Really need to find out what LOCAL STATE LAW says. But as far as federal work law, the second they clock in they are no longer on there time but yours. Doesn't matter if you have 45 minutes to job site from shop or 2 minutes, they are on the clock.
Now to solve this problem and depending how far the work is spread, you can limit the down time by requiring your labors to show up at first job or central location and then using modern options, have them clock in at that point like construction job sites do.
Sucks, but really treat the employee the way you would expect to be treated......all those service industries such as sears, roto rooter.......their employees are on the clock the second they clock in and stay on the clock till they clock out. If you are giving your time, you expect to be compensated for it too. Dont be a cheapskate, will come back and bite you.