If, as an independent contractor*, you go to 2 or more job sites in a single day then the 2nd trip is deductible as an business expense. For example you leave home drive 5 miles to the 1st job then drive 10 miles to the 2nd site. At the end of the day you drive 15 miles home. Only the 10 miles between jobs is deductible. The first trip in the morning, from home and the last trip at night back home are considered commuting (even for the self employed) and are not deductible.
*W-2 employees are not allowed any mileage deductions (effective with the 2018 tax law changes).
