In the state of California, do employers have to pay an employees for the time they are driving from one location to another?

We have a couple of employees that start the day at one of our offices and head up to another one of our offices about half way through the day to complete further work. Both offices are under the same company, but our program supervisor wants the employees to clock in and out at one location and clock in and out at the other location. They offer mileage reimbursement but here in the city and given the time they leave, they are only driving within 15 miles but it ends up taking them 30 to 45 minutes to drive there because of traffic so many of our employees are missing out on $15 a day from just their drive time, $10 if they are reimbursed five days a week. I just want to know if it is legal for them to not pay the drive time even if they are still working but just going from one location to another.