The actual rules about this can get quite complex, but to oversimplify: if you travel during the work day, when you'd otherwise be working, the travel itself is work and you need to be paid for it. If you travel outside your normal workdays or work hours, the company does not need to pay you for it, similar to how they don't need to pay you for your commute to/from work. There are some wrinkles to it and some exceptions, and to get a definitive answer about your exact situation, consult with an employment law attorney in detail, but the above is a general statement of the rule.