The US Centre for Disease Control and Prevention issued an Executive Order on January 29, 2021, requiring passengers on all forms of public transportation in the US to wear masks, with a specific exemption for commercial vehicle drivers who are the vehicle’s sole occupant.

The CDC Order is here. The exemption for truck drivers is on page 5, as follows:

This Order exempts the following categories of conveyances, including persons on board such conveyances:

…Commercial motor vehicles or trucks as these terms are defined in 49 CFR 390.5, if the driver is the sole occupant of the vehicle or truck;

The order applies to persons travelling into and within the US and applies primarily to passenger conveyances, but with initial applicability limitations that are broad enough to include all travellers, hence the exemption for commercial vehicles crossing either international or state borders.