Four-per-car rule

When employees are using state-appropriated funds, they must coordinate travel when they share the same itinerary. For instance, when four or fewer employees travel on the same trip, only one of the employees may be reimbursed for mileage. Moreover, when more than four employees travel on the same trip then only one out of every four employees may be reimbursed for mileage.

Source: Texas Government Code Section 660.044