Final answer:
A bus may use an HOV lane when carrying more than two passengers, excluding the driver. This policy is in place to improve transportation systems' efficiency and environmental sustainability.
Step-by-step explanation:
Under circumstances where High Occupancy Vehicle (HOV) lane regulations apply, a bus may use an HOV lane c. When carrying more than two passengers, excluding the driver. These lanes are designed to maximize vehicle efficiency and reduce traffic by encouraging carpooling and the use of public transit. The specific rules can vary by location, but typically, buses are permitted in HOV lanes irrespective of their occupancy levels, as they serve as a form of mass transportation. This policy aims to support buses in maintaining a timely schedule which is beneficial for both the transportation system efficiency and the environment.