When is it illegal to drive in the carpool lane?

Answer: B — When you do not have the required number of passengers

Driving in the carpool lane is specifically designed to encourage ride-sharing and reduce traffic congestion. It is illegal to use this lane unless you meet the established requirements for occupancy. This typically means having the requisite number of passengers in your vehicle, which is often two or more people, depending on the specific regulations of the area. Therefore, if a driver is alone or does not meet the passenger requirement, they are violating the law by using the carpool lane. The other options present various scenarios that do not accurately represent the rules surrounding carpool lane usage. Driving during rush hour might be permitted if the vehicle meets the passenger requirement. The rules regarding nighttime driving in carpool lanes can vary—some areas allow solo drivers to use the lane at night, making option C unclear. Lastly, driving when traffic is light does not justify the use of carpool lanes if the vehicle is not occupied by the required number of passengers. Hence, for legal compliance and safe driving, it is crucial to adhere to the passenger requirements prescribed for carpool lane usage.