To understand why a car with a light trailer must drive in the right lane, even though it can use any of the three, it is important to consider the traffic regulations on multi-lane roads.

- **Lanes on roads outside cities:** Vehicles weighing more than 3,500 kilograms or vehicle combinations measuring more than seven meters in length are required to drive in the right lane and can only use the adjacent lane under certain conditions. However, a car with a light trailer measuring less than seven meters is not subject to this strict restriction.

- **Flexibility for smaller vehicles:** Since the car with a light trailer does not exceed seven meters in length, it is not required to remain exclusively in the right lane. This allows it to drive in any of the lanes, as long as it does not obstruct traffic.

- **Traffic and road conditions:** Although it can use any of the lanes, it is advisable to primarily drive in the right lane, as it is the natural lane for vehicles that do not need to overtake or change direction.

In summary, a car with a light trailer has the flexibility to use any of the lanes, but should prioritize using the right lane to facilitate traffic flow and avoid obstructions.