Under what condition must you NOT pass a vehicle on the right?

Passing a vehicle on the right is specifically prohibited when the vehicle ahead is turning left because it poses a significant safety risk. When a vehicle is indicating a left turn, it has effectively committed to that action and may require the space on its left to complete the maneuver. Attempting to pass on the right in this scenario could lead to a collision because the turning vehicle may not anticipate another vehicle overtaking it on that side. This condition is designed to ensure the safety of all road users by preventing potentially hazardous situations that could arise from unexpected movements.

In contrast, passing on the right can be permissible under other conditions, depending on the traffic regulations and roadway design. For example, passing a slower vehicle might be allowed if it can be done safely without switching lanes in a way that risks an accident. The road's width or presence of oncoming traffic also plays a critical role in determining safe passing conditions, but when a vehicle is already preparing to turn left, that factors out any possibility of safe passing on the right.