When braking, a bus will have the most traction when:

When braking, a bus will have the most traction when the wheels are rolling, just short of locking. This is because maintaining wheel rotation allows for better contact with the road surface, which enhances grip and control. If a wheel locks up, it starts to skid, losing traction and control. In this skidding scenario, there's significantly less friction between the tire and the road, which can lead to a longer stopping distance and a higher chance of losing control of the vehicle.

On the other hand, when the wheels are locked, the bus has minimal traction, significantly increasing the risk of skidding and decreasing the effectiveness of braking. Applying the emergency brake might not be the most effective method for slowing or stopping a large vehicle like a bus, especially when the emergency brake is not designed to work in conjunction with standard braking for rapid deceleration. Thus, the best method for maintaining control and maximizing stopping power is to brake just before wheel lock-up, ensuring the wheels continue to roll and maintain traction on the road surface.