To yield the right of way to a **priority vehicle**, the most important factor is the **traffic circumstances**. This is because:

- Vehicles in **emergency service** (such as ambulances, police cars, and fire trucks) have priority on all roads when they are in an emergency. They must be recognized by their **sirens and lights**. In these situations, other drivers and pedestrians must yield the right of way to them, regardless of traffic signals or the presence of traffic officers.

- The drivers of these vehicles can even **ignore certain traffic signals** and drive faster than allowed, as long as they do not endanger others. This demonstrates that traffic circumstances, such as the presence of an emergency vehicle, are more decisive than signals.

- Additionally, in situations where a vehicle not equipped for emergencies needs priority (for example, transporting a seriously ill person to the hospital), the driver can use the **horn or emergency lights** to indicate urgency. Here again, the traffic circumstances dictate that other drivers must yield the right of way.

Therefore, the **traffic circumstances** are the key factor in determining how to yield the right of way to a priority vehicle.