The sign "Tunnel" indicates that you are approaching a tunnel. This sign is typically a blue circle with a white depiction of a tunnel entrance.

When you see this sign, it serves as a warning to prepare for entering a tunnel. Here are some important aspects to keep in mind:

1. **Speed Reduction**: You might need to reduce your speed before entering the tunnel, especially if there are changes in traffic patterns or conditions.

2. **Use of Lights**: It is often required to switch on your headlights when driving through a tunnel, even during the day, to enhance visibility.

3. **Emergency Procedures**: Familiarize yourself with the emergency procedures for the tunnel, such as how to exit in case of emergency or where to locate emergency phones.

4. **Ventilation and air quality**: Be aware that tunnels can have lower ventilation than open roads, so it's vital to maintain a safe distance from the vehicle in front of you to account for this.

5. **Traffic Signs Inside the Tunnel**: Once inside the tunnel, pay attention to any additional traffic signs, as they may indicate changes in speed limits or lane usage.

Always remain alert and adhere to any specific regulations while navigating through tunnels to ensure your safety and the safety of others. If you have any other questions or need more information, feel free to ask!