Design of the Proposed Mechanism

The proposed traffic control mechanism utilizes three functions, which are (1) moving path arrangement, (2) evacuation message delivery, and (3) dynamic traffic lights control, to make emergency vehicle drive fast and smoothly. Designs of these three functions are described below.

(1) Moving Path Arrangement

The traffic control center will arrange a moving path for the emergency vehicle to reach the accident scene. This moving path must be the shortest-time path to avoid

emergency vehicle driving on the longer-time path to the accident scene. The shortest-time path generation is based on the A* algorithm. A* algorithm is a best-first, graph search algorithm that can find the path with the minimum cost from the source to the destination.

The procedure of generating shortest-time path for emergency vehicles is described below.

1. When Wan-Fang hospital receives an emergency call for an accident, it reports the location of the accident to the traffic control center.

2. When the traffic control center receives the report, it will generate a shortest-time path from the source (Wan-Fang hospital) to the destination (the reported location of the accident). The traffic control center then transmits the path back to the hospital so that the emergency vehicle can drive based on the shortest-time path.

28.

(2) Evacuation Message Delivery

The evacuation message encapsulated in the WSM (WAVE Short Message) is used to inform the nearby vehicles of the approaching of emergency vehicles so that vehicles can make their way for emergency vehicles. The procedure of evacuation message delivery is described below.

1. The emergency vehicle periodically broadcasts the evacuation request message encapsulated in the WSM to the nearby RSU. The message contains the ID and the location of the emergency vehicle.

2. While the RSU near the emergency vehicle receives the evacuation request message, it delivers this message to the traffic control center through the wired link.

3. When the traffic control center receives the message, because it knows the moving path of the specified emergency vehicle ID, it sends a new evacuation message to the RSU which is installed at the intersection in front of the specified emergency vehicle. This new evacuation message contains the current and next road segment IDs which the emergency vehicle will pass through.

4. When the RSU which is installed at the intersection in front of the emergency vehicle receives the new evacuation message, it broadcasts the message using WSM to inform the nearby vehicles.

5. When the vehicle receives the WSM message, if it may block the way of the emergency vehicle based on the road segment IDs contained in the message, it will make the way for the emergency vehicle. So that the emergency vehicle can drive without been obstructed by other vehicles.

29.

Fig. 4.1 A Simple Case of Evacuation Message Delivery

For example, as one can see in Fig. 4.1, the emergency vehicle (node 7) periodically broadcasts the evacuation request message using WSM which contains its vehicle ID and location. The RSU (node 4) installed on the nearest intersection will receive this WSM message and then deliver this message to the traffic control center (node 5). When the traffic control center receives the evacuation request, it will find out the current and next road segment IDs of the specified emergency vehicle based on the shortest-time path. Then, the traffic control center will sends the evacuation message which contains the current and next road segment IDs of the emergency vehicle to the RSU (node 3) which is installed in front of the specified emergency vehicle. The RSU (node 3) will then broadcast the evacuation message using WSM to nearby vehicles (node 8, 9, 10, and 11). When vehicles (node 8 and 11) receive the broadcasted WSM message from the RSU and find out they may block the

30.

way of emergency vehicle (node 7), they will make the way for the emergency vehicle to pass through. As such, the emergency vehicle can drive on a clear lane fast and safely.

(3) Dynamic Traffic Lights Control

The traffic lights are controlled by the traffic control center. The emergency vehicle periodically broadcasts the WSM message which containing the request of traffic lights control, its vehicle ID and location to the nearby RSU. The RSU will receive and relay this message to the traffic control center. When the traffic control center receives the request of traffic lights control, it will change the traffic lights on the intersection, which is in front of the emergency vehicle, into green to pre-evacuate vehicles waiting for the traffic lights. By doing so, we can avoid the emergency vehicle stuck in the traffic jam in front of the intersection.

Fig. 4.2 A Simple Case of Dynamic Traffic Lights Control

31.

For instance, as one can see in Fig. 4.2, the emergency vehicle (node 7) periodically broadcasts a WSM message which contains the request of traffic light control, vehicle ID, and location. The nearby RSU (node 4) receives the WSM message and relays this message to the traffic control center (node 5). Because the traffic light control center has the shortest-time path information of the specified emergency vehicle ID, it changes the traffic lights in front of the emergency vehicle into green. As such, vehicles (node 8, 9, 10, and 11) can be pre-evacuated and will not make the emergency vehicle stuck in the traffic jam.