SYSTEM DESIGN
A. VANET-based Information Distribution and Estimation
The network topology considered here is shown in Figure
2. There are two traffic information sources. The first one is
to use the on-board GPS device and the electronic road
database for the vehicle to identify its current GPS location
and record the information like the instantaneous driving
speed in its memory when the vehicle starts to drive on the
road. As the flow shown in Figure 3, the on-board GPS
device detects whether it has passed through an intersection,
which indicates an end of the road segment. If yes, all
recorded driving speeds are averaged and stored in the
AverageSpeed field for traffic information of this road
segment, as listed in Table 1, with correct values in the
RoadSegmentID and RecordTime fields, which specifies the
time when the vehicle leaves this road segment.
Origin
V1
I
1 I4 I7
I
2 I5 I8
I
3 I6 I9
Road Segment
Vehicle
RSU
The Original
Route
The Route
after
Replanning
V3
Broadcast Safety message
V2
Dst
Access Google Maps Traffic
Information via RSU
Exchange Recorded Traffic
Information with Contacted
Vehicles
Figure 2. Network Topology for VBA*
Figure 3. The Traffic Information Recording Flow of the Vehicle
TABLE I. THE THREE-FIELD FORMAT FOR TRAFFIC INFORMATION
OF A ROAD SEGMENT
Field Name Description
RoadSegmentID The unique ID of the road segment between two
adjacent intersections.
RecordTime The time when traffic information is recorded.
AverageSpeed The average driving speed of this road segment.
The second traffic information source is from the Google
Maps real-time and historic traffic information, which
divides an hour into 4 time zones and each time zone lasts
for 15 minutes. The road segment colored in green, yellow,
red or red-black means its driving speed is fast, modest, slow
or congested, respectively. However, Google Maps does not
provide traffic information of all road segments but major
roads in the urban area or highways.
As mentioned above, the vehicle continues collecting the
traffic information when it is driving on the road. Whenever
another vehicle enters the IEEE 802.11p transmission range
of a vehicle, the VANET-based traffic information
distribution mechanism proposed in this paper is activated.
As shown in Figure 4, by exchanging IEEE 802.11p frames
that contain the collected traffic information for recent time
zones of each road segment, which the vehicle has passed
through, with neighbor vehicles, each vehicle can be notified
of the historic traffic information of the road segment that it
may not have driven through.
Figure 4. The Traffic Information Distribution Flow
As soon as the vehicle driver intends to plan a route to
the destination with the proposed VBA* algorithm, the
traffic information, i.e., the driving speed, of next time zone
for each road segment should be estimated first with the flow
shown in Figure 5. The recorded traffic information is
classified into those in the corresponding 15-minute time
zone of Google Maps. Then, the average recorded driving
speed of the road segment for each time zone is calculated. If
Google Maps provides historic traffic information for the
same road segment that already has its average recorded
driving speed been calculated, these two values, i.e., the
average recorded driving speed and the speed retrieved from
Google Maps, are averaged by a weighting factor α to
represent the observed driving speed of each time zone for a
road segment, which is formulated by (7) where ,