Probe Cars

The report policy of probe cars is the core of our solution. When we drive into a traffic jam on a highway, we may first brake fast and then drive in a stop-and-go fashion. According to the experience, a probe car observes its acceleration and speed at the same time to determine the boundaries of a traffic jam. Note that not all traffic jams cause fast braking, the stop-and-go moving pattern alone can also determine the beginning of a traffic jam.

Table 3-2 lists the notation and definition of variables used in our probe car report policy. The measurement rate of the GPS receiver and acceleration sensors is once per second, i.e., we can get the speed, location information and acceleration of the vehicle

Table 3-2 Parameters used in the probe car report policy.

Notation Definition Value

StatusPC The status of the vehicle 0 = in free flow 1 = in traffic jam LPC The current location of the probe car obtained

from GPS

-- VPC The current speed of the probe car -- A_PC The current acceleration of the probe car

paralleled to the heading direction

--

W The buffer size Constant

VSMS The space mean speed of the of the probe car -- V_in The speed threshold to determine the existence

of a traffic jam

-- A_in The acceleration threshold to determine the

existence of a traffic jam

A_in < 0 Vout The speed threshold to determine probe car out

of traffic

Vout > Vin

Dmod The distance threshold to to modify the start and end positions of a traffic jam

-- Dgen The distance threshold to determine to generate

a new traffic jam or eliminate a traffic jam

Dgen > Dmod

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each second. In addition, a probe car records the measurements with timestamps.

Measurements are stored in a buffer of size W and are used to compute the space mean speed. Let Locationn denote the current location, and Locationn-W denote the location of the probe car W seconds before. The space mean speed can be obtained as follows,

(3.1)

While the probe car first appears in the road network, it would receive the traffic information from the TIC. According the traffic information, the probe car determines whether it is in a traffic jam or not by its location and initializes its status StatusPC, If the probe car is in a traffic jam, Status_PC would be 1; otherwise, Status_PC would be 0.

A probe car determines that it drives into a traffic jam based on the following two conditions:

(1) VPS < Vin and APS < Ain

(2) VSMS < Vin

Condition 1 checks if the probe car brakes fast and its speed drop below Vin, a speed threshold of a traffic jam. Condition 2 checks if the space mean speed in the last W seconds is blow V_in.

Fig. 3-2 depicts the flow chart of a probe car after it get the measurements of its location, speed and acceleration. If the probe car is not in a traffic jam, i.e., Status_PC equals to 0, it first checks if condition 1 is met. If so, the probe car has just entered a traffic jam with fast braking applied, and the start position of the traffic jam measured by the probe car is L_PC. The probe car would record the current time, discard all traffic

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(1)Clean the data in the buffer

Fig. 3-2 A Flow chart of a probe car detecting traffic jams

data in the buffer and add the current one in the window. The probe car would also check if it is a new traffic jam or it is an old traffic jam, which has shifted its start location. According to the report policy, which will be discussed later, the probe car may report the change to the TIC. If condition 1 is not met, the probe car checks it condition 2 is met. If so, the probe car has entered a traffic jam W seconds ago without applying fast braking. The start position of the traffic jam is the first record stored in the buffer. The probe car sets StatusPC to 1, and performs the report policy to determine if it needs to send a report to the TIC.

On the other hand, if StatusPC equals to 1, the probe car checks if VSMS is larger than V_out, a space mean speed threshold, which determines whether a probe car has it has driven out of a traffic jam or not. Noted that is measured for the last W

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seconds. If the condition is met, the time when the probe drove out of the traffic jam is W seconds ago, and the end position of the traffic jam is also stored in the record W

seconds ago. With the recorded time of getting into the traffic jam, the probe car can easily compute the travel time of the current traffic jam. In order to confirm a probe car getting into and out of a stop-and-go traffic condition, we formulate the condition of getting into a traffic jam is stricter than that of getting out of a traffic jam. Simply stated, the V_in is smaller than V_out.

After a probe car detects the start location or the end location of a traffic jam, the probe car compares the detected information with that broadcasted from the TIC to determine if it needs to inform the TIC to modify the traffic information. When the start location of a traffic jam is detected by a probe car, let LMS denote the start location detected by the probe, and L_start denote the start positon of the nearest traffic jam broadcasted by the TIC. The probe car compares LMS and Lstart, and two distance thresholds are used to determine if the probe car needs to send a report and the type of report to be sent. First, if the distance between LMS and Lstart is less than Dmod, a modification threshold, the probe does not report the change to the TIC. The reason is because GPS location measurements are of unavoidable errors and minor changes of the start location do not need to be reported. Second, if the distance between LMS and L_start is larger than D_mod, bust less than D_gen, the probe car informs the TIC that the start location of the traffic jam have shifted to LMS. Last, if the distance between LMS and Lstart

is larger than Dgen, the probe car sends a report of a new detected traffic jam to the TIC.

When a probe car detects the end position of a traffic jam and computes the travel time of the traffic jam, if the traffic jam is a new one detected by the probe car, the probe car reports the end location and travel time of the traffic jam to the TIC. Otherwise, if the

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distance between LMS and Lend is larger than Dmod, the probe car reports to the TIC that the end position has changed.

In addition, if a probe is in a smooth traffic and passes the start location LMS of the traffic jam over D_gen, it means the probes pass the traffic jam for a certain distance and don’t detect the traffic jam. It notifies the TIC to remove the traffic jam. If a probe is in a traffic jam and passes the start location LMS of the next traffic jam, that means there are some traffic jams need to merge. It notifies the TIC to merge the traffic jams.

Furthermore, if the difference between the travel time in passing through the traffic jam measured by the probe car and that broadcast by the TIC is more than 5%, the probe car informs the TIC of the measured travel time.

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