Algorithm of Traffic Information Center

The TIC is designed to be an event trigger system. Table 3-2 lists notation that used in our TIC system. The events that the TIC system needs to deal with can be classified into two cases. First is when receiving T_max (T_min) report, second, when a traffic light cycle ends. The Figs 3-5(a) to (b) describe the flow chart of the two events above in our TIC system.

Table 3-2 Parameters used in the TIC system.

Notation Definition

Tmax

The report value of the maximum travel time

Tmin

The report value of the minimum travel time

adoption_flag A flag indicated that the report is adopted by the TIC or not

significant_change

A flag indicated that the report has a significant difference with the prediction value

steadyArg Argument for determine the trend is steady or not, initial = 0.05

ϒ Argument for calculate the prediction value, initial = 1/3

AdjustArg Argument for modify the prediction value, initial = 0.04

Δ The suitable range for Tmax_p and Tmin_p, initial = 40 ~ 125s

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true in the previous 120 seconds

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Broadcast the maximum T_{max_p} and the minimum T_{min_p} from prediction table for each road segment

End When a Traffic Light Cycle Ends

Is there are any T_{max_p} and T_{min_p} generated in this

cycle for each segment? Yes

Fig 3-5(b) The flow chart of the TIC when a Traffic Light Cycle ends Fig 3-5(a) describes the flow chart when the TIC receives a Tmax (Tmin) report.

When the TIC receives a Tmax (Tmin) report, it first checks the report information (i.e.

segment ID, Tmax or Tmin) and then stores the report in the corresponding reported table. In our TIC system, each road segment has its own report table for storing Tmax

and Tmin reported by PCs. Then, the TIC checks if the Tmax (Tmin) is of significant change of 15 percent by comparing with the broadcast value. If so, The TIC sets a flag

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called significant_change true, checks if there is another report of significant change in the previous 120 seconds. If not, the TIC does not adopt the report and sets the report’s adoption_flag false. Otherwise, it sets the adoption_flag true and goes on to determine trend procedure. In this step, we reject an isolated report of significant change.

To determine the changing trend of traffic condition, the TIC computes 4 factors based on the reports of each traffic light cycle as follows,

M_max = The maximum of Tmax

M_avg = The average of Tmax

m_min = The minimum of T_min m_avg = The average of T_min

Then, the TIC divides the four factors of the current cycle divides by those of the previous 2 cycle (e.g.𝑀_max (𝑇−2)^{𝑀_max (𝑇)} , T denotes the current cycle), respectively, to obtain four trend factors. A threshold, steadyArg, is used to determine the trend of each trend factor; the trend may be rising, declining or stable. If a trend factor is larger than (1+

steadyArg), it indicates a rising trend. If it is smaller than (1- steadyArg), a declining

trend. Otherwise, it is a stable trend. After that, the final trend is determined by majority vote of the four trend factors.

After the TIC determines the trend, it checks whether the current report matches

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the trend or not. If the report matches the trend, the TIC adopts this report, and vice versa. The conditions of adopting a report are in Table 3-3. A Tmax report is adopted during a rising or stable trend, if it’s the maximum of all Tmax in the previous 120 seconds, but it is adopted immediately during a declining trend. A Tmin report is adopted during a declining or stable trend, if it’s the minimum of all Tmin in the previous 120 seconds, but it is adopted immediately during a rising trend. The TIC also sets the report’s adoption_flag accordingly. If the report is adopted, the TIC goes on to prediction procedure.

Table 3-3 The conditions of adopting a report.

Adoption table

Condition Rising Trend Declining Trend Stable Trend T_max maximum value in

120 seconds adopt immediately maximum value in 120 seconds T_min adopt immediately minimum value in

120 seconds

minimum value in 120 seconds In the prediction procedure, the TIC uses the average factor related to the report type (i.e. M_avg for a Tmax report and m_avg for a Tmin report to generate the predicted

Tmin-p or Tmin-p as follows.

_ = + (M_ vg( ) − M_ vg( − 2)) ∗ ϒ _ = + ( _ vg( ) − _ vg( − 2)) ∗ ϒ (3.1)

If M_avg(T-2) or m_avg(T-2) factor is not available, then the predicted value will be the reported Tmax (Tmin). After that, the TIC adds the Tmax_p (Tmin_p) to the

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corresponding prediction table with the current timestamp.

Fig 3-5(b) shows the operation flow chart of the TIC when a traffic light cycle ends. In this event, the TIC first checks if there are any Tmax_p and Tmin_p generated in this cycle for each segment. If so, the TIC broadcasts the maximum of Tmax_p and the minimum of Tmin_p generated in this cycle for each segment. If not, the TIC checks if there are any reports received in this cycle. If so, the TIC set the new Tmax_p (Tmin_p) to be the maximum (minimum) of the non-adopted Tmax (Tmin) in this cycle. If there is no report in this cycle, then the TIC adjusts the previous prediction with an AdjustArg. In adjusting Tmax_p, the TIC multiplies the previous Tmax_p with (1- AdjustArg), and in adjusting T_{min_p}, the TIC multiplies the previous T_{min_p} with (1+ AdjustArg). After generating the new T_{max_p} (T_{min_p}), the TIC use Δ to adjust the generated value to ensure that the difference of T_{max_p} and T_{min_p} is in a suitable range. After that, the TIC broadcasts the new T_{max_p} and T_{min_p} generated above for each segment.

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