Probe Cars

Each probe car needs to measure and maintain its own speed-related value—the maximum speed in the current period denoted by Vcmax. Note that the speed we considered is not the instantaneous speed, but the average speed over a short interval of time (e.g., 5 sec.). To update V_cmax, each probe car has to check if its current average speed exceeds Vcmax. If so, Vcmax is set to such value. It should be noted that V_cmax is calculated on a per period basis, i.e., V_cmax is reset whenever each probe car receives Vmax from the TIC.

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The report policy used in the system is an approach that provides fast feedback to the TIC while each probe car selectively reports V_cmax to reduce the report traffic. The basic principle of our policy is to make sure that faster probe cars report earlier than slower ones. To illustrate the report policy, we describe the operation of each probe car based on the following five scenarios:

(1) When receiving Vmax

(2) When receiving V_m-tmp

(3) When Vcmax increases

(4) When entering a new segment

(5) When at the time of report

In the first scenario, each probe car sets up timer TC immediately when it receives V_max from the TIC. Let t_begin denote the time when the probe car receives V_max; in other words, tbegin stands for the begging of current period. Let tcurrent denote the current time, and then V_cmax represents the maximum speed between t_begin and t_current. Thus the timeout period of timer TC is set as follows:

max max

where α is an adjustable parameter. Equation (3.1) was chosen so that the higher V_cmax induces the shorter length of timer TC. Using Equation (3.1) we can set the report time of each probe car as:

 

max ,

report begin C current

t  t T t (3.2)

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Both Equation (3.1) and Equation (3.2) were chosen with the objective that the probe cars with faster speed would report their speed to the TIC earlier than those with slower speed. Fig. 3-4 illustrates the relationship between the timeout period of T_C and V_cmax. If the value of V_cmax is greater than or equal to αV_max, then t_report is set to be tcurrent, i.e., the probe car must report immediately. On the other hand, if the value of Vcmax is 0, then treport is set to be tbegin plus T, i.e., the probe car would report at the end of current period, which means it is impossible to report. Consequently, the faster the probe car is, the earlier it reports.

Vmax Vmax

T

Vcmax ≥ αVmax 0

Fig. 3-4 The relationship between the timeout period ofT_C and V_cmax.

In the second scenario, each probe car disables timer TC when it receives Vm-tmp

broadcasted by the TIC; in other words, each probe car sets t_report to be infinite. The objective is to prevent slower probe cars which have longer timer TC from reporting.

Therefore, the amount of messages sent to the TIC is reduced. This can be done by considering the fact that slower probe cars have longer timer T_C, which means they have not reported before they receive Vm-tmp. When they receive Vm-tmp, however, they do not have to report.

The third scenario happens when Vmax increases. Since a probe car driver is unlikely to drive at the same speed, we need to take the speed increment into account.

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The third scenario was handles differently depending on whether the probe car has received V_m-tmp or not. Fig. 3-5 depicts the flow chart of a probe car when it receives Vm-tmp. If not having received Vm-tmp in the current period, the probe car shortens the timeout period of timer T_C and thus reports earlier. The objective is to reflect the speed change as soon as possible. On the other hand, if having received Vm-tmp the probe car compares Vcmax with Vm-tmp. If Vcmax is greater than Vm-tmp, then the probe sets up timer T_m-tmp. Let t_recv denote the time when the probe car receives V_m-tmp. Let t_end be tbegin plus T; in other words, tend stands for the end of the current period. The timeout period of timer T_m-tmp is set as follows:

where β is an adjustable parameter. Using Equation (3.3) we can set the report time of the probe car as:

 

max ,

report recv m tmp current

t  t T _ t (3.4)

Both Equation (3.3) and Equation (3.4) were chosen so that the probe cars that accelerate faster (i.e. Vcmax－V_m-tmp) would report their speed to the TIC earlier than those that accelerate slower. Another objective is to prevent a probe car from reporting frequently when it steadily accelerates. For example, a probe car without the control of timer T_m-tmp is likely to report very frequently while the traffic light turns green.

The probe car is steadily accelerating in this situation, and it reports Vcmax to the TIC repeatedly no matter what its acceleration rate is. Nevertheless, a probe car with timer T_m-tmp has to wait for a larger moment to report when its acceleration is slow. Fig. 3-6 illustrates the relationship between the timeout period of Tm-tmp and Vcmax. If the value

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of Vcmax is greater than or equal to βVm-tmp, then treport is set to be tcurrent, i.e., the probe car has to report immediately. On the other hand, if the value of V_cmax is equal to Vm-tmp, then treport is set to be tend, that is, the probe car would report at the end of current period, which means it is impossible to report. Consequently, the faster the probe car accelerates, the earlier it reports.

Start

V_cmax > V_m-tmp?

Sets up T_m-tmp

YES Has received

V_m-tmp?

YES

Adjusts T_c NO

NO

End

Fig. 3-5 A flow chart of when a probe car receives Vm-tmp.

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Vmax Vmax

Vcmax ≥ βVm-tmp

T Vm-tmp

T^'

Vcmax = Vm-tmp

Fig. 3-6 The relationship between the timeout period ofT_m-tmp and V_cmax. In the fourth scenario, each probe car acts in the same way as in the third scenario, which means, depending on whether it has received Vm-tmp or not. The objective is to avoid missing potential speed reports; the TIC can obtain the real V_max. To simplify the complexity of system, we assume that the traffic information of adjacent road segments is available to the probe cars. Therefore, we can focus on the operation in one road segment.

At the time of report, each probe car transmits its Vcmax to the TIC via wireless communications. To put it differently, each probe car reports V_cmax when timer T_C or timer Tm-tmp expires.