Patent Publication Number: US-10319222-B2

Title: Traffic condition monitoring system, method, and storage medium

Description:
TECHNICAL FIELD 
     The present invention relates to a system, a method, and a storage medium for monitoring traffic conditions. 
     BACKGROUND ART 
     Conventional traffic condition monitoring systems collect data on road conditions from vehicle detectors, traffic monitoring cameras (CCTVs: Closed Circuit Televisions), and the like and transmit the data to an information processing apparatus, which determines traffic jam or calculates the transit time based upon the collected data. 
     In such a traffic condition monitoring system, however, vehicle detectors, CCTVs, and equipment for transmitting data collected from those devices need to be installed on roads. As a result, conventional traffic condition monitoring systems suffer from an increased cost and an enlarged scale. 
     Meanwhile, use of the communication infrastructure such as the existing cellular phones has been proposed to monitor traffic conditions. 
     For example, Patent Literature 1 discloses a “traffic information collection system” configured to wirelessly transmit information including an ID of a mobile station, the type of a vehicle, the current position, the current time, and a vehicle speed from the mobile station to a base station and to transmit that information via a telephone network from the base station to a fixed station, which obtain traffic information, such as the volume of traffic or traffic jam on a road on which the mobile station is present, based upon such information. 
     In Patent Literature 1, the mobile station has a car navigation device and a cellular phone connected to the car navigation device via a communication adapter, so that information obtained by the car navigation device can wirelessly be transmitted to the base station with the cellular phone while the mobile station is moving. The car navigation device includes a driver control part for playing back a map data stored in such device as a CD-ROM, a GPS receiver for receiving positional information from artificial satellites via an antenna, and a processing unit for displaying the current position of the mobile station on the map and the traveling conditions with a display screen based upon the map data from the driver control part, the positional data from the GPS receiver, the vehicle speed data from a vehicle speed sensor for autonomous navigation, and azimuthal data from an azimuthal sensor. The cellular phone can establish a dial-up connection with the base station and wirelessly transmit data such as a position, a vehicle speed, and a road image received from the processing unit of the car navigation device via the communication adapter to the base station. Furthermore, the cellular phone can transmit the ID assigned to the mobile station, the type of the vehicle, and the current time with a dial number when the cellular phone has a dial-up connection with the base station. In each of the mobile stations, the cellular phone connects to the base station every certain period of time or every certain distance and wirelessly transmits information including data such as the ID, the type of the vehicle, the current time, the position, the vehicle speed, and the road image to the base station. Patent Literature 1 also describes that various types of position measurement devices capable of measuring the position of a mobile station can be used as a position measurement part. 
     Patent Literature 2 discloses a “data processing apparatus for probe traffic information” that estimates the position of a cellular phone from connection status between a cellular phone and base stations, uses the positional information as probe data, and generates traffic jam information by matching of the probe data with road map data. In this Patent Literature 2, the total traveling distance and time of a cellular phone are measured by detecting principal positions between a base station to which a cellular phone first connected to begin a call and a base station from which a cellular phone disconnected to terminate a call, and transit time at which a cellular phone passed these principal positions. The velocity (speed per hour) of the vehicle is calculated from those traveling distance and traveling time. 
     Patent Literature 3 discloses a “traffic information processing system” that uses communication records of cellular phones from vehicles traveling on roads to collect and distribute traffic information. In Patent Literature 3, a cellular phone with a GPS function has means for acquiring positional information such as the latitude and longitude of its location by wirelessly communicating with artificial satellites. This cellular phone with a GPS function has traffic information distribution request means for requesting a center apparatus to distribute traffic information and positional information transmission means for transmitting its positional information that has been acquired by the GPS function to the center apparatus. The center apparatus has vehicle information collection means for creating traveling data of a vehicle and creating and updating vehicle information data based upon the position and the communication time of a cellular phone having a GPS function. When a cellular phone with a GPS function is operated to start to use a traffic information distribution service, the traffic information distribution request means of the cellular phone transmits a traffic information distribution request to the center apparatus via a base station. At the same time, the positional information transmission means of the cellular phone determines the position of the cellular phone with use of artificial satellites and transmits the positional information to the center apparatus. 
     Meanwhile, in a communication system for cellular phones or the like, a frequency shift can be estimated from a transmission frequency of a communication signal received by a base station. For example, Non-Patent Literature 1 has proposed a method of estimating a frequency shift by observing the phase rotation of a pilot symbol inserted in a communication signal. 
     Furthermore, Patent Literature 4 discloses an “adaptive control apparatus” that adaptively controls the directivity of an array antenna so as to track each of mobile terminals in a continuous manner. This adaptive control apparatus disclosed in Patent Literature 4 has a setting part for setting parameters required to calculate a weighting factor based upon a received signal before or after combining. This setting part includes a Fourier transform part for performing a Fourier transform on a signal before combining that has been received by one or more antenna elements to calculate a frequency spectrum and a velocity calculation part for calculating a Doppler frequency based upon this frequency spectrum and calculating a relative speed of the mobile terminal. 
     Moreover, Patent Literature 5 discloses a “traveling speed detection apparatus of a mobile station” in which a base station detects a traveling speed of the mobile station by calculating the maximum Doppler frequency of a received signal from a mobile station. This traveling speed detection apparatus disclosed in Patent Literature 5 has envelope calculation means for calculating an envelope of radio waves received from the mobile station, intersection count calculation means for calculating the number of intersections over a certain period of time between the envelope calculated by the envelope calculation means and each of a plurality of predetermined levels of the received power, maximum value detection means for detecting the maximum value from among the intersection counts of respective predetermined levels that have been calculated by the intersection count calculation means, and traveling speed calculating means for calculating a traveling speed of the mobile station with use of the maximum value of the intersection counts that has been detected by the maximum value detection means. 
     PRIOR ART REFERENCES 
     Patent Literature 
     
         
         Patent Literature 1: JP-A 10-307993 
         Patent Literature 2: JP-A 2006-285567 
         Patent Literature 3: JP-A 2008-134957 
         Patent Literature 4: WO2004/066523 
         Patent Literature 5: JP-A 06-242225 
       
    
     Non-Patent Literature 
     
         
         Non-Patent Literature 1: Kato and Sasaoka, “Frequency Offset Compensation Method for QAM in Land Mobile Radio Communications,” Journal of the Institute of Electronics, Information and Communication of Engineers of Japan, Volume J74-B2, pp. 493-496. 
       
    
     SUMMARY OF INVENTION 
     Problem(s) to be Solved by Invention 
     However, Patent Literatures 1 to 5 and Non-Patent Literature 1 suffer from the following problems. 
     In Patent Literature 1, the cellular phone is connected to the car navigation device via the communication adapter. The positional information received by the GPS receiver provided in the car navigation device and the vehicle speed data detected by the vehicle speed sensor are transmitted to the base station via the cellular phone. Therefore, according to Patent Literature 1, traffic information cannot be collected for mobile stations (vehicles) having no car navigation device. Furthermore, Patent Literature 1 describes that the position of a mobile station may be measured with various kinds of position measurement devices. However, each of mobile stations needs to have a vehicle speed sensor in order to measure a vehicle speed of the mobile station. Moreover, Patent Literature 1 does not consider a traveling direction of a mobile station. 
     Patent Literature 2 determines traffic information from a base station that receives communication information transmitted by a cellular phone and from a coverage area of a receiving sector of the base station. Therefore, Patent Literature 2 can only use rough positional information or a rough vehicle speed, lowering the accuracy of the traffic information. Furthermore, Patent Literature 2 does not consider a traveling direction of a vehicle. 
     In Patent Literature 3, a cellular phone with a GPS function transmits its positional information to a center apparatus only during use of a traffic information distribution service. Furthermore, Patent Literature 3 does not calculate a traveling speed of a vehicle or consider a traveling direction of a vehicle. Therefore, according to Patent Literature 3, only the position of a cellular phone (vehicle) can be obtained as traffic information. 
     Non-Patent Literature 1 merely proposes a method of estimating a frequency shift. 
     Patent Literature 4 merely discloses calculating a relative speed of a mobile terminal with use of a Doppler frequency. Patent Literature 4 fails to disclose or suggest any association between a calculated relative speed of a mobile terminal and traffic conditions such as traffic jam. Patent Literature 4 does not consider a traveling direction of a mobile terminal. 
     Patent Literature 5 merely discloses an example of a method of calculating a traveling speed of a mobile station from a Doppler frequency. Patent Literature 5 also fails to disclose or suggest any association between a calculated traveling speed of a mobile station and traffic conditions such as traffic jam. Patent Literature 5 does not consider a traveling direction of a mobile station. 
     In such traffic condition monitoring systems, it is important to know not only a traveling speed of each vehicle, but also a traveling direction of each vehicle in order to evaluate traffic conditions more precisely. 
     It is, therefore, an object of the present invention to provide a small-scale, low-cost traffic condition monitoring system that can more precisely evaluate a traffic condition by using cellular phone infrastructure, and a method and a storage medium for such a traffic condition monitoring system. 
     Means for Solving Problem(s) 
     A traffic condition monitoring system according to the present invention is provided in a base station installed by a side of a road for monitoring a traffic condition based upon communication signals transmitted from mobile terminals in a plurality of vehicles traveling on the road. The system has a receiver part operable to receive the communication signals transmitted from the mobile terminals of the respective vehicles to output received signals, a speed/direction estimation part operable to estimate traveling speeds and traveling directions of the respective vehicles based upon the received signals, and a traffic condition analysis part operable to determine effective traveling speeds of the respective vehicles in each traveling direction, to calculate an average traveling speed of a group of vehicles in each traveling direction, and to analyze a traffic condition in each traveling direction based upon the average traveling speed of the group of vehicles in each traveling direction. 
     A traffic condition monitoring method according to the present invention is used in a traffic condition monitoring system provided in a base station installed by a side of a road, for monitoring a traffic condition based upon communication signals transmitted from mobile terminals in a plurality of vehicles traveling on the road. The method includes a speed/direction estimation step of estimating traveling speeds and traveling directions of the respective vehicles based upon received signals of a receiver part that has received the communication signals transmitted from the mobile terminals in the vehicles, and a traffic condition analysis step of determining effective traveling speeds of the respective vehicles in each traveling direction. of calculating an average traveling speed of a group of vehicles in each traveling direction, and of analyzing a traffic condition in each traveling direction based upon the average traveling, speed of the group of vehicles in each traveling direction. 
     A storage medium according to the present invention is a computer-readable storage medium having a program recorded thereon for allowing a computer of a traffic condition monitoring system provided in a base station installed by a side of a road to monitor a traffic condition based upon communication signals transmitted from mobile terminals in a plurality of vehicles traveling on the road. The program causes the computer to implement a speed/direction estimation function of estimating traveling speeds and traveling directions of the respective vehicles based upon received signals of a receiver part that has received the communication signals transmitted from the mobile terminals in the vehicles, and a traffic condition analysis function of determining effective traveling speeds of the respective vehicles in each traveling direction, of calculating an average traveling speed of a group of vehicles in each traveling direction, and of analyzing a traffic condition in each traveling direction based upon the average traveling speed of the group of vehicles in each traveling direction. 
     Advantageous Effects of Invention 
     A traffic condition monitoring system according to the present invention can more precisely evaluate a traffic condition by using cellular phone infrastructure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a traffic condition monitoring system according to a first example of the present invention; 
         FIG. 2  is a schematic plan view showing an example of a sector arrangement of antennas of a base station installed by the side of a road; 
         FIG. 3  is a schematic plan view showing an example of an angle between a line connecting a vehicle to the base station and a traveling direction of the vehicle and an angle between a central direction of a sector and the road; 
         FIG. 4  is a flow chart explanatory of an operation of a traffic condition determination part used in the traffic condition monitoring system illustrated in  FIG. 1 ; 
         FIG. 5  is a block diagram showing a configuration of a traffic condition monitoring system according to a second example of the present invention; 
         FIG. 6  is a schematic plan view explanatory of an operation of an arrival angle determination part used in the traffic condition monitoring system illustrated in  FIG. 5 , which shows the relationship between vehicles traveling on a road and directional beams of array antennas of a base station; 
         FIG. 7  is a block diagram showing a configuration of a traffic condition monitoring system according to a third example of the present invention; 
         FIG. 8  is a block diagram showing a configuration of a traffic condition monitoring system according to a fourth example of the present invention; and 
         FIG. 9  is a schematic plan view explanatory of an operation of calculating a traveling distance with a traveling distance/traveling direction calculation part used in the traffic condition monitoring system illustrated in  FIG. 8 , which shows the relationship between vehicles traveling on a road and a base station. 
     
    
    
     MODE FOR CARRYING OUT INVENTION 
     Exemplary embodiments of the present invention will be described below. 
     Some features of exemplary embodiments of the present invention will be described. 
     Certain embodiments of the present invention employ the cellular phone infrastructure such that a base station calculates a frequency shift of a communication signal transmitted from a mobile terminal of each of vehicles, estimates a traveling speed and a traveling direction of each of the vehicles, and thus provides a small-scale, low-cost traffic condition monitoring system. 
     The traffic condition monitoring system employs the existing communication infrastructure such as cellular phones and collects traffic conditions with communication signals transmitted from mobile terminals mounted on vehicles traveling on roads or from mobile terminals held by passengers of vehicles. A frequency shift of a mobile terminal can be estimated by a base station. The estimation results of the frequency shifts are collected from base stations and analyzed, so that traveling speeds and traveling directions of the vehicles can be measured. Therefore, a small-scale, low-cost traffic condition monitoring system can be provided merely by adding, to the existing base stations for cellular phones or the like that have been installed by the side of roads, a device operable to calculate and analyze a traveling speed and a traveling direction of each vehicle based upon an estimation result of a frequency shift. 
     Next, the principle of certain exemplary embodiments of the present invention will be described. 
     It has been known that the frequency of a communication signal transmitted from a mobile terminal approaching directly toward or moving directly away from a base station shifts due to the Doppler effect. When the transmission frequency of a transmission signal (communication signal) transmitted from a mobile terminal is defined as f 0 , the traveling speed of a vehicle having a mobile terminal mounted thereon or a vehicle in which a passenger has a mobile terminal is defined as v, and the speed of light is defined as c, then the received frequency of a received signal at the base station shifts from the transmission frequency by Δf, which is calculated by the following formula 1. 
     
       
         
           
             
               
                 
                   
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                   Formula 
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                   1 
                 
               
             
           
         
       
     
     When the mobile terminal is approaching the base station, Δf will be a positive value. When the mobile terminal is moving away from the base station, Δf will be a negative value. 
     In practice, as shown in  FIG. 3 , a base station  200  is installed at a position that is remote from a road  210  to some extent. Therefore, when an angle (included angle) between a line connecting a vehicle to the base station and a direction in which the road extends is defined as θ, then Δf is expressed by the following formula 2. 
     
       
         
           
             
               
                 
                   
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     Such a relationship is utilized to estimate a frequency shift of a communication signal transmitted from a mobile terminal. Thus, a traveling speed and a traveling direction of a vehicle can be estimated. 
     Specifically, traveling speeds and traveling directions of a plurality of vehicles can be calculated by estimating frequency shifts of communication signals transmitted from mobile terminals mounted on the vehicles or mobile terminals held by passengers in the vehicles. Thus, traffic conditions can be determined. 
     Furthermore, another exemplary embodiment of the present invention employs the existing cellular phone infrastructure, calculates a traveling speed and a traveling direction of a vehicle from a communication signal including positional information that is periodically transmitted from a mobile terminal of a vehicle traveling on a road, and analyzes traffic conditions. Thus, a low-cost, small-scale traffic condition monitoring system is provided. 
     Example 1 
     Referring to  FIG. 1 , a traffic condition monitoring system according to a first example of the present invention will be described. The illustrated traffic condition monitoring system is provided in a base station  200  (see  FIG. 2 ) installed by the side of a road  210  (see  FIG. 2 ). The traffic condition monitoring system monitors traffic conditions based upon communication signals transmitted from mobile terminals in a plurality of vehicles traveling on the road  210 . 
     The illustrated traffic condition monitoring system has a receiver part  1000 , a speed/direction estimation part  2000 , and a traffic condition analysis part  3000 . The receiver part  1000  is operable to receive communication signals transmitted from the mobile terminals of the vehicles to output received signals. The speed/direction estimation part  2000  is operable to estimate traveling speeds and traveling directions of the respective vehicles based upon the received signals. The traffic condition analysis part  3000  is operable to determine effective traveling speeds of the respective vehicles in each traveling direction, calculate an average traveling speed of a group of vehicles in each traveling direction, and analyze traffic conditions in each traveling direction based upon the average traveling speed of the group of vehicles in each traveling direction. 
     The receiver part  1000  includes first to Nth sector antennas  100 - 1  to  100 -N (where N is an integer not less than 2) operable to receive communication signals (radio signals) from mobile terminals (not shown) mounted on the respective vehicles (mobile stations) or mobile terminals (not shown) of passengers riding in the respective vehicles (mobile stations) to output first to Nth radio signals  101 - 1  to  101 -N, and first to Nth radio receiver parts  102 - 1  to  102 -N operable to convert the first to Nth radio signals  101 - 1  to  101 -N received into first to Nth baseband signals  103 - 1  to  103 -N. The first to Nth baseband signals  103 - 1  to  103 -N are output as the received signals. 
     The speed/direction estimation part  2000  includes a sector selection part  104 , a frequency shift estimation part  107 , a frequency shift storage device  109 , a frequency shift average part  111 , a traveling direction determination part  113 , a traveling speed calculation part  115 , and a traveling speed correction part  117 . 
     The sector selection part  104  is operable to select a received signal (baseband signal) of a sector having the best reception quality from among the first to Nth baseband signals  103 - 1  to  103 -N to output a received signal  106  of the selected sector and the selected sector number  105 . The frequency shift estimation part  107  is operable to estimate a frequency shift from the received signal (baseband signal) of the selected sector to output a frequency shift signal  108  indicative of the estimated frequency shift. The frequency shift storage device  109  is operable to store the frequency shift signal  108  indicative of the estimated frequency shift in a continuous manner. The frequency shift average part  111  is operable to read frequency shift signals  110  stored for each average period from the frequency shift storage device  109 , calculate an average frequency shift to output an average frequency shift signal  112  indicative of the average frequency shift. The traveling direction determination part  113  is operable to determine a traveling direction of the vehicles from the selected sector number  105  and the average frequency shift, which is represented by the average frequency shift signal  112 , to output a traveling direction signal  114  indicative of the traveling direction of the vehicles. The traveling speed calculation part  115  is operable to calculate a relative traveling speed of the vehicles with respect to the base station  200  ( FIG. 2 ) from the average frequency shift, which is represented by the average frequency shift signal  112 , to output a relative traveling speed signal  116  indicative of the relative traveling speed of the vehicles. The traveling speed correction part  117  is operable to correct the relative traveling speed of the vehicles, which is represented by the relative traveling speed signal  116 , based upon the selected sector number  105 , thus calculate a traveling speed of the vehicles to output a traveling speed signal  118  indicative of the traveling speed of the vehicles. 
     A combination of the frequency shift estimation part  107 , the frequency shift storage device  109 , and the frequency shift average part  111  functions as a frequency shift calculation part operable to estimate frequency shifts of the respective vehicles from the received signals  106  of the selected sector and calculate an average frequency shift by averaging the frequency shifts. 
     The traffic condition analysis part  3000  includes a traveling speed variation calculation part  119 , a traveling speed deviation calculation part  121 , an effective traveling speed determination part  123 , a traveling speed storage device  125 , a vehicle group average traveling speed calculation part  127 , and a traffic condition determination part  129 . 
     The traveling speed variation calculation part  119  is operable to calculate a variation of the current traveling speed from the last traveling speed. The traveling speed deviation calculation part  121  is operable to calculate a deviation of the traveling speed. The effective traveling speed determination part  123  is operable to determine that the traveling speed data are effective when the traveling speed and the variation and deviation thereof are not more than predetermined threshold values. The traveling speed storage device  125  is operable to store effective traveling speed signals  124  for the respective vehicles in each traveling direction. The vehicle group average traveling speed calculation part  127  is operable to read effective traveling speed signals of all vehicles in each traveling direction from the traveling speed storage device  125 , calculate an average of the effective traveling speeds of all vehicles to calculate an average traveling speed of a group of vehicles in each traveling direction, and output a vehicle group average traveling speed signal  128  in each traveling direction that indicates the average traveling speed of the group of vehicles in each traveling direction. The traffic condition determination part  129  determines traffic conditions in each traveling direction based upon the average traveling speed of the group of vehicles in each traveling direction, which is represented by the vehicle group average traveling speed signal  128  in each traveling direction. 
     Specifically, the traveling speed variation calculation part  119  reads an effective traveling speed signal of a vehicle that was calculated the last time, as the last traveling speed signal  126  indicative of the last traveling speed, from the traveling speed storage device  125  and calculates a variation  120  of the current traveling speed of the vehicle from the last traveling speed. The traveling speed deviation calculation part  121  calculates a deviation  122  of the current traveling speed of the vehicle with respect to the average traveling speed of a group of vehicles in each traveling direction, which is represented by the vehicle group average traveling speed signal  128  in each traveling direction. The effective traveling speed determination part  123  determines whether or not the current traveling speed of the vehicle, which is represented by the traveling speed signal  118 , is effective based upon the variation  120  and the deviation  122  and output an effective traveling speed signal  124  when it determines that the current traveling speed of the vehicle is effective. 
     Therefore, a combination of the traveling speed variation calculation part  119 , the traveling speed deviation calculation part  121 , and the effective traveling speed determination part  123  functions as an effective speed determination part operable to determine, for each vehicle, whether or not the current traveling speed of the vehicle, which is represented by the traveling speed signal  118 , is effective, in cooperation with the traveling speed storage device  125 , to output the effective traveling speed signal  124  indicative of an effective traveling speed. 
     The base station  200  with the first to Nth sector antennas  100 - 1  to  100 -N for N sectors where N is integer not less than 2 is installed, for example, with a sector arrangement as shown in  FIG. 2 , by the side of the road  210 . 
     In the example illustrated in  FIG. 2 , the road  210  extends from north to south, and the base station  200  is arranged by the west side of the road  210 . Furthermore, in the illustrated example, a plurality of sectors are arranged at equal angular intervals of 60 degrees about the base station  200 . The number of the sectors N is six. The sectors 1 to 6 are sequentially arranged clockwise about the base station  200  from the sector 1, whose central direction is directed to the north with respect to the base station  200 , to the sector 6. In other words, the central direction of the sector 2 is directed to the east-northeast with respect to the base station  200 . The central direction of the sector 3 is directed to the east-southeast. The central direction of the sector 4 is directed to the south with respect to the base station  200 . The central direction of the sector 5 is directed to the west-southwest with respect to the base station  200 . The central direction of the sector 6 is directed to the west-northwest with respect to the base station  200 . The first to sixth sector antennas  100 - 1  to  100 - 6  are provided for the sectors 1 to 6, respectively. 
     Furthermore, in the example of  FIG. 2 , a vehicle A is on the road  210  within the sector 1 in the north of the base station  200  and is travelling southward on the road  210 . A vehicle B is on the road  210  within the sector 4 in the south of the base station  200  and is traveling northward on the road  210 . 
     In this case, as shown in  FIG. 2 , only the first to fourth sector antennas  100 - 1  to  100 - 4  for the sectors 1 to 4 can receive communication signals from mobile terminals of the vehicles A and B on the road  210 . Thus, for first to fourth baseband signals (received signals)  103 - 1  to  103 - 4  that have been received by the first to fourth sector antennas  100 - 1  to  100 - 4  for the sectors 1 to 4 and output by the first to fourth radio receiver parts  102 - 1  to  102 - 4 , the sector selection part  104  calculates the received power, for example, as an index of the reception quality for each mobile terminal, selects a received signal (baseband signal) from the sector having the highest received power to output that received signal (baseband signal). 
     In the example of  FIG. 2 , for the vehicle A within the sector 1, the sector selection part  104  determines that the first baseband signal  103 - 1  that has been transmitted from the mobile terminal of the vehicle A, received by the first sector antenna  100 - 1 , and output by the first radio receiver part  102 - 1  is a received signal (baseband signal) of a sector having the highest received power, selects the sector 1 as a selected sector, and outputs a selected sector number  105  indicative of the sector 1. Furthermore, for the vehicle B within the sector 4, the sector selection part  104  determines that the fourth baseband signal  103 - 4  that has been transmitted from the mobile terminal of the vehicle B, received by the fourth sector antenna  100 - 4 , and output by the fourth radio receiver part  102 - 4  is a received signal (baseband signal) of a sector having the highest received power, selects the sector 4 as a selected sector, and outputs a selected sector number  105  indicative of the sector 4. 
     Furthermore, the sector selection part  104  outputs a received signal (baseband signal) only when the highest received power is nor less than a preset threshold value. Thus, baseband signals with low reception quality are discarded. Therefore, communication signals transmitted from distant mobile terminals that are not on the nearby road  210  can be excluded. 
     For the received signal (baseband signal)  106  of the selected sector, the frequency shift estimation part  107  performs a channel estimation on, for example, a pilot symbol inserted in the received signal to calculate a phase rotation of the symbol and estimate a frequency shift. The frequency shift estimation part  107  stores a frequency shift signal  108  indicative of the estimated frequency shift in the frequency shift storage device  109 . The frequency shift average part  111  reads frequency shift signals  110  from the frequency shift storage device  109  for each preset average period, calculates an average of the frequency shift signals  110  to output the average to a subsequent stage, and then deletes the contents of the frequency shift storage device  109 . Thus, frequency shifts are averaged over a plurality of reception periods. Therefore, influence of an estimation error of the frequency shift can be reduced. 
     With use of the Doppler effect that the frequency shift becomes positive when a vehicle is approaching the base station  200  and becomes negative when a vehicle is moving away from the base station  200 , the traveling direction determination part  113  determines the traveling direction of a vehicle based upon the polarity of the average frequency shift. However, the relationship between the polarity of the frequency shift and the traveling direction differs depending upon the selected sector. Therefore, the traveling direction determination part  113  should determine the traveling direction of a vehicle depending upon the selected sector number  105  output by the sector selection part  104 . 
     In the example of  FIG. 2 , the sectors 1 and 2 are directed northward with respect to the base station  200 . Therefore, when a frequency shift of the first and second radio signals  101 - 1  and  101 - 2  received by the first and second sector antennas  100 - 1  and  100 - 2  for the sectors 1 and 2 is positive, the vehicle is traveling southward. When the frequency shift is negative, the vehicle is traveling northward. On the other hand, the sectors 3 and 4 are directed southward with respect to the base station  200 . Therefore, when a frequency shift of the third and fourth radio signals  101 - 3  and  101 - 4  received by the third and fourth sector antennas  100 - 3  and  100 - 4  for the sectors 3 and 4 is positive, the vehicle is traveling northward. When the frequency shift is negative, the vehicle is traveling southward. 
     The traveling speed calculation part  115  calculates a relative traveling speed v′ with respect to the base station  200  from the relationship of the Doppler effect between the traveling speed and the frequency shift by the following formula 3 using an average frequency shift, which is represented by the average frequency shift signal  112 . Wherein, the transmission frequency set at a mobile terminal is defined as f 0 , the speed of light is defined as c, and the average frequency shift is defined as Δf′. All of those values are known at the base station  200 . 
     
       
         
           
             
               
                 
                   
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     The term Δf′ in the above formula 3 represents an average frequency shift observed by the base station  200 . The relative traveling speed v′ becomes equal to the traveling speed of a vehicle only when the vehicle moves directly toward or away from the base station  200 . In practice, as shown in  FIG. 3 , the base station  200  is installed at a position that is remote from the road  210  to some extent. Thus, the vehicle C moves at an angle θ with respect to the base station  200 . Accordingly, the traveling speed correction part  117  needs to correct the traveling speed of the vehicle C by using the following formula 4. 
     
       
         
           
             
               
                 
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     Wherein, as shown in  FIG. 3 , θ is an angle (included angle) between a line connecting the vehicle C to the base station  200  and the traveling direction of the vehicle C. However, a sector antenna cannot obtain the exact angle θ. Therefore, the traveling speed correction part  117  corrects the traveling speed with the following formula 5 using an angle (included angle) θ′ between the central direction of the sector selected by the sector selection part  104  (the sector 3 in the example of  FIG. 3 ) and the longitudinal direction of the road  210 . 
     
       
         
           
             
               
                 
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                   5 
                 
               
             
           
         
       
     
     The traveling speed variation calculation part  119  reads the traveling speed signal of the vehicle that was calculated last time, as the last traveling speed signal  126  indicative of the last traveling speed, from the traveling speed storage device  125  and calculates a variation  120  of the current traveling speed of the vehicle, which is represented by the traveling speed signal  118 , from the last traveling speed. The traveling speed deviation calculation part  121  calculates a deviation  122  of the current traveling speed of the vehicle, which is represented by the traveling speed signal  118 , with respect to the vehicle group average traveling speed signal  128  in each traveling direction, which represents an average traveling speed of all vehicles in each traveling direction. 
     In order to enhance the reliability of the traveling speed data, the effective traveling speed determination part  123  determines that the current traveling speed represented by the traveling speed signal  118  is an effective traveling speed only if each of the current traveling speed of the vehicle, which is represented by the traveling speed signal  118 , and the variation  120  and the deviation  122  of the traveling speed is not more than a preset threshold value. The effective traveling speed determination part  123  stores an effective traveling speed signal  124 , which represents an effective traveling speed, in the traveling speed storage device  125  for each vehicle in each traveling direction. 
     Wherein, a threshold value is individually set for each of the traveling speed of the vehicle, which is represented by the traveling speed signal  118 , the variation  120 , and the deviation  122 . For example, since a usual vehicle is unlikely to travel at a speed higher than 200 km/h, a threshold value for the traveling speed is set at 200 km/h so that a traveling speed higher than 200 km/h is discarded. Furthermore, a variation of the traveling speed is unlikely to be extremely large. For example, a usual speed variation as compared to the last traveling speed is unlikely to be larger than 10 km/h. Therefore, a threshold value for the variation is set at 10 km/h so that a traveling speed having a speed variation larger than 10 km/h is discarded. Moreover, for when the vehicle group average traveling speed  128  is 40 km/h, a vehicle traveling at a speed of 80 km/h may possibly be an exceptional vehicle such as a vehicle traveling on a shoulder of a road during traffic jam. Additionally, a mobile terminal traveling at a speed of 5 km/h is likely to be a mobile terminal of a pedestrian, rather than a mobile terminal of a vehicle. Therefore, a traveling speed having a deviation to the vehicle group average traveling speed  128  that is larger than 30 km/h is discarded. 
     The vehicle group average traveling speed calculation part  127  reads the effective traveling speed signals  126  of all vehicles in each traveling direction from the traveling speed storage device  125  and calculates an average of the effective traveling speeds of all vehicles. The transmission frequency of mobile terminals suffers from errors caused by individual differences of internal oscillators. Traveling speeds of a plurality of vehicles are averaged in the above manner so that influences from such errors can be eliminated. Furthermore, the vehicle group average traveling speed calculation part  127  calculates a time average of the calculated average traveling speed of all vehicles for a predetermined period of time so as to smooth the traveling speed. Thus, the vehicle group average traveling speed calculation part  127  outputs a vehicle group average traveling speed signal  128  in each traveling direction, which indicates a vehicle group average traveling speed in each traveling direction. Wherein, the reliability of traveling speed data can further be enhanced by not using traveling speed data during a specific period for which received signals from mobile terminals other than mobile terminals mounted on vehicles are expected to increase. For example, when a railway runs near the road  210  to be monitored, traveling speeds of vehicles that are received during periods of time when trains pass are excluded from the averaging samples. 
     The traffic condition determination part  129  compares the vehicle group average traveling speed signal  128  in each traveling direction with a threshold value for determining traffic conditions and determines traffic conditions in each traveling direction with reference to pre-inputted event information such as traffic accidents. For example, the traffic condition determination part  129  determines traffic conditions of each traveling direction among “smooth,” “spontaneously congested,” “spontaneously jammed,” “congested by accident,” and “jammed by accident” with use of a congestion threshold value, a jam threshold value, and accident occurrence information as illustrated in a flow chart of  FIG. 4 . 
     Referring to  FIG. 4 , a traffic condition determination operation by the traffic condition determination part  129  will be described. 
     First, the traffic condition determination part  129  compares the vehicle group average traveling speed signal  128  in each traveling direction with the congestion threshold value (Step S 401 ). If the vehicle group average traveling speed signal  128  in each traveling direction is not less than the congestion threshold value (Yes at Step S 401 ), then the traffic condition determination part  129  determines the traffic conditions as “smooth” (Step S 402 ). 
     If the vehicle group average traveling speed signal  128  in each traveling direction is less than the congestion threshold value (No at Step S 401 ), then the traffic condition determination part  129  compares the vehicle group average traveling speed signal  128  in each traveling direction with the jam threshold value (Step S 403 ). If the vehicle group average traveling speed signal  128  in each traveling direction is not less than the jam threshold value (Yes at Step S 403 ), then the traffic condition determination part  129  determines from the accident occurrence information whether or not any accident has occurred (Step S 404 ). If no accident has occurred (No at Step S 404 ), the traffic condition determination part  129  determines the traffic conditions as “spontaneously congested” (Step S 405 ). If any accident has occurred (Yes at Step S 404 ), the traffic condition determination part  129  determines the traffic conditions as “congested by accident” (Step S 406 ). 
     If the vehicle group average traveling speed signal  128  in each traveling direction is less than the jam threshold value (No at Step S 403 ), then the traffic condition determination part  129  determines from the accident occurrence information whether or not any accident has occurred (Step S 407 ). If no accident has occurred (No at Step S 407 ), the traffic condition determination part  129  determines the traffic conditions as “spontaneously jammed” (Step S 408 ). On the other hand, if any accident has occurred (Yes at Step S 407 ), the traffic condition determination part  129  determines the traffic conditions as “jammed by accident” (Step S 409 ). 
     In the example of  FIG. 4 , the traffic condition determination part  129  determines the traffic conditions of each traveling direction among “smooth,” “spontaneously congested,” “spontaneously jammed,” “congested by accident,” and “jammed by accident,” based upon the congestion threshold value, the jam threshold value, and the accident occurrence information. Nevertheless, the accident occurrence information may be omitted. In such a case, Steps S 404  and S 407  in  FIG. 4  can be eliminated. Thus, the traffic condition determination part  129  determines the traffic conditions of each traveling direction among “smooth,” “congested,” and “jammed,” based upon the congestion threshold value and the jam threshold value. Furthermore, one of the congestion threshold value and the jam threshold value may be eliminated from the congestion threshold value, the jam threshold value, and the accident occurrence information. 
     Next, an operation of the traffic condition monitoring system illustrated in  FIG. 1  will be described below. 
     The first to Nth radio signals  101 - 1  to  101 -N received by the first to Nth sector antennas  100 - 1  to  100 -N are respectively converted into the first to Nth baseband signals  103 - 1  to  103 -N by the first to Nth radio receiver parts  102 - 1  to  102 -N. The sector selection part  104  calculates the reception quality of a mobile terminal for each of the sectors with the first to Nth baseband signals  103 - 1  to  103 -N. If the reception quality of a sector having the best reception quality is not less than a threshold value, the sector selection part  104  outputs the baseband signal (received signal)  106  of the selected sector and outputs the selected sector number  105  to the traveling direction determination part  113  and the traveling speed correction part  117 . If the reception quality of the selected sector is less than the threshold value, the sector selection part  104  discards the baseband signal of the mobile terminal so that subsequent processes are not performed. 
     The frequency shift estimation part  107  estimates a frequency shift with respect to the baseband signal  106  of the selected sector and stores a frequency shift signal  108  indicative of the estimated frequency shift in the frequency shift storage device  109 . The frequency shift average part  111  reads frequency shift signals  110  for each average period from the frequency shift storage device  109 , calculates an average frequency shift to output an average frequency shift signal  112  indicative of the average frequency shift, and then deletes the contents of the frequency shift storage device  109 . 
     The traveling direction determination part  113  determines a traveling direction of the vehicle from the selected sector number  105  and the polarity of the average frequency shift signal  112  to output a traveling direction signal  114  indicative of the traveling direction of the vehicle. The traveling speed calculation part  115  calculates a relative traveling speed of the vehicle with respect to the base station  200  from the average frequency shift signal  112  to output a relative traveling speed signal  116  indicative of the relative traveling speed of the vehicle. The traveling speed correction part  117  corrects the relative traveling speed signal  116  based upon the selected sector number  105  to output a traveling speed signal  118  indicative of the corrected traveling speed of the vehicle. 
     The traveling speed variation calculation part  119  reads the effective traveling speed signal that was calculated last time as the last traveling speed signal  126  indicative of the last traveling speed from the traveling speed storage device  125 . The traveling speed variation calculation part  119  calculates and outputs a variation  120  of the current traveling speed of the vehicle with respect to the last traveling speed. The traveling speed deviation calculation part  121  calculates and outputs a deviation  122  of the current traveling speed of the vehicle with respect to the average traveling speed of a group of vehicles in each traveling direction, which is represented by the vehicle group average traveling speed signal  128  in each traveling direction. 
     The effective traveling speed determination part  123  compares the traveling speed signal  118  and the variation  120  and the deviation  122  of the traveling speed with the threshold values. The effective traveling speed determination part  123  stores the effective traveling speed signal  124  for each vehicle in the traveling direction, which is represented by the traveling direction signal  114 , in the traveling speed storage device  125  only if none of the traveling speed signal  118 , the variation  120 , and the deviation  122  are greater than the threshold value. If any of the traveling speed, the variation, and the deviation is greater than the threshold value, the effective traveling speed determination part  123  discards the traveling speed so that subsequent processes are not performed. 
     The vehicle group average traveling speed calculation part  127  reads from the traveling speed storage device  125  the effective traveling speed signals  126  of all vehicles in the traveling direction, which is represented by the traveling direction signal  114 , and calculates an average of the effective traveling speeds of all vehicles. Furthermore, the vehicle group average traveling speed calculation part  127  calculates a time average to obtain an average traveling speed of the group of vehicles in the traveling direction to output a vehicle group average traveling speed signal  128  in the traveling direction that indicates the average traveling speed of the group of vehicles in the traveling direction. 
     As shown in  FIG. 4 , the traffic condition determination part  129  compares the vehicle group average traveling speed signal  128  in the traveling direction with the traffic condition threshold values to determine traffic conditions in the traveling direction. 
     The advantageous effects of the first example of the present invention will be described below. 
     According to the first example of the present invention, with the existing cellular phone infrastructure, a traveling speed and a traveling direction of a vehicle moving (traveling) on a road can be calculated from a frequency shift of a mobile terminal of the vehicle. Thus, traffic conditions can be analyzed. Therefore, a low-cost, small-scale traffic condition monitoring system can be provided. 
     The speed/direction estimation part  2000  and the traffic condition analysis part  3000  can be implemented by a computer. As is known in the art, the computer includes a central processing unit (CPU), a storage device (RAM) for storing data, and a program memory (ROM) for storing programs. The storage device (RAM) serves as the frequency shift storage device  109  and the traveling speed storage device  125 . The CPU reads the programs stored in the program memory (ROM) and implements functions of the speed/direction estimation part  2000  and the traffic condition analysis part  3000 . 
     Example 2 
     Referring to  FIG. 5 , a traffic condition monitoring system according to a second example of the present invention will be described. The traffic condition monitoring system according to the second example calculates a traveling speed of a vehicle and determines a traveling direction of the vehicle by forming a directional beam and estimating a frequency shift. In the following description, components having the same function as those in the traffic condition monitoring system shown in  FIG. 1  according to the first example of the present invention are denoted by the same reference numerals, and the explanation thereof is omitted herein for brevity. Thus, the following description focuses on differences from the traffic condition monitoring system shown in  FIG. 1  according to the first example of the present invention. 
     The traffic condition monitoring system according to the second example differs in the following points from the traffic condition monitoring system according to the first example. Specifically, the first to Nth sector antennas  100 - 1  to  100 -N have been changed into M antenna elements of first to Mth array antennas  500 - 1  to  500 -M (where M is an integer not less than 2). The sector selection part  104  has been changed into an arrival angle calculation part  501 , an arrival angle determination part  503 , and a directional beam formation part  505 . The traveling direction determination part  113  has been changed into a traveling direction determination part  507  operable to determine a traveling direction based upon an average frequency shift, which is represented by an average frequency shift signal  112 , and a determined arrival angle, which is represented by an arrival angle signal  504 . The traveling speed correction part  117  has been changed into a traveling speed correction part  508  operable to correct a relative traveling speed based upon the determined arrival angle, which is represented by the arrival angle signal  504 . Thus, the receiver part is denoted by the reference numeral  1000 A, and the speed/direction estimation part is denoted by the reference numeral  2000 A. 
     For first to Mth baseband signals  103 - 1  to  103 -M that have been received by the M antenna elements, the arrival angle calculation part  501  calculates the received power, for example, as an index of the reception quality for each mobile terminal, estimates an arrival angle having the best reception quality, and outputs an arrival angle signal  502  indicative of the estimated arrival angle to the arrival angle determination part  503 . However, if the reception quality of the estimated arrival angle is less than the preset threshold value of the reception quality, then the arrival angle calculation part  501  discards the arrival angle signal  502  so that subsequent processes are not performed. Therefore, communication signals transmitted from distant mobile terminals that are not on a nearby road can be excluded. 
     The arrival angle determination part  503  determines whether or not the estimated arrival angle, which is represented by the arrival angle signal  502 , corresponds to the direction of the road. If the arrival angle determination part  503  determines that the estimated arrival angle corresponds to the direction of the road, then it outputs an arrival angle signal  504  indicative of the determined arrival angle to the directional beam formation part  505 , to the traveling direction determination part  507 , and to the traveling speed correction part  508 . On the other hand, if the arrival angle determination part  503  determines that the estimated arrival angle does not correspond to the direction of the road, then it discards the arrival angle signal  502 , which indicates the estimated arrival angle, so that subsequent processes are not performed. 
     Specifically, a combination of the arrival angle calculation part  501  and the arrival angle determination part  503  serves as an arrival angle estimation/determination part operable to estimate and determine an arrival angle of a received signal having the best reception quality among the first to Mth baseband signals  103 - 1  to  103 -M. 
     For example, as shown in  FIG. 6 , it is assumed that a base station  200 A is arranged by the west side of a road  210  extending from north to south. A vehicle A is on the road  210  in the north of the base station  200 A and is traveling southward on the road  210 . A vehicle B is on the road  210  in the south of the base station  200 A and is traveling northward on the road  210 . In such a case, when the northern direction is defined as 0°, the arrival angle determination part  503  determines that arrival angles within a range of 0° to 180° correspond to the direction of the road and discards other arrival angles. 
     The directional beam formation part  505  forms a directional beam corresponding to the determined arrival angle, which is represented by the arrival angle signal  504  to output a baseband signal  506  after the formation of the directional beam. Unlike the case of the sector antennas, the directional beam can continuously track a vehicle. Therefore, determination of whether or not a received signal has been transmitted from a vehicle on the road  210  can be made with higher precision as compared to the first example. 
     The traveling direction determination part  507  determines the traveling direction of the vehicle based upon the polarity of a frequency shift, which is represented by the frequency shift signal  112 , and the determined arrival angle, which is represented by the arrival angle signal  504 . In the example of  FIG. 6 , the traveling direction determination part  507  determines that the vehicle A is moving (traveling) southward if the arrival angle is within a range of 0° to 90° like the vehicle A while the frequency shift is positive or if the arrival angle is within a range of 90° to 180° while the frequency shift is negative. On the other hand, the traveling direction determination part  507  determines that the vehicle B is moving (traveling) northward if the arrival angle is within a range of 0° to 90° while the frequency shift is negative or if the arrival angle is within a range of 90° to 180° like the vehicle B while the frequency shift is positive. 
     The traveling speed correction part  508  converts the determined arrival angle, which is represented by the arrival angle signal  504 , into an angle (included angle) θ between a line connecting the vehicle to the base station  200 A and the longitudinal direction of the road  210 . The traveling speed correction part  508  substitutes θ for the following formula 6 to correct the relative traveling speed of the vehicle. Thus, the traveling speed correction part  508  calculates a corrected traveling speed to output a traveling speed signal  118  indicative of the traveling speed of the vehicle. 
     
       
         
           
             
               
                 
                   v 
                   = 
                   
                     
                       v 
                       ′ 
                     
                     
                       cos 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       θ 
                     
                   
                 
               
               
                 
                   Formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   6 
                 
               
             
           
         
       
     
     Wherein, it is not necessary to approximate the included angle θ by using the included angle θ′ unlike the first example. Therefore, the traveling speed of a vehicle can be corrected with higher precision. 
     Accordingly, in the second example, traffic conditions can be determined more accurately as compared to the first example. 
     The speed/direction estimation part  2000 A and the traffic condition analysis part  3000  can be implemented by a computer. As is known in the art, the computer includes a central processing unit (CPU), a storage device (RAM) for storing data, and a program memory (ROM) for storing programs. The storage device (RAM) serves as the frequency shift storage device  109  and the traveling speed storage device  125 . The CPU reads the programs stored in the program memory (ROM) and implements functions of the speed/direction estimation part  2000 A and the traffic condition analysis part  3000 . 
     Example 3 
     Referring to  FIG. 7 , a traffic condition monitoring system according to a third embodiment of the present invention will be described. Components having the same function as those in the traffic condition monitoring system shown in  FIG. 5  according to the second example of the present invention are denoted by the same reference numerals, and the explanation thereof is omitted herein for brevity. The following description focuses on differences from the traffic condition monitoring system shown in  FIG. 5  according to the second example of the present invention. 
     The traffic condition monitoring system according to the third example differs in the following points from the traffic condition monitoring system according to the second example. A last arrival angle storage device  700  operable to store a current arrival angle signal  504  indicative of the current arrival angle to output a last arrival angle signal  701  indicative of the last arrival angle has been added in the traffic condition monitoring system according to the third example. The traveling direction determination part  507  has been replaced with a traveling direction determination part  702  operable to determine a traveling direction of a vehicle based upon the last determined arrival angle, which is represented by the last arrival angle signal  701 , and the currently determined arrival angle, which is represented by the current arrival angle signal  504 . Thus, the speed/direction estimation part is denoted by the reference numeral  2000 B. 
     The traveling direction determination part  702  compares the last arrival angle, which is represented by the last arrival angle signal  701  read from the last arrival angle storage device  700 , with the current arrival angle, which is represented by the current arrival angle signal  504 , to determine a traveling direction of a vehicle. For example, in the example of  FIG. 6 , when the current arrival angle is greater than the last arrival angle, the traveling direction determination part  702  determines that the vehicle is traveling southward. In contrast, when the current arrival angle is smaller than the last arrival angle, the traveling direction determination part  702  determines that the vehicle is traveling northward. 
     Furthermore, after the traveling direction determination part  702  reads the last arrival angle signal  701  from the last arrival angle storage device  700 , it stores the current arrival angle signal  504  in the last arrival angle storage device  700  for the next determination. 
     According to the third example, a traveling direction of a vehicle can be determined by changes of the determined arrival angle instead of the frequency shift. 
     The speed/direction estimation part  2000 B and the traffic condition analysis part  3000  can be implemented by a computer. As is known in the art, the computer includes a central processing unit (CPU), a storage device (RAM) for storing data, and a program memory (ROM) for storing programs. The storage device (RAM) serves as the frequency shift storage device  109 , the last arrival angle storage device  700 , and the traveling speed storage device  125 . The CPU reads the programs stored in the program memory (ROM) and implements functions of the speed/direction estimation part  2000 B and the traffic condition analysis part  3000 . 
     Example 4 
     Referring to  FIG. 8 , a traffic condition monitoring system according to a fourth example of the present invention will be described. The traffic condition monitoring system according to the fourth example periodically receives positional information, which has been obtained with a positional measurement function such as a GPS (Global Positioning System) mounted on a mobile terminal of a vehicle, at a base station  200 B (see  FIG. 9 ) to calculate a traveling speed and a traveling direction of a vehicle. Components having the same function as those in the traffic condition monitoring system shown in  FIG. 1  according to the first example of the present invention are denoted by the same reference numerals, and the explanation thereof is omitted herein for brevity. The following description focuses on differences from the traffic condition monitoring system shown in  FIG. 1  according to the first example of the present invention. 
     The traffic condition monitoring system according to the fourth example differs in the following points from the traffic condition monitoring system according to the first example. A processing part (speed/direction estimation part)  2000  relating to selection of a sector and calculation of a traveling speed with estimation of a frequency shift has been changed to a processing part (speed/direction estimation part)  2000 C using positional information. No traveling speed correction part  117  is required in the traffic condition monitoring system according to the fourth example. Furthermore, unlike the traffic condition monitoring systems according to the first and second examples, the configuration of the antenna  100  is not limited to a specific one in the traffic condition monitoring system according to the fourth example. 
     The speed/direction estimation part  2000 C includes a positional information receiver part  800 , a positional information determination part  802 , a traveling distance/traveling direction calculation part  807 , a last positional information storage device  805 , and a traveling speed calculation part  809 . 
     The positional information receiver part  800  calculates, for example, a received power as an index of the reception quality for each mobile terminal. If the calculated reception quality is not less than a preset threshold value of the reception quality, then the positional information receiver part  800  extracts positional terminal information  801  from the baseband signal (received signal)  103  for each mobile terminal to output the positional terminal information  801 . If the calculated reception quality is less than the threshold value, then the positional information receiver part  800  discards the received signal  103  so that subsequent processes are not performed. Thus, the reliability of the positional information data can be enhanced. 
     Specifically, the positional information receiver part  800  calculates the reception quality of a received signal (baseband signal output by the radio receiver part  102 )  103  for each mobile terminal, extracts positional terminal information  801  indicative of the position of the mobile terminal from the received signal having a reception quality higher than the preset reception quality to output the positional terminal information. 
     The positional information determination part  802  compares the preset positional information of the road  210  with the positional terminal information  801  of the mobile terminal. If the position of the mobile terminal matches with any position on the road  210  ( FIG. 9 ), then the positional information determination part  802  outputs current vehicle positional information  804  and also transmits a positional information reception cycle  803  to the traveling speed calculation part  809 . If the position of the mobile terminal does not matches with any position on the road  210  ( FIG. 9 ), then the positional information determination part  802  discards the positional terminal information  801  so that subsequent processes are not performed. 
     In other words, the positional information determination part  802  compares the preset positional information of the road  210  with the positional terminal information  801  to output current vehicle positional information  804  and a positional information reception cycle  803  when the position of the mobile terminal matches with any position on the road  201 . 
     Therefore, a combination of the positional information receiver part  800  and the positional information determination part  802  serves as a positional information identification part operable to identify a current vehicle position on the road  201  from communication signals including the positional information to output current vehicle positional information  804  indicative of the current vehicle position and a positional information reception cycle  803 . 
     The last positional information storage device  805  stores the current vehicle positional information  804  to output last vehicle positional information  806  indicative of the last vehicle position. 
     The traveling distance/traveling direction calculation part  807  reads the last vehicle positional information  806  from the last positional information storage device  805 , calculates a traveling distance of each vehicle and a traveling direction of each vehicle from the last vehicle position, which is represented by the last vehicle positional information  806 , and the current vehicle position, which is represented by the current vehicle positional information  804  to output a traveling distance signal  808  indicative of the traveling distance of each vehicle and a traveling direction signal  114  indicative of the traveling direction of each vehicle, and stores the current vehicle positional information  804  in the last positional information storage device  805  for the next calculation. 
     In the example of  FIG. 9 , a base station  200 B is arranged by the west side of a road  210  extending from north to south. A vehicle located on the south side of the base station  200 B is traveling northward on the road  210  so as to approach the base station  200 B. In this example, when the vehicle moves from the last vehicle position 1 (x 1 , y 1 ) to the current vehicle position 2 (x 2 , y 2 ), the traveling distance D can be calculated by the following formula 7.
 
 D =√{square root over (( x   2   −x   1 ) 2 +( y   2   −y   1 ) 2 )}  Formula 7
 
     In this formula, x 1  and x 2  are position coordinates in the east-west direction, and y 1  and y 2  are position coordinates in the north-south direction. 
     If an angle with respect to the north-south direction is defined as φ, the traveling direction of a vehicle can be calculated by the following formula 8. 
     
       
         
           
             
               
                 
                   ϕ 
                   = 
                   
                     
                       tan 
                       
                         - 
                         1 
                       
                     
                     ⁡ 
                     
                       ( 
                       
                         
                           
                             x 
                             2 
                           
                           - 
                           
                             x 
                             1 
                           
                         
                         
                           
                             y 
                             2 
                           
                           - 
                           
                             y 
                             1 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   8 
                 
               
             
           
         
       
     
     The traveling speed calculation part  809  calculates a traveling speed of each vehicle with use of the traveling distance, which is represented by the traveling distance signal  808 , and a positional information reception cycle  803  as shown by the following formula 9 and outputs a traveling speed signal  118  indicative of the traveling speed of each vehicle to a subsequent stage. Wherein, the traveling speed is defined as v, the traveling distance is defined as D, and the positional information reception cycle is defined as T.
 
 v=D/T   Formula 9
 
     Since the traffic condition monitoring system according to the fourth example can identify the position of a vehicle, it is not necessary to correct the traveling speed with use of angular information unlike the traffic condition monitoring systems according to the first to third examples. In the traffic condition monitoring systems according to the first to third examples, signals transmitted from the mobile terminals located relatively near the base station and in the same direction as the road but not on the road cannot be excluded completely. In contrast, the traffic condition monitoring system according to the fourth example can identify signals from vehicles on the road with reference to the positional information of the road. Therefore, the traveling speed and the traveling direction of vehicles can be estimated with higher accuracy. Accordingly, the traffic condition monitoring system according to the fourth example can determine traffic conditions more accurately as compared to the traffic condition monitoring systems according to the first to third examples. 
     The fourth example of the present invention employs the existing cellular phone infrastructure, can calculate a traveling speed and a traveling direction of a vehicle from a communication signal including positional information that is periodically transmitted from a mobile terminal of a vehicle traveling on a road, and can analyze traffic conditions of each traveling direction. Therefore, a low-cost, small-scale traffic condition monitoring system can be provided. 
     The speed/direction estimation part  2000 C and the traffic condition analysis part  3000  can be implemented by a computer. As is known in the art, the computer includes a central processing unit (CPU), a storage device (RAM) for storing data, and a program memory (ROM) for storing programs. The storage device (RAM) serves as the last positional information storage device  805  and the traveling speed storage device  125 . The CPU reads the programs stored in the program memory (ROM) and implements functions of the speed/direction estimation part  2000 C and the traffic condition analysis part  3000 . 
     While the present invention has been particularly shown and described with reference to examples thereof, the present invention is not limited to the aforementioned examples. It will be understood by those of ordinary skill in the art that various modifications in form and details may be made therein without departing from the sprit and scope of the present invention as defined by the claims. Furthermore, the present invention is not limited to use of the cellular phone infrastructure, which has been assumed in the above examples. The present invention is applicable to communication systems having any communication form. 
     The whole or part of the exemplary embodiments disclosed above can be described as, but not limited to, the following supplementary notes. 
     (Supplementary note 1) A traffic condition monitoring system provided in a base station installed by a side of a road for monitoring a traffic condition based upon communication signals transmitted from mobile terminals in a plurality of vehicles traveling on the road, the system comprising: 
     a receiver part operable to receive the communication signals transmitted from the mobile terminals of the respective vehicles to output received signals; 
     a speed/direction estimation part operable to estimate traveling speeds and traveling directions of the respective vehicles based upon the received signals; and 
     a traffic condition analysis part operable to determine effective traveling speeds of the respective vehicles in each traveling direction, to calculate an average traveling speed of a group of vehicles in each traveling direction, and to analyze a traffic condition in each traveling direction based upon the average traveling speed of the group of vehicles in each traveling direction. 
     (Supplementary note 2) The traffic condition monitoring system as recited in Supplementary note 1, wherein the receiver part includes: 
     first to Nth sector antennas disposed for N sectors where N is an integer not less than 2; and 
     first to Nth radio receiver parts operable to convert first to Nth radio signals received by the first to Nth sector antennas into first to Nth baseband signals to output the first to Nth baseband signals as the received signals, 
     wherein the speed/direction estimation part includes: 
     a sector selection part operable to select a sector in which the received signal has the best reception quality from among the first to Nth baseband signals to output the received signals of the selected sector and a selected sector number; 
     a frequency shift calculation part operable to estimate frequency shifts from the received signals of the selected sector and to calculate an average frequency shift by averaging the frequency shifts; 
     a traveling direction determination part operable to determine a traveling direction of the vehicle from the selected sector number and the average frequency shift; 
     a traveling speed calculation part operable to calculate a relative traveling speed of the vehicle with respect to the base station from the average frequency shift; and 
     a traveling speed correction part operable to correct the relative traveling speed of the vehicle based upon the selected sector number to calculate a traveling speed of the vehicle. 
     (Supplementary note 3) 
     The traffic condition monitoring system as recited in Supplementary note 2, wherein the frequency shift calculation part includes: 
     a frequency shift estimation part operable to estimate frequency shifts from the received signals of the selected sector in a continuous manner; 
     a frequency shift storage device operable to store frequency shift signals indicative of the estimated frequency shifts in a continuous manner; and 
     a frequency shift average part operable to read the frequency shift signals stored for each average period from the frequency shift storage device and to calculate the average frequency shift. 
     (Supplementary note 4) The traffic condition monitoring system as recited in Supplementary note 1, wherein the receiver part includes: 
     first to Mth array antennas including M antenna elements where M is an integer not less than 2; and 
     first to Mth radio receiver parts operable to convert first to Mth radio signals received by the first to Mth array antennas into first to Mth baseband signals and to output the first to Mth baseband signals as the received signals, 
     wherein the speed/direction estimation part includes: 
     an arrival angle estimation/determination part operable to estimate and determine arrival angles of the received signals each having the best reception quality from among the first to Mth baseband signals; 
     a directional beam formation part operable to form directional beams corresponding to the determined arrival angles and to output baseband signals after the formation of the directional beams; 
     a frequency shift calculation part operable to estimate frequency shifts from the baseband signals after the formation of the directional beam and to calculate an average frequency shift by averaging the frequency shifts; 
     a traveling direction determination part operable to determine a traveling direction of the vehicle from the arrival angle and the average frequency shift; 
     a traveling speed calculation part operable to calculate a relative traveling speed of the vehicle with respect to the base station from the average frequency shift; and 
     a traveling speed correction part operable to correct the relative traveling speed of the vehicle based upon the arrival angle to calculate a traveling speed of the vehicle. 
     (Supplementary note 5) A traffic condition monitoring system as recited in Supplementary note 4, wherein the arrival angle estimation/determination part includes: 
     an arrival angle calculation part operable to estimate an arrival angle of the received signal having the best reception quality for each mobile terminal from among the first to Mth baseband signals; and 
     an arrival angle determination part operable to determine whether or not the estimated arrival angle corresponds to a direction of the road and to output an arrival angle signal indicative of the determined arrival angle when it determines that the estimated arrival angle corresponds to the direction of the road. 
     (Supplementary note 6) The traffic condition monitoring system as recited in Supplementary note 4 or 5, wherein the frequency shift calculation part includes: 
     a frequency shift estimation part operable to estimate frequency shifts in a continuous manner from baseband signals after the formation of the directional beam; 
     a frequency shift storage device operable to store frequency shift signal indicative of the estimated frequency shifts in a continuous manner; and 
     a frequency shift average part operable to read the frequency shift signals stored for each average period from the frequency shift storage device and to calculate the average frequency shift. 
     (Supplementary note 7) The traffic condition monitoring system as recited in Supplementary note 1, wherein the receiver part includes: 
     first to Mth array antennas including M antenna elements where M is an integer not less than 2; and 
     first to Mth radio receiver parts operable to convert first to Mth radio signals received by the first to Mth array antennas into first to Mth baseband signals to output the first to Mth baseband signals as the received signals, 
     wherein the speed/direction estimation part includes: 
     an arrival angle estimation/determination part operable to estimate and determine current arrival angles of the received signals each having the best reception quality from among the first to Mth baseband signals; 
     a directional beam formation part operable to form directional beams corresponding to the current arrival angles to output baseband signals after the formation of the directional beams; 
     a frequency shift calculation part operable to estimate frequency shifts from the baseband signals after the formation of the directional beams and to calculate an average frequency shift by averaging the frequency shifts; 
     a last arrival angle storage device operable to store a current arrival angle signal indicative of the current arrival angle to output a last arrival angle signal indicative of a last arrival angle; 
     a traveling direction determination part operable to determine a traveling direction of the vehicle from the last arrival angle and the current arrival angle; 
     a traveling speed calculation part operable to calculate a relative traveling speed of the vehicle with respect to the base station from the average frequency shift; and 
     a traveling speed correction part operable to correct the relative traveling speed of the vehicle based upon the current arrival angle to calculate a traveling speed of the vehicle. 
     (Supplementary note 8) The traffic condition monitoring system as recited in Supplementary note 7, wherein the arrival angle estimation/determination part includes: 
     an arrival angle calculation part operable to estimate a current arrival angle of the received signal having the best reception quality for each mobile terminal from among the first to Mth baseband signals; and 
     an arrival angle determination part operable to determine whether or not the estimated current arrival angle corresponds to a direction of the road to output a current arrival angle signal indicative of the current arrival angle when it determines that the estimated current arrival angle corresponds to the direction of the road. 
     (Supplementary note 9) The traffic condition monitoring system as recited in Supplementary note 7 or 8, wherein the frequency shift calculation part includes: 
     a frequency shift estimation part operable to estimate frequency shifts in a continuous manner from baseband signals after the formation of the directional beams; 
     a frequency shift storage device operable to store frequency shift signals indicative of the estimated frequency shifts in a continuous manner; and 
     a frequency shift average part operable to read the frequency shift signals stored for each average period from the frequency shift storage device and to calculate the average frequency shift. 
     (Supplementary note 10) The traffic condition monitoring system as recited in Supplementary note 1, wherein the receiver part periodically receives, as the communication signals, signals including positional information from the mobile terminals in the respective vehicles to output the received signals, 
     wherein the speed/direction estimation part includes: 
     a positional information identification part operable to identify a current vehicle position on the road from the signal including the positional information to output current vehicle positional information indicative of the current vehicle position and a positional information reception cycle, 
     a last positional information storage device operable to store the current vehicle positional information to output last vehicle positional information indicative of a last vehicle position; 
     a traveling distance/traveling direction calculation part operable to calculate a traveling distance of each vehicle and a traveling direction of each vehicle from the last vehicle position and the current vehicle position; and 
     a traveling speed calculation part operable to calculate a traveling speed of each vehicle with use of the traveling distance and the positional information reception cycle. 
     (Supplementary note 11) The traffic condition monitoring system as recited in Supplementary note 10, wherein the positional information identification part includes: 
     a positional information receiver part operable to calculate a reception quality of received signal for each mobile terminal, to extract positional terminal information indicative of a position of the mobile terminal from the received signals having a reception quality higher than a preset value, and to output the positional terminal information; and 
     a positional information determination part operable to compare preset positional information of the road with the positional terminal information to output the current vehicle positional information and the positional information reception cycle when the position of the mobile terminal matches with any position of the road. 
     (Supplementary note 12) The traffic condition monitoring system as recited in any one of Supplementary notes 1 to 11, wherein the traffic condition analysis part includes: 
     an effective speed determination part operable to determine whether or not a current traveling speed of each vehicle is effective to output an effective traveling speed signal indicative of the effective traveling speed; 
     a traveling speed storage device operable to store the effective traveling speed signal of each vehicle in each traveling direction; 
     a vehicle group average traveling speed calculation part operable to read the effective traveling speed signals of all vehicles in each traveling direction from the traveling speed storage device, and to calculate an average of the effective traveling speeds of all vehicles to obtain the average traveling speed of the group of vehicles in each traveling direction; and 
     a traffic condition determination part operable to determine the traffic condition in each traveling direction based upon the average traveling speed of the group of vehicles in each traveling direction. 
     (Supplementary note 13) The traffic condition monitoring system as recited in Supplementary note 12, wherein the effective speed determination part includes: 
     a traveling speed variation calculation part operable to read an effective traveling speed signal of the vehicle that was calculated last time, as a last traveling speed signal indicative of the last traveling speed, from the traveling speed storage device and to calculate a variation of the current traveling speed of the vehicle with respect to the last traveling speed; 
     a traveling speed deviation calculation part operable to calculate a deviation of the current traveling speed of the vehicle with respect to the average traveling speed of the group of vehicles in each traveling direction; and 
     an effective traveling speed determination part operable to determine whether or not the current traveling speed of the vehicle is effective based upon the variation and the deviation to output the effective traveling speed signal. 
     (Supplementary note 14) The traffic condition monitoring system as recited in Supplementary note 12 or 13, wherein the traffic condition determination part is operable to determine the traffic condition in each traveling direction from among a smooth condition, a spontaneously congested condition, a spontaneously jammed condition, a congested condition by accident, and a jammed condition by accident, based upon the average traveling speed of the group of vehicles in each traveling direction, a congestion threshold value, a jam threshold value, and accident occurrence information.
 
(Supplementary note 15) A traffic condition monitoring method used in a traffic condition monitoring system provided in a base station installed by a side of a road, for monitoring a traffic condition based upon communication signals transmitted from mobile terminals in a plurality of vehicles traveling on the road, the method comprising:
 
     a speed/direction estimation step of estimating traveling speeds and traveling directions of the respective vehicles based upon received signals of a receiver part that has received the communication signals transmitted from the mobile terminals in the vehicles; and 
     a traffic condition analysis step of determining effective traveling speeds of the respective vehicles in each traveling direction, of calculating an average traveling speed of a group of vehicles in each traveling direction, and of analyzing a traffic condition in each traveling direction based upon the average traveling speed of the group of vehicles in each traveling direction. 
     (Supplementary note 16) The traffic condition monitoring method as recited in Supplementary note 15, wherein the receiver part includes first to Nth sector antennas disposed for N sectors where N is an integer not less than 2, and first to Nth radio receiver parts operable to convert first to Nth radio signals received by the first to Nth sector antennas into first to Nth baseband signals to output the first to Nth baseband signals as the received signals, 
     wherein the speed/direction estimation step includes: 
     a sector selection step of selecting a sector in which the received signal has the best reception quality from among the first to Nth baseband signals and outputting the received signals of the selected sector and a selected sector number; 
     a frequency shift calculation step of estimating frequency shifts from the received signals of the selected sector and of calculating an average frequency shift by averaging the frequency shifts; 
     a traveling direction determination step of determining a traveling direction of the vehicle from the selected sector number and the average frequency shift; 
     a traveling speed calculation step of calculating a relative traveling speed of the vehicle with respect to the base station from the average frequency shift; and 
     a traveling speed correction step of correcting the relative traveling speed of the vehicle based upon the selected sector number to calculate a traveling speed of the vehicle. 
     (Supplementary note 17) The traffic condition monitoring method as recited in Supplementary note 16, wherein the frequency shift calculation step includes: 
     a frequency shift estimation step of estimating frequency shifts from the received signals of the selected sector in a continuous manner; 
     a step of storing frequency shift signals indicative of the estimated frequency shifts in a frequency shift storage device in a continuous manner; and 
     a frequency shift averaging step of reading the frequency shifts stored for each average period from the frequency shift storage device and of calculating the average frequency shift. 
     (Supplementary note 18) The traffic condition monitoring method as recited in Supplementary note 15, wherein the receiver part includes first to Mth array antennas including M antenna elements where M is an integer not less than 2, and first to Mth radio receiver parts operable to convert first to Mth radio signals received by the first to Mth array antennas into first to Mth baseband signals to output the first to Mth baseband signals as the received signals, 
     wherein the speed/direction estimation step includes: 
     an arrival angle estimation/determination step of estimating and determining arrival angles of the received signals each having the best reception quality from among the first to Mth baseband signals, 
     a directional beam formation step of forming directional beams corresponding to the determined arrival angles and of producing baseband signals after the formation of the directional beams; 
     a frequency shift calculation step of estimating frequency shifts from the baseband signals after the formation of the directional beams and of calculating an average frequency shift by averaging the frequency shifts; 
     a traveling direction determination step of determining a traveling direction of the vehicle from the arrival angle and the average frequency shift; 
     a traveling speed calculation step of calculating a relative traveling speed of the vehicle with respect to the base station from the average frequency shift, and 
     a traveling speed correction step of correcting the relative traveling speed of the vehicle based upon the arrival angle to calculate a traveling speed of the vehicle. 
     (Supplementary note 19) The traffic condition monitoring method as recited in Supplementary note 18, wherein the arrival angle estimation/determination step includes: 
     an arrival angle calculation step of estimating an arrival angle of the received signal having the best reception quality for each mobile terminal from among the first to Mth baseband signals; and 
     an arrival angle determination step of determining whether or not the estimated arrival angle corresponds to a direction of the road and of producing an arrival angle signal indicative of the determined arrival angle when it determines that the estimated arrival angle corresponds to the direction of the road. 
     (Supplementary note 20) The traffic condition monitoring method as recited in Supplementary note 18 or 19, wherein the frequency shift calculation step includes: 
     a frequency shift estimation step of estimating frequency shifts in a continuous manner from baseband signals after the formation of the directional beam; 
     a step of storing frequency shift signals indicative of the estimated frequency shifts in a frequency shift storage device in a continuous manner; and 
     a frequency shift averaging step of reading the frequency shift signals stored for each average period from the frequency shift storage device and of calculating the average frequency shift. 
     (Supplementary note 21) The traffic condition monitoring method as recited in Supplementary Note 15, wherein the receiver part includes first to Mth array antennas including M antenna elements where M is an integer not less than 2, and first to Mth radio receiver parts operable to convert first to Mth radio signals received by the first to Mth array antennas into first to Mth baseband signals to output the first to Mth baseband signals as the received signals, 
     wherein the speed/direction estimation step includes: 
     an arrival angle estimation/determination step of estimating and determining current arrival angles of the received signals each having the best reception quality from among the first to Mth baseband signals; 
     a directional beam formation step of forming directional beams corresponding to the current arrival angles to output baseband signals after the formation of the directional beams; 
     a frequency shift calculation step of estimating frequency shifts from the baseband signals after the formation of the directional beams and of calculating an average frequency shift by averaging the frequency shifts; 
     a step of storing a current arrival angle signal indicative of the current arrival angle in a last arrival angle storage device to output a last arrival angle signal indicative of a last arrival angle from the last arrival angle storage device; 
     a traveling direction determination step of determining a traveling direction of the vehicle from the last arrival angle and the current arrival angle; 
     a traveling speed calculation step of calculating a relative traveling speed of the vehicle with respect to the base station from the average frequency shift; and 
     a traveling speed correction step of correcting the relative traveling speed of the vehicle based upon the current arrival angle to calculate a traveling speed of the vehicle. 
     (Supplementary note 22) The traffic condition monitoring method as recited in Supplementary note 21, wherein the arrival angle estimation/determination step includes: 
     an arrival angle calculation step of estimating a current arrival angle of the received signal having the best reception quality for each mobile terminal from among the first to Mth baseband signals; and 
     an arrival angle determination step of determining whether or not the estimated current arrival angle corresponds to a direction of the road to output a current arrival angle signal indicative of the current arrival angle when it determines that the estimated current arrival angle corresponds to the direction of the road. 
     (Supplementary note 23) The traffic condition monitoring method as recited in Supplementary note 21 or 22, wherein the frequency shift calculation step includes: 
     a frequency shift estimation step of estimating frequency shifts in a continuous manner from baseband signals after the formation of the directional beams; 
     a step of storing frequency shift signals indicative of the estimated frequency shifts in a frequency shift storage device in a continuous manner; and 
     a frequency shift averaging step of reading the frequency shift signals stored for each average period from the frequency shift storage device and of calculating the average frequency shift. 
     (Supplementary note 24) The traffic condition monitoring method as recited in Supplementary note 15, wherein the receiver part periodically receives, as the communication signals, signals including positional information from the mobile terminals in the respective vehicles to output the received signals, 
     wherein the speed/direction estimation step includes: 
     a positional information identification step of identifying a current vehicle position on the road from the signal including the positional information to output current vehicle positional information indicative of the current vehicle position and a positional information reception cycle; 
     a step of storing the current vehicle positional information in a last positional information storage device to output last vehicle positional information indicative of a last vehicle position from the last positional information storage device; 
     a traveling distance/traveling direction calculation step of calculating a traveling distance of each vehicle and a traveling direction of each vehicle from the last vehicle position and the current vehicle position; and 
     a traveling speed calculation step of calculating a traveling speed of each vehicle with use of the traveling distance and the positional information reception cycle. 
     (Supplementary note 25) The traffic condition monitoring method as recited in Supplementary note 24, wherein the positional information identification step includes: 
     a positional information receiver step of calculating a reception quality of the received signal for each mobile terminal, of extracting positional terminal information indicative of a position of the mobile terminal from the received signals having a reception quality higher than a preset value, and of producing the positional terminal information; and 
     a positional information determination step of comparing preset positional information of the road with the positional terminal information to output the current vehicle positional information and the positional information reception cycle when the position of the mobile terminal matches with any position of the road. 
     (Supplementary note 26) The traffic condition monitoring method as recited in any one of Supplementary notes 15 to 25, wherein the traffic condition analysis step includes: 
     an effective speed determination step of determining whether or not a current traveling speed of each vehicle is effective to output an effective traveling speed signal indicative of the effective traveling speed; 
     a step of storing the effective traveling speed signal of each vehicle in each traveling direction in a traveling speed storage device; 
     a vehicle group average speed calculation step of reading the effective traveling speed signals of all vehicles in each traveling direction from the traveling speed storage device and of calculating an average of the effective traveling speeds of all vehicles to obtain the average traveling speed of the group of vehicles in each traveling direction; and 
     a traffic condition determination step of determining the traffic condition in each traveling direction based upon the average traveling speed of the group of vehicles in each traveling direction. 
     (Supplementary note 27) The traffic condition monitoring method as recited in Supplementary note 26, wherein the effective speed determination step includes: 
     a traveling speed variation calculation step of reading an effective traveling speed signal of the vehicle that was calculated last time, as a last traveling speed signal indicative of the last traveling speed, from the traveling speed storage device and of calculating a variation of the current traveling speed of the vehicle with respect to the last traveling speed; 
     a traveling speed deviation calculation step of calculating a deviation of the current traveling speed of the vehicle with respect to the average traveling speed of the group of vehicles in each traveling direction; and 
     an effective traveling speed determination step of determining whether or not the current traveling speed of the vehicle is effective based upon the variation and the deviation to output the effective traveling speed signal. 
     (Supplementary note 28) The traffic condition monitoring method as recited in Supplementary note 26 or 27, wherein the traffic condition determination step includes determining the traffic condition in each traveling direction from among a smooth condition, a spontaneously congested condition, a spontaneously jammed condition, a congested condition by accident, and a jammed condition by accident, based upon the average traveling speed of the group of vehicles in each traveling direction, a congestion threshold value, a jam threshold value, and accident occurrence information.
 
(Supplementary note 29) A computer-readable storage medium having a program recorded thereon for allowing a computer of a traffic condition monitoring system provided in a base station installed by a side of a road to monitor a traffic condition based upon communication signals transmitted from mobile terminals in a plurality of vehicles traveling on the road, the program causing the computer to implement:
 
     a speed/direction estimation function of estimating traveling speeds and traveling directions of the respective vehicles based upon received signals of a receiver part that has received the communication signals transmitted from the mobile terminals in the vehicles; and 
     a traffic condition analysis function of determining effective traveling speeds of the respective vehicles in each traveling direction, of calculating an average traveling speed of a group of vehicles in each traveling direction, and of analyzing a traffic condition in each traveling direction based upon the average traveling speed of the group of vehicles in each traveling direction. 
     (Supplementary note 30) The computer-readable storage medium as recited in Supplementary note 29, wherein the receiver part includes first to Nth sector antennas disposed for N sectors where N is an integer not less than 2, and first to Nth radio receiver parts operable to convert first to Nth radio signals received by the first to Nth sector antennas into first to Nth baseband signals to output the first to Nth baseband signals as the received signals, 
     wherein the speed/direction estimation function causes the computer to implement: 
     a sector selection function of selecting a sector in which the received signal has the best reception quality from among the first to Nth baseband signals to output the received signals of the selected sector and a selected sector number; 
     a frequency shift calculation function of estimating frequency shifts from the received signals of the selected sector and of calculating an average frequency shift by averaging the frequency shifts; 
     a traveling direction determination function of determining a traveling direction of the vehicle from the selected sector number and the average frequency shift; 
     a traveling speed calculation function of calculating a relative traveling speed of the vehicle with respect to the base station from the average frequency shift; and 
     a traveling speed correction function of correcting the relative traveling speed of the vehicle based upon the selected sector number to calculate a traveling speed of the vehicle. 
     (Supplementary note 31) The computer-readable storage medium as recited in Supplementary note 30, wherein the frequency shift calculation function causes the computer to implement: 
     a frequency shift estimation function of estimating frequency shifts from the received signals of the selected sector in a continuous manner; 
     a function of storing frequency shift signals indicative of the estimated frequency shifts in a frequency shift storage device in a continuous manner; and 
     a frequency shift averaging function of reading the frequency shifts stored for each average period from the frequency shift storage device and of calculating the average frequency shift. 
     (Supplementary note 32) The computer-readable storage medium as recited in Supplementary note 29, wherein the receiver part includes first to Mth array antennas including M antenna elements where M is an integer not less than 2, and first to Mth radio receiver parts operable to convert first to Mth radio signals received by the first to Mth array antennas into first to Mth baseband signals to output the first to Mth baseband signals as the received signals, 
     wherein the speed/direction estimation function causes the computer to implement: 
     an arrival angle estimation/determination function of estimating and determining arrival angles of the received signals each having the best reception quality from among the first to Mth baseband signals, 
     a directional beam formation function of forming directional beams corresponding to the determined arrival angles and of producing baseband signals after the formation of the directional beams; 
     a frequency shift calculation function of estimating frequency shifts from the baseband signals after the formation of the directional beam and of calculating an average frequency shift by averaging the frequency shifts; 
     a traveling direction determination function of determining a traveling direction of the vehicle from the arrival angle and the average frequency shift; 
     a traveling speed calculation function of calculating a relative traveling speed of the vehicle with respect to the base station from the average frequency shift; and 
     a traveling speed correction function of correcting the relative traveling speed of the vehicle based upon the arrival angle to calculate a traveling speed of the vehicle. 
     (Supplementary note 33) The computer-readable storage medium as recited in Supplementary note 32, wherein the arrival angle estimation/determination function causes the computer to implement: 
     an arrival angle calculation function of estimating an arrival angle of the received signal having the best reception quality for each mobile terminal from among the first to Mth baseband signals; and 
     an arrival angle determination function of determining whether or not the estimated arrival angle corresponds to a direction of the road and of producing an arrival angle signal indicative of the determined arrival angle when it determines that the estimated arrival angle corresponds to the direction of the road. 
     (Supplementary note 34) The computer-readable storage medium as recited in Supplementary note 32 or 33, wherein the frequency shift calculation function causes the computer to implement: 
     a frequency shift estimation function of estimating frequency shifts in a continuous manner from baseband signals after the formation of the directional beam; 
     a function of storing frequency shift signals indicative of the estimated frequency shifts in a frequency shift storage device in a continuous manner; and 
     a frequency shift averaging function of reading the frequency shift signals stored for each average period from the frequency shift storage device and of calculating the average frequency shift. 
     (Supplementary note 35) The computer-readable storage medium as recited in Supplementary note 29, wherein the receiver part includes first to Mth array antennas including M antenna elements where M is an integer not less than 2, and first to Mth radio receiver parts operable to convert first to Mth radio signals received by the first to Mth array antennas into first to Mth baseband signal to output the first to Mth baseband signals as the received signals, 
     wherein the speed/direction estimation function causes the computer to implement: 
     an arrival angle estimation/determination function of estimating and determining current arrival angles of the received signals each having the best reception quality from among the first to Mth baseband signals; 
     a directional beam formation function of forming directional beams corresponding to the current arrival angles to output baseband signals after the formation of the directional beams; 
     a frequency shift calculation function of estimating frequency shifts from the baseband signals after the formation of the directional beams and of calculating an average frequency shift by averaging the frequency shifts; 
     a function of storing a current arrival angle signal indicative of the current arrival angle in a last arrival angle storage device to output a last arrival angle signal indicative of a last arrival angle from the last arrival angle storage device; 
     a traveling direction determination function of determining a traveling direction of the vehicle from the last arrival angle and the current arrival angle; 
     a traveling speed calculation function of calculating a relative traveling speed of the vehicle with respect to the base station from the average frequency shift; and 
     a traveling speed correction function of correcting the relative traveling speed of the vehicle based upon the current arrival angle to calculate a traveling speed of the vehicle. 
     (Supplementary note 36) The computer-readable storage medium as recited in Supplementary note 35, wherein the arrival angle estimation/determination function causes the computer to implement: 
     an arrival angle calculation function of estimating a current arrival angle of the received signal having the best reception quality for each mobile terminal from among the first to Mth baseband signals, and 
     an arrival angle determination function of determining whether or not the estimated current arrival angle corresponds to a direction of the road to output a current arrival angle signal indicative of the current arrival angle when it determines that the estimated current arrival angle corresponds to the direction of the road. 
     (Supplementary note 37) The computer-readable storage medium as recited in Supplementary note 35 or 36, wherein the frequency shift calculation function causes the computer to implement: 
     a frequency shift estimation function of estimating frequency shifts in a continuous manner from baseband signals after the formation of the directional beams, 
     a function of storing frequency shift signals indicative of the estimated frequency shifts in a frequency shift storage device in a continuous manner; and 
     a frequency shift averaging function of reading the frequency shift signals stored for each average period from the frequency shift storage device and of calculating the average frequency shift. 
     (Supplementary note 38) The computer-readable storage medium as recited in Supplementary note 29, wherein the receiver part periodically receives, as the communication signals, signals including positional information from the mobile terminals in the respective vehicles to output the received signals, 
     wherein the speed/direction estimation function causes the computer to implement: 
     a positional information identification function of identifying a current vehicle position on the road from the signal including the positional information to output current vehicle positional information indicative of the current vehicle position and a positional information reception cycle; 
     a function of storing the current vehicle positional information in a last positional information storage device to output last vehicle positional information indicative of a last vehicle position from the last positional information storage device; 
     a traveling distance/traveling direction calculation function of calculating a traveling distance of each vehicle and a traveling direction of each vehicle from the last vehicle position and the current vehicle position; and 
     a traveling speed calculation function of calculating a traveling speed of each vehicle with use of the traveling distance and the positional information reception cycle. 
     (Supplementary note 39) The computer-readable storage medium as recited in Supplementary note 38, wherein the positional information identification function causes the computer to implement: 
     a positional information receiver function of calculating a reception quality of the received signal for each mobile terminal, of extracting positional terminal information indicative of a position of the mobile terminal from the received signals having a reception quality higher than a preset value, and of producing the positional terminal information; and 
     a positional information determination function of comparing preset positional information of the road with the positional terminal information to output the current vehicle positional information and the positional information reception cycle when the position of the mobile terminal matches with any position of the road. 
     (Supplementary note 40) The computer-readable storage medium as recited in any one of Supplementary notes 29 to 39, wherein the traffic condition analysis function causes the computer to implement: 
     an effective speed determination function of determining whether or not a current traveling speed of each vehicle is effective to output an effective traveling speed signal indicative of the effective traveling speed; 
     a function of storing the effective traveling speed signal of each vehicle in each traveling direction in a traveling speed storage device; 
     a vehicle group average speed calculation function of reading the effective traveling speed signals of all vehicles in each traveling direction from the traveling speed storage device and of calculating an average of the effective traveling speeds of all vehicles to obtain the average traveling speed of the group of vehicles in each traveling direction; and 
     a traffic condition determination function of determining the traffic condition in each traveling direction based upon the average traveling speed of the group of vehicles in each traveling direction. 
     (Supplementary note 41) The computer-readable storage medium as recited in Supplementary note 40, wherein the effective speed determination function causes the computer to implement: 
     a traveling speed variation calculation function of reading an effective traveling speed signal of the vehicle that was calculate last time, as a last traveling speed signal indicative of the last traveling speed, from the traveling speed storage device and of calculating a variation of the current traveling speed of the vehicle with respect to the last traveling speed; 
     a traveling speed deviation calculation function of calculating a deviation of the current traveling speed of the vehicle with respect to the average traveling speed of the group of vehicles in each traveling direction; and 
     an effective traveling speed determination function of determining whether or not the current traveling speed of the vehicle is effective based upon the variation and the deviation to output the effective traveling speed signal. 
     (Supplementary note 42) The computer-readable storage medium as recited in Supplementary note 40 or 41, wherein the traffic condition determination function causes the computer to implement determining of the traffic condition in each traveling direction from among a smooth condition, a spontaneously congested condition, a spontaneously jammed condition, a congested condition by accident, and a jammed condition by accident, based upon the average traveling speed of the group of vehicles in each traveling direction, a congestion threshold value, a jam threshold value, and accident occurrence information. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
         
           
               100  . . . antenna 
               100 - 1  to  100 -N . . . sector antenna 
               102  . . . radio receiver part 
               102 - 1  to  102 -N . . . radio receiver part 
               102 - 1  to  102 -M . . . radio receiver part 
               103 - 1  to  103 -N . . . baseband signal 
               103 - 1  to  103 -M . . . baseband signal 
               104  . . . sector selection part 
               105  . . . selected sector number 
               106  . . . received signal (baseband signal) of the selected sector 
               107  . . . frequency shift estimation part 
               108  . . . frequency shift signal 
               109  . . . frequency shift storage device 
               110  . . . frequency shift signal 
               111  . . . frequency shift average part 
               112  . . . average frequency shift signal 
               113  . . . traveling direction determination part 
               114  . . . traveling direction signal 
               115  . . . traveling speed calculation part 
               116  . . . relative traveling speed signal 
               117  . . . traveling speed correction part 
               118  . . . traveling speed signal 
               119  . . . traveling speed variation calculation part 
               120  . . . variation of the current traveling speed 
               121  . . . traveling speed deviation calculation part 
               122  . . . deviation of the current traveling speed 
               123  . . . effective traveling speed determination part 
               124  . . . effective traveling speed signal 
               125  . . . traveling speed storage device 
               126  . . . last traveling speed signal (effective traveling speed signal) 
               127  . . . vehicle group average traveling speed calculation part 
               128  . . . vehicle group average traveling speed signal 
               129  . . . traffic condition determination part 
               200 ,  200 A,  200 B . . . base station 
               210  . . . road 
               500 - 1  to  500 -M . . . array antenna 
               501  . . . arrival angle calculation part 
               502  . . . arrival angle signal 
               503  . . . arrival angle determination part 
               504  . . . arrival angle signal (current arrival angle signal) 
               505  . . . directional beam formation part 
               506  . . . baseband signal after formation of directional beam 
               507  . . . traveling direction determination part 
               508  . . . traveling speed correction part 
               700  . . . last arrival angle storage device 
               701  . . . last arrival angle signal 
               702  . . . traveling direction determination part 
               800  . . . positional information receiver part 
               801  . . . positional terminal information 
               802  . . . positional information determination part 
               803  . . . positional information reception cycle 
               804  . . . current vehicle positional information 
               805  . . . last positional information storage device 
               806  . . . last vehicle positional information 
               807  . . . traveling distance/traveling direction calculation part 
               808  . . . traveling distance signal 
               809  . . . traveling speed calculation part 
               1000 ,  1000 A,  1000 B . . . receiver part 
               2000 ,  2000 A,  2000 B,  2000 C . . . speed/direction estimation part 
               3000  . . . traffic condition analysis part 
           
         
       
    
     This application is based upon and claims the benefit of priority from Japanese patent application No. 2011-093706, filed on Apr. 20, 2011, the disclosure of which is incorporated herein in its entirety by reference.