1.  Field of the Invention
The present invention relates to an electronic toll collection system (hereinafter also referred to as the ETC system in short).  More particularly, the present invention is concerned with an ETC apparatus which is mounted on a motor vehicle (this apparatus will hereinafter be referred to as the vehicle-onboard ETC apparatus), which apparatus is so designed as to be capable of setting a timing for communication of electronic toll collection information or ETC information with a toll gate station equipped with the ETC system on the basis of a vehicle-speed detection signal of the motor vehicle. 
2.  Description of Related Art
For having better understanding of the principle underlying the present invention, description will first be made in some detail of the conventional vehicle-onboard ETC apparatus known heretofore. 
FIG.  6 of the accompanying drawings is a block diagram showing schematically a general arrangement of a hitherto known vehicle-onboard ETC apparatus.  Referring to the figure, reference numeral 1 denotes an antenna mounted on a motor vehicle (hereinafter referred to as the vehicle-mounted antenna) for interchanging or exchanging (i.e., receiving/sending) the ETC-relevant information such as toll gate position information, toll charge/payment information, ID code information inherent or specific to the motor vehicle and the like with a toll collection transaction antenna (also referred to as the on-road overhead antenna) installed at a toll gate station of a toll road (not shown).  Further, reference numeral 2 denotes a cable for the vehicle-onboard ETC apparatus (hereinafter this cable will be referred to as the vehicle-mounted cable) through which the ETC information is conveyed from the vehicle-mounted antenna 1 to a vehicle-onboard ETC apparatus 10 described below and vice versa. 
As can be seen in FIG.  6, the vehicle-onboard apparatus 10 is comprised of a transmitter/receiver unit 3 for interchanging by radio waves the ETC information with a corresponding unit of the ETC system installed at the toll gate station through the medium of the vehicle-mounted antenna 1 and the on-road overhead antenna (not shown), a control unit 4 for generating the ETC information to be sent to the toll gate station through the transmitter/receiver unit 3 while processing the ETC information received through the transmitter/receiver unit 3 into a signal for displaying a corresponding image, a manipulating unit 8 for performing signal input/output operations, input/output signal gain setting operations and others for the control unit 4, a display device 5 designed for displaying the received ETC information as an image while displaying the ETC information sent to the toll gate from the vehicle-onboard ETC apparatus as an image for the monitoring purpose, a voice-output unit 6 for outputting in voice the received ETC information through a speaker 9, and a power supply source 7. 
Parenthetically, the manipulating unit 8, the display device 5, the voice-output unit 6 and the speaker 9 cooperate to constitute a man-machine interface unit 11 which serves an intermediation medium for enabling communication between the vehicle-onboard ETC apparatus 10 and the driver of the motor vehicle. 
FIG.  7 is a block diagram showing generally a circuit arrangement of the transmitter/receiver unit 3 known in the art.  Incidentally, like reference numerals as those shown in FIG.  6 denote like or equivalent parts. 
Referring to the figure, the transmitter/receiver unit 3 is comprised of a detector diode D for detecting the ETC information signal emitted from the toll collection transaction antenna A1 (on-road overhead antenna) installed at a toll gate TG and received by the vehicle-mounted antenna 1, a detecting circuit 3-1 for detecting the level of the received signal as detected by the detector diode D, a demodulating circuit 3-2 for demodulating the received signal as detected to thereby restore the original ETC information which is then supplied to the control unit 4 as the received data, and a modulating circuit 3-3 for modulating the ETC information outputted from the control unit 4 into a signal which is then fed to the vehicle-mounted antenna 1. 
In this conjunction, it should be mentioned that the control unit 4 is so designed as to process the ETC information acquired after demodulation through the demodulating circuit 3-2 when the detection signal of a level higher than a predetermined level is received.  More specifically, the ETC information sent from the overhead antenna A1 is received by the vehicle-onboard ETC apparatus to be processed by the control unit 4, only when the electric field intensity of the radio waves carrying the ETC information is higher than a predetermined level.  In this way, communication between the vehicle-onboard ETC apparatus and the ETC equipment installed at the toll gate TG can be carried out within a narrow range of radio-wave coverage area (service area), while erroneous communication with an adjacent antenna (i.e., other on-road overhead antenna) can be excluded.  Thus, the toll collection or toll charge/payment transaction can be carried out essentially without fail. 
Next, description will turn to operation of the conventional vehicle-onboard ETC apparatus of the structure described above. 
By way of example, it is assumed that a motor vehicle or car equipped with the ETC apparatus 10 runs beneath the overhead antenna A1 installed at the toll gate TG.  Then, the vehicle-onboard ETC apparatus 10 receives the radio wave signal carrying the information concerning the toll gate identifier which the motor vehicle currently is passing through and the toll collection information code from the ETC equipment installed at the toll gate through the overhead antenna A1 and the vehicle-mounted antenna 1.  The control unit 4 incorporated in the vehicle-onboard ETC apparatus 10 processes the received signal to recognize discriminatively the toll gate identifier as well as the toll collection information code, the results of which are displayed as visible information or image on the display device 5 or alternatively messaged as the voice information from the speaker 9 driven by the voice-output unit 6. 
The driver of the car can thus confirm that his or her car is passing through a toll gate station concerned and that the toll collection processing (i.e., toll charge/payment processing) has been started by observing the screen image on the display device 5 or hearing the voice message outputted from the speaker 9. 
Furthermore, upon identification of the toll collection code being received, the control unit 4 reads out the information such as the ID number intrinsic to the motor vehicle or car concerned from an internal memory (not shown), which information is then supplied to the modulating circuit 3-3 to be thereby modulated for transmission to the toll collection overhead antenna A1 of the toll gate TG from the vehicle-mounted antenna 1. 
The ETC equipment installed at the toll gate TG recognizes discriminatively the information such as the ID number or the like, whereon the processing for settlement of the toll charge/payment is automatically executed for the bank account of the owner of the motor vehicle passing through the toll gate TG. 
As will now be appreciated from the foregoing description, with the conventional vehicle-onboard ETC apparatus, the ETC information transferred between the ETC apparatus of the motor vehicle and the ETC equipment of the toll gate station through radio-wave communication can be recognized by the driver by viewing the images generated on the display device or hearing the voice information outputted from the speaker.  This however means that the driver's attention tends to be attracted to the information concerning the transactions with the toll gate station, depriving the driver of his or her attention in the forward direction in which the motor vehicle is running, which of course is undesirable particularly when the motor vehicles have to pass through the toll gate station of the toll road at a relatively high speed in order to avoid traffic jam. 
The coverage of the radio wave emitted from the toll collection antenna (i.e., service area of the toll gate station over which communication of the ETC information is possible) is set narrow on the order of 3 m×4 m or so with a view to avoiding interference with the service area of other toll gate station.  As a consequence, there may arise such situation that the motor vehicle passes through the service area without conducting communication with the toll gate station for a sufficient time although it depends on the speed of the motor vehicle entering the service area, thus giving rise to a problem that the ETC information can not be sufficiently exchanged or transferred between the motor vehicle and the toll gate station. 
On the other hand, when the speed of the motor vehicle entering the service area is low due to traffic jam, a lot of time will be taken for the motor vehicle to pass through a communication-disabled region which may take place due to change of the field strength or intensity in the vicinity of the boundary between the communication-capable area and the communication-incapable area under the influence of ratio-wave disturbance phenomenon such as Rayleigh fading.  When the communication-disabled period continues long and when the number of times the communication is retried exceeds a predetermined value, then the communication between the ETC equipment of the toll gate station and the vehicle-onboard ETC apparatus can no more be restored.  Moreover, the facility of the motor vehicle for the ETC information communication is invalidated in succession, to another great disadvantage.