FR2864626A1 - Motor vehicle`s speed measuring method, involves detecting moments at which vehicle crosses parallel planes, from sound measured by transducers, and establishing speed from time interval elapsed between moments and distance between planes - Google Patents

Abstract

The method involves measuring a sound such that two acoustic transducers generate acoustic signals representing sound waves generated by a vehicle during its movement. Two passage moments at which the vehicle crosses respective parallel median planes is detected from the sound measured by the transducers. Speed of the vehicle is established from a time interval elapsed between the moments and a distance separating the planes. Independent claims are also included for the following: (A) an information recording medium having instruction for executing a method of measuring speed of a vehicle (B) a system for measuring speed of a vehicle.

Description

The invention relates to a method and a system for measuring the speed of a

  vehicle, and a recording medium for their implementation.

  More specifically, the invention relates to a method for measuring the speed of a vehicle using acoustic transducers aligned parallel to the same common axis, this method comprising a step of measuring the noise so that each transducer generates an acoustic signal representative of the sound wave generated by the vehicle during its movement.

  Such a method is, for example, described in the French patent application FR 2 812 402. The method described in FR 2 812 402 consists in calculating an intercorrelation function between two acoustic signals measured respectively by a first and a second acoustic transducer. . The intercorrelation function makes it possible to estimate the delay time between the signal measured by the first transducer and that measured by the second transducer. This delay time is representative of the angular position of the vehicle relative to the axis passing through the two acoustic transducers. More specifically, this delay time represents the value of the angle between a first straight line passing through the middle of the two transducers and the position of the vehicle and a second straight line perpendicular to the axis on which these two acoustic transducers are aligned. Therefore, to calculate the linear speed of the vehicle from the evolution of its angular position it is necessary to know the distance D between the vehicle lane of the axis on which are aligned the two transducers. The speed thus measured can only be obtained accurately if, on the one hand, the alignment axis of the two transducers is strictly parallel to the traffic lane and, on the other hand, the distance D is accurately known. .

  During installation, these two conditions make it necessary to position the two transducers with respect to the traffic lane with great precision and to measure the distance D. In practice, this complicates the installation of the transducers.

  The invention aims to remedy this drawback by proposing a method for measuring the speed of a vehicle using acoustic transducers that can be implemented without the distance separating the travel path of the acoustic transducers being known.

  The subject of the invention is therefore a method for measuring the speed as described above, characterized in that: at least three acoustic transducers are used, these transducers being grouped together to form at least a first and a second pair of transducers, the first and second pairs of transducers respectively defining first and second median planes perpendicular to the common axis, the first median plane being located midway between the two transducers of the first pair, and the second median plane being located half-way between the two transducers of the second pair, and - in that the method comprises: - a step of detecting a first and a second instants of passage at which the vehicle passes respectively the first and the second second median planes from the noise measured respectively by the first and second pairs of transducers, and - a step of establishing the speed of the vee at least from the interval of time between the first and second passage times and the distance between the corresponding first and second median planes.

  Since the median planes used in the above method are parallel, the time interval between the moment the vehicle crosses the foreground and the point where the vehicle crosses the second plane is independent of the distance separating the vehicle. automotive acoustic transducers. Therefore, this method can be implemented without knowing the distance D separating the taxiway of the vehicle acoustic transducers.

  According to other additional features of the measuring method according to the invention, it is characterized in that: it comprises a step of multiplication between them of the two acoustic signals measured by each pair of transducers so as to obtain a signal representative of the noise power in the median plane defined by this pair, and in that the detection step further comprises identifying the instant at which each plane signal is maximum, those times when the plane signals are maximum corresponding to the moments of passage of the vehicle in the median planes; it comprises a step of filtering the acoustic signals used during the detection step to eliminate from these signals the frequency components having a wavelength greater than four times the distance between two acoustic transducers; the step of establishing the speed of the vehicle comprises a step of establishing a regression line whose slope is proportional or inversely proportional to the speed of the vehicle; it comprises a step of identifying one or more pure sinusoids in one or more of the acoustic signals measured so as to detect the use of a buzzer; it comprises a step of calculating the power of one or more of the measured acoustic signals and a step of comparing the or each calculated power with a preset threshold for detecting the noise of a collision of the vehicle against an obstacle.

  it comprises a step of incrementing a vehicle counter when the speed of this vehicle is measured.

  The invention also relates to an information recording medium, characterized in that it comprises instructions for the implementation of a method for measuring the speed of a vehicle according to the invention, when these instructions are executed by an electronic calculator.

  The subject of the invention is also a system for measuring the speed of a vehicle capable of executing a method for measuring this speed according to any one of the preceding claims, this system comprising: acoustic transducers aligned in parallel with an axis 30 common to measure the noise generated by the vehicle when passing in front of these transducers, and - a calculator able to process the acoustic signals measured to deduce the speed of the motor vehicle, characterized in: - in that the system comprises at least three said transducers being grouped to form at least first and second pairs of transducers, the first and second pairs of transducers respectively defining first and second median planes perpendicular to the common axis, the first midplane being located midway between distance between the two transducers of the first pair, and the second median plane being located halfway between the two transducers of the second pair, and - in that the computer is able to execute: - a step of detecting a first and a second instants of passage at which the vehicle crosses respectively the first and the second median planes from the noise measured respectively by the first and second pairs of transducers, and - a step of setting the vehicle speed at least from the time interval elapsed between the first and the second instants and the distance separating the corresponding first and second median planes.

  According to another additional feature of the measuring system according to the invention, it is characterized in that the transducers are distributed at regular intervals along the common axis.

  The invention will be better understood on reading the following description given solely by way of example and with reference to the drawings in which: FIG. 1 is a schematic illustration of the architecture of a measurement system the speed of a vehicle according to the invention, - Figure 2 is a flowchart of a speed measuring method of a vehicle according to the invention, and - Figure 3 is a graph representing the distance traveled by a vehicle according to measured passage times.

  FIG. 1 represents a system 2 for measuring the speed of a motor vehicle 4 traveling on a traffic lane 6.

  The vehicle 4 is a conventional vehicle that when traveling on a traffic lane continuously generates a sound wave.

  The system 2 comprises at least three acoustic transducers connected to a unit 10 for acquiring and processing the acoustic signals measured by these transducers.

  Here, all the acoustic transducers of the system 2 are aligned on a common axis 12 substantially parallel to the axis of the channel 6. In the particular embodiment described here, these acoustic transducers are arranged at regular intervals on this common axis 12 The distance separating two consecutive acoustic transducers is noted here 2e.

  As an illustration, the system 2 comprises five acoustic transducers referenced from left to right by the symbols CI to C5.

  Preferably, the distance e is chosen as large as possible so as to improve the resolution of the system 2. However, to limit the size of the system 2, this distance e is here chosen to be between 5 and 15 centimeters.

  Each transducer C; is able to measure the sound wave generated by the vehicle 4 during its displacement and to transmit an acoustic signal if (t) corresponding to the unit 10 for treatment.

  In FIG. 1, seven dashed lines referenced from left to right by the symbols PI to P, each represent a median plane between two transducers C; of a pair of acoustic transducers. More specifically, each pair of transducers C; defines the position of a median plane perpendicular to the axis 12 and located midway between these two transducers C. Here, since the transducers C; are arranged at regular intervals along the axis 12, the same median plane may correspond to several pairs of different transducers. For example, the median plane P4 corresponds to that defined by the pair of transducers C2, C4 and the pair of transducers CI, C5.

  The distance separating two consecutive parallel median planes is equal to the distance e.

  The unit 10 is able to acquire the different acoustic signals measured by the transducers C; and to process these signals so as to furthermore measure the speed of the vehicle 4. For this purpose, the unit 10 comprises an analog-to-digital converter 20 suitable for converting the analog signals s; (t) into digital signals. Typically, the sampling frequency of this converter 20 is of the order of 6000 samples per second and the samples of each acoustic signal if (t) are measured at the same times so as not to introduce a phase difference between these different acoustic signals. .

  Then, the unit 10 comprises a high pass filter 22 to eliminate digitized acoustic signals, the frequency components that are insensitive to the direction of the sound wave generated by the vehicle 4. For this purpose, the cutoff frequency fc of this filter is chosen so as to eliminate the frequency components of which a quarter of the wavelength is greater than the distance 2e separating two consecutive transducers. Thus, the cut-off frequency fc is chosen so as to respect the following relation: where c is the speed of sound in the air.

  Here, the cutoff frequency fc is equal to c / 8e.

  Indeed, the frequency components whose wavelength is greater than four times the distance 2e between transducers always arrive with a phase shift less than rr / 2 regardless of the position of the vehicle. They therefore correspond to a product P, (t), explained below, always positive and thus form a background noise which deteriorates the accuracy of the speed measurement if it is not eliminated.

  The acoustic signal if (t) after being filtered by the filter 22 is denoted sh; (t).

  A multiplier 24 is provided in the unit 10 to multiply between them the signals sh; (t) from the two acoustic transducers Ci and Ck of the same pair of acoustic transducers. At the output, the multiplier 24 thus delivers n (n-1) / 2 signals, here called plane signals PC (t), obtained thanks to the following relation: Pi (t) = sh; (t) .shk (t) n is the number of transducers. The index I corresponds to a number identifying the pair of transducers from which the signal PO (t) is constructed.

  A low pass filter 26 is connected to the output of the multiplier 24 so as to smooth the plane signals P, (t) obtained. This filter 26 is capable of outputting n (n-1) / 2 filtered plane signals Q, (t) obtained by calculating the rolling average of each plane signal P, (t) over a time interval T. is for example chosen equal to 5 to 20 times the sampling period. The signal QI (t) is for example obtained using the following relation: Q1 (t) TE aPi (t + a) where a is a variable which varies in incremental steps equal to the sampling period of T / 2 to T / 2.

  Finally, the unit 10 comprises an electronic calculator 28 able to process the signals Q1 (t) so as to further determine the speed of the vehicle 4 as well as other information, as described with reference to FIG. the computer 28 is able to execute the method of FIG. 2.

  The computer 28 is also able to control various peripheral devices associated with the unit 10 such as, for example, a camera 40 and a radio transmitter / receiver 42.

  The computer 28 is, for example, a programmable conventional calculator capable of executing instructions stored in a memory 44. Here, the memory 44 includes the instructions necessary for the execution of the method of FIG. 2.

  The camera 40 is able to photograph the vehicle 4 and in particular its license plate during its passage.

  The radio transmitter / receiver 42 is intended to transmit, via a wireless network, information acquired and processed by the unit 10 to other equipment. By way of illustration, here, the transmitter / receiver 42 is able to directly transmit control instructions to a red light 48 installed near the unit 10. The transmitter / receiver 42 is also able, here, to transmit the information acquired and processed to a center 52 for road traffic management.

  The operation of the system 2 will now be described with reference to FIG.

  In operation, each transducer C; continuously measuring, during a step 60, the sound waves coming from the channel 6 and transmitting the acoustic signal s; (t) corresponding to the unit 10.

  The converter 20 digitizes, during a step 62, all the acoustic signals if (t) transmitted by the transducers C. The digitized acoustic signals are filtered, during a step 64, by the filter 22 and the signals obtained sh; (t) are transmitted to the multiplier 24.

  In a step 66, the multiplier 24 continuously calculates for each pair of transducers C; the corresponding plane signal Pl (t). Here, the multiplier 24 thus continuously establishes ten signals P1 (t) since there are ten pairs of possible transducers in the system 2.

  Each signal PO (t) is then filtered, in a step 68, by the low-pass filter 26 so as to obtain the signal QC (t).

  Steps 60 to 68 are performed continuously and continuously for each measured signal s (t).

  From each signal QC (t) the computer 28 detects during a step 70, the moment of passage tpl of the vehicle 4 in the median plane defined by the pair of transducers corresponding to the index I. It is recalled that the index I corresponds to the number of a pair of transducers and not to the index of a median plane. The moment of passage in a median plane defined by a pair of transducers C; corresponds to the moment when the plan signal has a peak power or high energy. Indeed, when the vehicle 4 crosses a median plane, the sound wave that it generates at this moment there takes exactly the same time to reach the two transducers C; of the pair of transducers that defines this median plane. Thus, the plan signal has a maximum at this time. For example, to determine the time of passage tp, the property is used according to which the value of this moment of passage tpl is the value which minimizes the dispersion of the values of the signal Q, (t) over a time window W around this moment. The value of the time tpl is, for example, given by the following relation: tPt = aa.Qi (t + a) 1 / til aQi (t + a)] where: 2864626 9 - a is a variable varying by not incremental equal to the sampling period over the interval [W / 2; W / 2], W being the amplitude of the considered time window, and - t is the time.

  The index I of the instant tp ,, is equal to the index I of the signal Q, (t) corresponding and therefore corresponds to a pair of transducers C; known.

  The value W for the time window is chosen according to the average speed of the vehicles observed on the track 6.

  It should be noted that since in this embodiment, several pairs of transducers Ci can define the same median plane, several instants of passage in the same median plane are calculated from different signals Q, (t). It is therefore understood that there is a redundancy of the measured passage times, which increases the robustness of the method described here with respect to parasitic noise such as, for example, vis-à-vis the noise generated by reflective fixed elements the sound wave of the vehicle 4.

  Then, the calculator 28 establishes, during a step 72, for each instant tpi a confidence index Sb ,. This confidence index is established, for example, using the following relation: Sbi = QI (t + 0) 1 / BI where Bi corresponds to the power of a residual noise.

  The residual noise B is the noise measured by the pair of index transducers 1 between two motor vehicle passages on the channel 6. Typically, the power of the residual noise is estimated by averaging the minimums of the signals QC (t). over a period longer than the time window W. For example this period will be at least 5W.

  The calculator 28 then establishes, during a step 74, the speed of the vehicle 4 from the times of passage tpi and the distances separating the different median planes.

  By way of illustration, in the graph of FIG. 3, the round points represent the position of the vehicle at the instant tpi. This position is deduced from the index I. Indeed, each index I is associated with a pair of transducers C; and therefore corresponds to a known median plane. Here, the position of each median plane is measured from the position of the IC transducer. Therefore, the median plane PK is at the distance k.e of the transducer CI.

  If the measurements and calculations were perfect, the round points would be aligned on the same straight line whose slope is equal to the speed of the vehicle 4. However, because of measurement errors and various parasitic phenomena, the round points are not, in reality, never perfectly aligned. Therefore, here, to determine the speed of the vehicle 4, the calculator 28 determines, during an operation 76, the regression line passing closer to these round points. This regression line corresponds to the square points in FIG. 3. This line is determined by a known method such as, for example, the least squares method. The slope of this line then corresponds to the speed measured by the system 2. Here, the measured speed v is established using the following formula: v / e = 12E kk.tpk / [(P -1) .P . (P + 1)] 6.E k tpk / [(P -1) .P] where: - P is the number of planes, - k is an index varying from 1 to P, and - tpk is the average of the instants tp, measured for the same plane Pk.

  It will be noted that since the distance between the planes is measured by taking as a reference point the position of the transducer IC, the speed is positive when the vehicle 4 moves from the transducer IC to the transducer C6 and negative otherwise.

  The calculator 28 also calculates, during a step 80, a confidence index Sb for the speed calculated during step 74. This confidence index Sb is calculated using the following relation: Sb = E l Sbl / P where I is an index varying from 1 to P. As the time window W is moved, the value of this confidence index Sb reaches a maximum before decreasing.

  During a step 82, the computer stores the speed of the vehicle 4 calculated, for example, in step 74 only when this confidence index Sb is maximum. During this step 82, the computer also memorizes the passage of this vehicle and, for example, increments a vehicle counter. Then, during a step 84, the computer compares the stored speed with a predefined speed threshold S. If the memorized speed exceeds the threshold Sv then, the computer controls, during a step 86, the camera 40 so as to take a picture of the vehicle 4.

  At regular intervals, the computer 28 transmits, during a step 88, the various information stored at the management center 52 via the transmitter 1 receiver 42.

  This calculator 28 also uses, if necessary, the stored information to control the activation of the red light 48, during a step 90.

  Steps 70 to 84 are performed continuously and continuously.

  In parallel with these steps, the computer executes a step 100 of detecting an accident such as, for example, the collision of the vehicle 4 against an obstacle. For this purpose, the computer 28 filters, during an operation 102 one or more signals s; (t) using a high pass filter for passing the shock wave. Next, the instantaneous power of the or each filtered acoustic signal is calculated, during an operation 104, and then compared, during an operation 106, with the average Ma of the instantaneous power of the same signal over at least one second. If the instantaneous power of one of the signals is greater than at least two times Ma, then the computer carries out an operation 108 for indicating this event. Typically, during the operation 108, the information that an accident has occurred is transmitted via the transceiver 42 to the management center 52.

  Also in parallel with step 100, the computer 28 performs a step 110 of identifying one or more pure sinusoids in the signals s; (t) to detect a buzzer. For this purpose, the calculator 28 calculates, during an operation 112, the correlation, for a time interval greater than one second, between one or more of the acoustic signals if (t) and N pure sinusoids whose frequencies are regularly distributed. for example between 200 and 400 Hertz. For this purpose, the computer 28 uses, for example, the following relationship: P (con,) _ t if (t). sin (comt) l + t if (t). COS (Wmt)} where: - w m is the pulsation of the pure sinusoid of index m, - s, (t) is the acoustic signal of the transducer of index i - P; (wm) is the value of the correlation between the pulsating sinusoid w m and the signal sr.

  Then, each signal P; (wm) is compared during an operation 114 to a predetermined threshold S. If the acoustic signal is very strongly correlated with one of the N pure sinusoids, then the value of the signal P; (Wm) is greater than the threshold SZ and the computer carries out an operation 116 of discrimination between a buzzer of a priority vehicle and that of any other vehicle. Otherwise, the process returns to operation 112.

  During the operation 116, the calculator 28 searches in a history the occurrence of a sounding detection persistent for S consecutive seconds corresponding to a first pulse Wi alternating with a second pulse W2. If so, the computer transmits during an operation 118 to the management center 52 information that the audible warning of a priority vehicle has been detected. In the opposite case, it is simply a horn of a vehicle and the computer 28 then proceeds to a step 120 during which, for example, the information that a buzzer of a motor vehicle has been detected is stored and transmitted to the management center 52.

  Since the median planes defined by the transducers C; are parallel to each other, it is not necessary using the method of Figure 2 to know the distance between the transducers C; and track 6 for measuring the linear speed of the vehicle 4.

  In addition, unlike the known systems, it is not necessary for the axis 12 to be strictly parallel to the axis of the channel 6. In fact, a slight misalignment has only a small effect on the speed measured. The table below gives some examples of the influence of an alignment error on the speed measurement. The alignment error is here measured between the axis 12 and the axis of the channel 6.

  Alignment error Measurement error on speed 0.4% 10 1.5% 15 3, 4% System 2 also has the advantage of being insensitive to the height at which it is placed with respect to track 6 .

  System 2 has been described in the particular case where only a line of transducers C; is used in order to reduce the bulk. In a variant, the system comprises several lines of transducers C. In this variant, the method of FIG. 2 is executed for each line of transducers C. Transducers C; have been described here as being aligned. However, this is not necessary to define several median planes parallel to each other. For example, several pairs of transducers C; can be shifted in height relative to each other while defining each of the median planes parallel to each other. For such an arrangement, the method of FIG. 2 is not modified.

  The system 2 has been described in the particular case where it has only five transducers. However, in a variant, so as to increase the resolution and the accuracy of the measurement of the speed and to limit the consequences of parasitic noise, the number of transducers C; is chosen greater than 10, 20 or 30. Indeed, the higher the number of transducers, the greater is the number of instants tpi passages calculated and the greater the precision and insensitivity to noise system noise. It will be noted in particular that the number of calculated instants of passage increases exponentially with the number of transducers C; and not in a linear fashion. It is therefore possible to calculate a large number of instants of passages using a reduced number of acoustic transducers.

  The system 2 has been described in the particular case where the transducers are distributed at regular intervals along the axis 12. This simplifies the calculations. However, alternatively, other distributions of the acoustic transducers C; along the axis 12 are possible. It is then necessary to adapt the calculation formulas exposed here to the arrangement of the acoustic transducers chosen.

  The system has been described here in the particular case where it includes a low-pass filter 26. However, alternatively, this low-pass filter 26 is removed and the corresponding step 68 of the method of Figure 2 is also deleted. The other steps remain unchanged.

  Alternatively, to suppress the echoes due to the reflection of the sound wave generated by the vehicle on fixed elements, the unit 10 comprises an additional filter placed between the filter 22 and the multiplier 24. This additional filter is an adaptive filter . In this variant, the method of FIG. 2 comprises an additional step, performed by the adaptive filter, inserted between steps 64 and 66. In this additional step, the signal sh; (t) is filtered by the adaptive filter which sets for this purpose, for example, the following relation: uh; (t) = sh; (t) -E, ak.uhl (t - k.â) where: Uh; (t) is the signal delivered by the adaptive filter at the input of the multiplier 24, - ak are coefficients which represent the proportion of reflected sound waves arriving with a delay equal to k times the sampling period a, and k is an index which varies between 1 and dmax, where d max is the maximum distance between the transducers and the parasitic reflective elements to be taken into account.

  The coefficients ak are calculated by conventional methods that minimize, for example, the quadratic error and using algorithms such as those of Levinson and Durbin, least squares or the stochastic gradient.

  The other steps of the method of Fig. 2 remain unchanged except that it is the signal uh; (t) which is processed instead of the signal sh; (t).

  Alternatively, in step 100, the computer 28 uses a fast Fourier transform to identify one or more pure sinusoids in the signals s; (t).

  The system has been described in the particular case of measuring the speed of a motor vehicle. However, the system 2 can be applied to the measurement of the speed of any object generating a sound wave during its displacement.

Claims (10)

  A method for measuring the speed of a vehicle continuously generating a sound wave during its displacement, this measurement being carried out using receiving acoustic transducers aligned in parallel with the same common axis, this method comprising a step (60). ) of the noise so that each transducer generates an acoustic signal representative of the sound wave generated by the vehicle during its displacement, characterized in that at least three acoustic transducers are used, these transducers being grouped together to form at least a first and a second pair of transducers, the first and second pairs of transducers respectively defining first and second median planes perpendicular to the common axis, the first midplane being located midway between the two transducers of the first pair, and the second median plane being located midway between the two transducers of the second pair, and - in that the method comprises: - a step (70) of detecting a first and a second passage instants at which the vehicle crosses respectively the first and the second median planes from the measured noise respectively by the first and second pairs of transducers, and - a step (74) of establishing the vehicle speed at least from the time interval elapsed between the first and second times of passage and the distance between the corresponding first and second median planes.   2. Method according to claim 1, characterized in that it comprises a step of multiplying between them the two acoustic signals measured by each pair of transducers so as to obtain a plane signal representative of the power of the noise in the defined median plane by this pair, and in that the detection step further comprises identifying the instant at which each plan signal is maximum, those instants where the plan signals are maximum corresponding to the instants of passage of the vehicle in the median planes.   3. Method according to any one of the preceding claims, characterized in that it comprises a step (69) for filtering the acoustic signals used during the detection step (70) to eliminate from these signals the frequency components having a wavelength greater than four times the distance between two acoustic transducers.   4. Method according to any one of the preceding claims, characterized in that the step (74) for establishing the speed of the vehicle comprises an operation (76) for establishing a regression line whose slope is proportional. or inversely proportional to the speed of the vehicle.   5. Method according to any one of the preceding claims, characterized in that it comprises a step (110) for identifying one or more pure sinusoids in one or more of the acoustic signals measured so as to detect the use of 'a buzzer.   6. Method according to any one of the preceding claims, characterized in that it comprises a step (104) for calculating the power of one or more of the measured acoustic signals and a step (106) for comparing the each calculated power to a predetermined threshold to detect the noise of a collision of the vehicle against an obstacle.   7. Method according to any one of the preceding claims, characterized in that it comprises a step (82) of incrementation of a vehicle meter when the speed of the vehicle is measured.   8. Support (44) for recording information, characterized in that it comprises instructions for the execution of a measurement method according to any one of the preceding claims, when these instructions are executed by a calculator electronic.   9. A system for measuring the speed of a vehicle capable of performing a method for measuring this speed according to any one of the preceding claims, this system comprising: acoustic transducers (C 1) aligned parallel to a common axis; (12) for measuring the noise generated by the vehicle when passing in front of these transducers, and - a computer (28) able to process the acoustic signals measured to deduce the speed of the motor vehicle, characterized in that: - the system comprises at least three transducers these transducers being grouped together to form at least a first and a second pair of transducers, the first and second pairs of transducers respectively defining first and second median planes perpendicular to the common axis, the first median plane being located halfway between the two transducers of the first pair, and the second midplane being mid-way between the two transducers of the second pair, and - in that the computer (28) is capable of executing: a step (70) of detecting a first and a second passage time at which the vehicle passes respectively the first and second median planes from the noise measured respectively by the first and second pairs of transducers, and - a step (74) of establishing the vehicle speed at least from the time interval elapsed between the first and second instants and the distance separating the corresponding first and second median planes.   10. System according to claim 9, characterized in that the transducers (4) are distributed at regular intervals along the common axis (12).