GB2046914A - Method of and apparatus for detecting and storing data concerning the movement of a vehicle - Google Patents

Abstract

A method and apparatus for detecting and storing data concerning the movement of a vehicle, in particular for the purpose of reconstructing the speed conditions of the vehicle prior to a certain instant of time or stopping point, for example an accident wherein at least two of the variables time, speed and distance are continuously detected and stored as measurement data and when the store is full the data which is oldest in time is selectively removed and fresh measurement data is entered in its place. Stores may be magnetic tapes, discs or FIFO shift registers. A clock pulse generator may generate clock pulses in dependence upon the distance travelled or the speed. The apparatus is so constructed that interference with the time detection unit e.g. a clock is rendered recognisable or impossible.

Description

SPECIFICATION Method of and apparatus for detecting and storing data concerning the movement of a vehicle The present invention relates to a method of and apparatus for detecting and storing data concerning the movement of a vehicle.

The invention relates particularly to a method of detecting, storing and under certain circumstances evaluating travelling data of vehicles, in particular for the purpose of reconstructing the speed conditions prior to an accident, wherein at least two of the variables time, speed and path are detected and stored as measurement data.

It is known to equip vehicles with travel recorders which record the course of travel on a storage medium, preferably a circular paper disc. The relationship between the measurement data and time is represented by the purely time-dependent rotary movement of the paper disc which serves as a store. Usually the speed is recorded; frequently a so-called vibration marker is additionally recorded which shows whether the vehicle has stood still or was in motion. Recording is effected by means of a writing pen.

It is a disadvantage of the known arrangement described above that the time scale must be relatively large in order that the consumption of storage sheet is not excessive.

As a result however the time resolution and thus the accuracy of evaluation is very limited.

For example, the speed conditions prior to an accident can be reconstructed only insofar as the maximum speed prior to the accident can be determined. However, it is not possible to determine with any certainty, even by integration of the recorded speed-time relationship, the distance from the location of the accident at which the vehicle had any given speed. The disc driving speed must not be increased because then too frequent a change of disc is necessary. Admittedly replacement of the disc by a paper tape would permit without difficulty an improvement of the accuracy without the need for too frequent a change of the data carrrier, but the expenditure for the data carrier and also for the winding and unwinding device therefor would be too high.It must be taken into account in this context that an accident is an event which is very rare in relation to the useful life of a vehicle and only a tiny fraction of the data detected during the life of a vehicle is required for the reconstruction of the vehicle's speed prior to an accident. For this reason the known method and apparatus serve more the purpose of permitting the observation of regulations by law to be monitored, such as regulations in respect of maximum speeds, driver working periods, and the like.

It is an object of the present invention to provide a method and apparatus which permit the speed of a vehicle prior to an accident to be reconstructed with a very much higher accuracy than was the case heretofore, without the expenditure for storage of detected data increasing steeply.

According to the present invention, there is provided a method of detecting and storing data concerning the movement of a vehicle to enable the data relating to a certain instant of time to be recovered, in which at least two of the variables time, speed and distance are measured to provide measurement data and the measurement data is stored, characterised in that the measurement data is continuously entered into a store the contents of which can be cancelled, when the store is full the stored data which is oldest in time is selectively removed, and fresh measurement data is entered into the store in place of the removed data.

The method according to the invention is based on the recognition that only a negligibly small fraction of the almost unlimited flood of measurement data produced during the life of a vehicle is required to reconstruct an accident. At the same time the detection and storage of all relevant measurement data is imperative. Surprisingly the invention has made it possible to always preserve the up-todata measurement data which alone are of interest. The employment of a store which can be cancelled and which has re-usable storage locations and into which continuous data entry is effected, and for which purposes storage locations are cleared, means that the desired information is always maintained despite a very limited total storage capacity. This is because the measurement data which is oldest in time are cancelled.This cancellation is completely free of objection because;the chronological distance from a possible accident of cancelled data is with certainty so large that it cannot be relevant to the events leading up to an accident and therefore is not required for the reconstruction of the vehicle speed conditions prior to the accident.

The method according to the invention is based on the recognition that measurement data is retained in the storage only if it is required for the reconstruction of the speed conditions prior to an accident in order to permit a relation to be obtained between the speed of the vehicle and the events during the accident or the cause of the accident. Obviously this reconstruction of the speed conditions is of interest not only in the case of accidents, but for example it is of interest also during speed controls performed by the police as the driver can prove the actual speed at which he travelled at a given instant in time, for example in the case of faulty measurements which can never be excluded. For this purpose, for example, he can switch the apparatus to a different store and thereby preserve the store content under consideration.A particularly precise reconstruction of events is possible if time data is stored which embraces also the data in addition to the clock time.

Instead of employment of an additional store, alternatively the arrangement may be switched off, in order to retain in the store data generated during the time interval of interest even when the vehicle continues its travel.

The preservation of the store content, or the switch-over of the store, becomes particularly simple when in accordance with a preferred embodiment of the method the measurement data is stored in a magnetic store, such as an endless tape or a disc. Insertion of a fresh disc or a fresh endless tape is then sufficient to ensure that the store content is preserved.

However, it is disadvantageous when magnetic stores of this type are used that they require mechanically moved parts; this requires considerable constructional expenditure and nevertheless the life of the arrangement is limited because of wear. In preferred realisations of the invention the measuring values are therefore stored in a digital store, in particular in FIFO shift register. In such a shift register the inserted data travels through in one direction and the values inserted first are also the first ones to issue again at the output.

Therefore the data flow within the shift register occurs always in the same direction (first in first out). Thereby it can be arranged in a desired manner that the oldest data disappears from the store when it reaches the output of the shift register, without any particular expenditure for the cancellation or transmission of the data to other storage locations.

For the purpose of evaluating them, the stored measuring values are reproduced, in accordance with a preferred form of performing the method, in a diagram or a table as associated speed-distance values, under certain circumstances with time as parameter. In this case reproduction in the form of a diagram is particularly clear as it permits a direct visual evaluation. In this case the production of the diagram may be effected in a manner known per se by connecting a plotter to the shift register, in which case under certain circumstances a computing operation is connected ahead thereof in order to obtain from the stored measuring values mutually associated speed and distance measurement data.

In the processing of digital measuring values, each step in the processing occurs in a unit of time, in order to obtain the desired manner of working. Usually this unit of time is purely time dependent and is produced by an independent oscillator as a fixed frequency.

In preferred embodiments of the invention, however, the unit of time is derived from the distance on speed data. This has the advantage that the arrangement "stops still" automatically when no distance or speed data is produced, that is to say for example when the vehicle is at standstill. Therefore the sync time transmitter need not be switched off upon standstill of the vehicle, in order to prevent the storage process for continuing and thereby cancelling the useful stored values.

The method according to the invention may be performed in such a manner that the oldest stored data is removed when new measurement data arrives at the store input. In contrast, in preferred embodiments the procedure is such that when the older data is removed from the store, intermediate-in-time data is removed and thereby the time interval between consecutive stored older data is increased. With a predetermined storage volume an enlargement of the detected path length is obtained thereby, wherein merely the time scale of the older data is different from that of the more recent data. However this has no disadvantageous consequences for the evaluation, because the accuracy demands for the older data are lower than for the more recent data.Independently of the fact whether the distance or the speed is detected as the measurement data, the speed of travel is of increasingly less interest as the distance from the standstill location or the location of the accident of the vehicle increases. For example, the most recent speed data may be stored in respect of one meter intervals, older speed data may be stored in respect of five meter intervals, and the oldest speed data may be stored in respect of ten meter intervals or more.

In other embodiments of the method according to the invention, the incoming measurement data is compared with the last stored measurement data, and the storage of the fresh measurement data is performed only if it is different from the last stored measurement data. In this way, too, storage space can be saved, without the useful data content being limited. If speed data is detected and stored, squandering of storage volume is avoided if the speed remains the same for a period. Otherwise data stored when the speed remains the same would not provide any additional useful information. When the data changes, it not only indicates the speed travelled at the respective instant of time. but also it indicates that at the last measuring instant preceding it the speed (within the measuring accuracy) was unchanged relative to the previously stored measurement data. If, in place of the speed, the distance travelled is detected and stored, it is attained in this way that no further storage is effected during standstill of the vehicle and therefore switching off of the sync time is unnecessary, in order to prevent further storage of data and thus cancellation of previously stored data during standstill of the vehicle.

In a preferred realisation form of the method according to the invention, the travel course is recorded by a video camera and the output signals of the video camera are stored in a digital store or on an endless magnetic tape. Such vido cameras of very small constructional magnitude are commercially available and may be accommodated conveniently in the vehicle, for example at the rear-view mirror. In this case the video camera is employed in place of or in addition to the distance or speed sensing means. Its output signals are stored in a digital store or on an endless magnetic tape. The storage cost can be kept relatively low, because it is possible to use a camera having a very coarse matrix, that is to say a relatively low number of lines.

The invention relates also to apparatus for performing the method explained above, namely apparatus for storing and under certain circumstances evaluating travel data of vehicles, in particular for the purpose of reconstructing the speed conditions of the vehicle prior to an accident.

The apparatus is required to provide, with a relatively small storage voiume, a high accuracy. This is obtained due to the fact that the measurement data of interest is detected and stored in a chronologically sufficiently close succession.

According to the invention, there is provided an apparatus for detecting and storing data concerning the movement of a vehicle to enable the data relating to a certain instant of time to be recovered, comprising means for sensing at least two of the variables time, speed and distance and a store for storing measurement data derived from the sensing means, characterised in that means are provided for entering measurement data continously into the store, for detecting when the store is full and thereafter removing stored data which is oldest in time, and for entering fresh measurement data into the store in place of the removed data.

Preferably, the apparatus comprises a clock for providing data and time signals, and in that the store has individually deletable storage locations each of which comprises storage for mutually associated time-distance or timespeed data. The method according to the invention may be performed with such apparatus and the advantageous inherent in the method realised thereby.

In this case the apparatus preferably comprises a digital store and a clock pulse generator as well as under certain circumstances an analogue-digital converter which is connected to the input of the digital store and behind the measurement data source. In this case the digital store may be of any desired type, provided it permits cancellation limited to individual locations of the store content. In this case either individual locations may be selectively deletable or the locations may be deletable individually one following the other.

In a preferred constructional form of the invention a video camera is provided as a distance or speed sensor or in addition to such sensors, and the store is connected thereto. Preferably this camera is provided in addition to a distance or speed sensor, because the camera alone frequently does not permit the speed to be reconstructed with sufficient accuracy upon interrogation of the store by an indicator device. On the other hand the use of a camera has the immeasurable advantage that the entire course of events prior to and up to an instant of time of interest is obtained in great completeness and clarity. At the same time it is of particular advantage that the camera may be so arranged that it permits the events to be detected and stored from the angle of sight and with the field of view of the driver.Thereby foreceful evidential statements may be derived therefrom in accidents as to whether and in how far the driver has acted in a culpable manner. A commerically available CID camera is preferred, that is to say a television camera is used which has no tubes and comprises exclusively solid state circuits, although alternatively a camera with a vidicon may be used.

In a preferred constructional form of the invention which comprises a distance sensor to which a pulse former and under certain circumstances a divider stage is connected, one input of the clock pulse transmitter is connected to the output of the pulse former or the distance sensor or the divider stage and the store is connected to the output of the clock pulse transmitter. This constructional form of the invention in which the store receives measurement data for storage only when the clockpulse transmitter delivers a clockpulse, has the advantage that upon failure of measurement data the clockpulse transmitter also stops and therefore the arrangement does not operate in the absence of measurement data.For example, if the vehicle is at standstill, the store content automatically remains unaltered and a cancellation upon insertion of fresh data can be effected only with the vehicle travelling.

Preferably the store is a FIFO shift register which is particularly suitable for the apparatus according to the invention because it does not require control units and addressing units of any kind. Moreover, the use of such a shift register is of additional advantage for the reason that the arrangement of the measurement data in shift registers reproduces directly the chronological sequence thereof. For example, when a chronologically unchangeable unit of time is used, it is unnecessary that simultaneously with each distance or speed measurement a time measurement is also stored, because the spacing in time of the measurement data is predetermined in any case by the fixed unit of time and it is sufficient that at least one of the stored data is associated with a statement of time.From this the statements of time of all the other measurement data may than be found merely on the basis of their arrangement in the shift register.

In preferred constructional forms of the in vention, the clockpulse transmitter output is connected to at least one divider stage the divided output of which releases the transmission of the stored measuring values from one part of the shift register to a succeeding part of the shift register. Thus the shift register is divided into at least two sections, or at least two shift registers are employed, wherein the first shift register receives all the measurement data and the second shift register, or the succeeding part of the shift register, is supplied with merely every second, third, fourth or fifth or the like measurement data, dependent upon the division ratio of the divider stage.It is possible in this way to store the data with at a larger chronological or spatial spacing in the succeeding shift register and thereby to increase the storable distance or travelling time. In spite of this the accuracy in the actual interesting region, namely in the region of the newest measurement data, does not suffer, since the same are always received and stored with the narrowest chronological and spatial spacing. Only after the expiry of a certain period of time or distance travelled a portion of the data is removed or not transmitted, respectively, and thereby a larger spacing between conseuctive stored data is produced.

In a further embodiment of the apparatus according to the invention the measurement data sensor is connected to a following comparator which when the measuring value 0 is detected blocks a gate which is connected in front of the store and through which the measurement data can be fed to the store.

Storage of the measurement data 0 and consequent cancellation of old data is prevented by means of this comparator. Obviously this comparator may be omitted when the clockpulse transmitter produces its clockpulses dependently upon the distance data, because then the same property is produced already.

Thus the comparator is employed when the clockpulse transmitter does not operate in a travel-dependent manner, that is to say for example it operates in a constant frequency manner. Such a clockpulse transmitter is employed when for example no distance measurement is effected, but a speed measurement.

In a preferred constructional form of the invention, the measurement data sensor is followed by two successive intermediate stores each having one storage location, and the two storage locations are connected to a comparator, the output of which, in the case of dissimilarity of the two stored values, opens to the store a gate in the transmission path for the older one of the two stored values. This arrangement serves for stopping the arrangement when the vehicle is at standstill, if a distance transmiiter is employed and the distance is measured; however, it serves for improved utilisation of the store content without loss of information content when a speed transmitter is employed and the respective speed is measured. Namely in this constructional form of the invention measurement data is stored only if it differs from the previously detected data.Thereby storage of data which is of no interest is avoided, for example during travel at a speed which is constant within the measuring accuracy.

Since the stored data are not directly visible, in contrast to the usual trip recorders, for example an interrogation unit is connectable to the store by means of which the individual storage locations can be interrogated and illustrated, for example by means of a numerical indication. The values read out of the individual storage locations are then either written down in tabular form, or manually processed to form a diagram. Preferably, however, the store content may be interrogated by means of an evaluation unit and this evaluation unit comprises a calculator unit for forming the quotient of distance and time differences of consecutive distance-time data and/ or for forming the product of speed and time difference of conseuctive speed-time data.The desired function, namely the travelled speed dependent upon the distance travelled through may be determined very quickly by means of such an evaluation unit. In this case the calculator unit is preferably followed by a printer or plotter which prints out a table or records a diagram, respectively. Furthermore, in this case the calculator unit is provided with a unit for associating the time value of measurement data with the product, or for associating the distance value of measurement data with the quotient. Thereby the time may be additionally stated as a parameter in the table or in the diagram.

The apparatus according to the invention may be connected to a variety of measurement data sensors. For example sensors may be employed which detect the rotary speed of at least one wheel of the vehicle. In this case a plurality of sensors may be provided and associated with different wheels, wherein a selector circuit is provided which always transmits the data to the apparatus according to the invention which data has been obtained in the same magnitude from at least two wheels It is attained thereby that in the case of braked or rotating wheels data is used -which has been derived from wheels which are not locked. This arrangement is reliably usable when the vehicle is provided with an antilocking device. If, however, the vehicle is equipped with a conventional brake, it cannot be excluded that even with the vehicle moving all wheels are locked, for example during emergency braking, and no signal can be derived from the wheels in spite of the vehicle movement. In order to be able to exclude these cases, sensors may be used which de tect the speed relative to the road surface directly. Such sensors are known.

In place of the usual sensors, such as find use also for detecting the rotary speed of a wheel in the known anti-locking system, anal ogue or digital speed sensors may be used alternatively. For example, the speed of the tachometer driving shaft may be used for this purpose. Thus, as explained, the apparatus according to the invention is not restricted to the employment of certain sensors or sensor types.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, which are in the form of simplified and dia grammatic illustrations omitting details which are not essential for understanding the invention. In the drawings: Figure 1 is a block circuit diagram of appa ratus comprising a distance sensor; Figure 2 is a block circuit diagram of appa ratus comprising an analogue speed transmitter; Figure 3 is a block circuit diagram of an evaluation unit for apparatus according to Fig.

1; and Figure 4 is the block circuit diagram of an evaluation unit for apparatus according to Fig.

2.

The apparatus illustrated in Fig. 1 comprises a pulse transmitter 1 which for example is an inductive pulse transmitter and delivers pulses via a lead 2 to a pulse former 3. The number of pulses delivered is proportional to the distance travelled. The pulse former 3 generates output pulses which have a shape independent of the frequency. The output pulses of the pulse former 3 are applied via a lead 4 to a store 5 which is formed by a FIFO (first in, first out) shift register. The output of the pulse former 3 is also connected to a clock pulse transmitter 6 the output 7 of which synchronises the store 5 and a digital clock 8.

At predetermined intervals the store 5 accepts into its foremost storage space a pulse from the pulse former 3 and time data from the clock 8. In this case the time data comprises the clock time and additionally preferably also the data, in order to anticipate forgery attempts. It will be understood that the clock 8 is accommodated so as to be protected against external interference, either by rendering such interference impossible or by rendering the performance of such interference recognisable.

If the transmitter 1 delivers more than one pulse for every desired unit of distance travelled, an appropriate divider stage is inserted in the lead 4 between the pulse former 3 and the store 5. As a result a pulse will appear at the output of the divider stage and thus at the input of the store 5 for each unit of distance, for example for each meter travelled. A delay member may also be inserted in the lead 4 to ensure that a pulse arrives at the store input when the store 5 has been prepared for receiving it by a clock pulse from the clock pulse transmitter 6.

For example, the store 5 may comprise 50 storage places to permit data concerning 50 meters of travel to be stored. A lead 9 con nects the store 5 to a store 10 which is also preferably constructed as a FIFO shift register and for example comprises 20 storage places.

A divider 11 having the division ratio 1:5 has its input connected to the clock pulse lead 7 and its output connected to the store 10 via a divider clock pulse lead 1 2. Therefore the store 10 accepts only every fifth one of the outputs which issue from the store 5 on lead 9. This is because the store 10 is prepared for the acceptance of data by the clock pulse appearing on the divider clock pulse lead 12, i.e. only every fifth output of the store 5 is accepted. Thus the measurement data contained in the store 10 comprises a spatial interval, referred to a distance moved of 5 meters, in contrast to the interval of one meter between the measurement data in the store 5.

In the same manner, the store 10 is connected by a lead 1 3 to a store 1 5 which is synchronised by a further divider 14 having the division ratio 1:2. The stored measurement data has therefore a relative spacing of 10 meters. If the stores 1 5 and 10 each comprise twenty storage locations, the store 1 5 covers a distance of 200 meters, and the store 10 a distance of 100 meters. Altogether, therefore, the three stores which together have 90 storage locations store a distance of 350 meters, which should always be sufficient in practice. Because of the subdivision in meters, the accuracy for the last 50 meters is so high that it is always sufficient in this region which is generally decisive for determining the course of an accident.

Thus for the first time an accurate and reliable reconstruction of the movement of a vehicle prior to an accident or prior to each stoppage of the vehicle is obtained at economical cost.

This has never been attained before.

In the embodiment illustrated in Fig. 2, a speed sensor 21 detects the instantaneous speed of the vehicle and supplies an analogue signal representative of the detected speed, for example in the form of an electrical potential the magnitude of which is proportional to the speed. The analogue signal is supplied to an analogue-digital converter 23 via a lead 22, the converter 23 in this case being a voltage-digital converter or a voltage-frequency converter. If the analogue-digital converter 23 comprises a voltage-frequency converter, or if the speed sensor 21 delivers a frequency which is proportional to speed, the pulse former 3 with the arrangement connected therebeyond in accordance with Fig. 1 may be connected in the former case to the converter 23 or in the latter case to the sensor 21, respectively.In other cases, however, a digital measurement signal is present at the output of the analogue-digital converter 23, when a clock pulse transmitter 26 has synchronised it by way of a lead 27. The clock pulse transmitter 26 in turn is controlled by a digital clock 28 via lead 24. Furthermore, the clockpulse transmitter 26 controls via lead 29 a comparator 30 the output of which is connected to a gate 31 and the input of which is connected to the output of the converter 23 by a lead 32. The comparator 30 compares the digital signal on the lead 32 to the value 0 and opens the gate 31 if this comparison is negative, that is to say the digital signal appearing on the lead 32 is different from 0.

The gate 31 opens when it receives a corresponding command from the comparator 30, and a clockpulse from the clockpulse lead 29.

When open, the gate 31 connects the output of the converter 23 via a lead 33 and a lead 34 to the input of a first intermediate store 35. The store 35 is synchronised via the clockpulse lead 29 and stores the digital sig nal which appears at its input on lead 34, together with the associated time signal which is supplied simultaneously by the digital clock 28 via a lead 36.

The two inputs of a comparator 37 are connected respectively to the first intermediate store 35 and a second intermediate store 38. The comparator 37 compares the speed measurement data in the stores. If the compared data is the same, a gate 39 controlled by the comparator 37 remains closed. The gate 39 is connected between the output of the first intermediate store 35 and the input of the second intermediate store 38. If, in contrast, the compared data is different, the comparator 37 opens the gate 39 and a further gate 40. The measurement data stored in the second intermediate store 38 is transmitted to a store 25 via lead 41 as soon as the next following clockpulse passes through the opened gate 40. The measurement data stored in the first intermediate store 35 is transmitted through the gate 39 into the second intermediate store 38.The store 25 comprises a FIFO shift register which is connected by a clockpulse lead 42 to the output of the gate circuit 40. Thus, only measurement data which differs from a preceding measurement data reaches the main store 25.

It will be understood that the lead 34 is connected directly to the input of the store 25 if it is not desired to enlarge the distance travelled covered by the store 25 when the vehicle travels at constant speed. Periods during which the speed is constant are of no interest when only the continuously changing speed prior to an accident is to be detected.

In this case stores 10 or 15, respectively, may obviously be connected to the store 25 in the manner known from Fig. 1.

An evaluation unit for evaulating the data stored in the stores 5, 10 and 1 5 comprises for example an interrogation unit 51 (Fig. 3) which is connected to the stores 5, 10 and 15 by a lead 50. The interrogation unit 51 always accepts two consecutive measurement values for distance and time and forms the difference of the two distance values and the difference of the two time values. These differences are transmitted by means of intermediate leads 52 and 53, respectively, to a division circuit 54 which forms the quotient of the two differences. The quotient is fed by a lead 55 to a delivery unit 56 which feeds an indicator, a printer or a plotter by way of an output lead 57.Additionally the difference between the two distance values or one distance value by itself may be transmitted by an additional lead 58 from the interrogation unit 51 to the delivery unit 56.

In the arrangement illustrated in Fig. 4, an interrogation unit 61 is employed which accepts measurement data from the store 25 by means of a lead 60 and transfers the accepted data by means of intermediate leads 62 and 63 to a multiplication circuit 64. The multiplication circuit 64 forms the product of speed and time difference (or speed difference and time difference) and feeds the computed value to a delivery unit 66 by means of a lead 65.

The calculated value which identifies a location or a distance, respectively, together with a time and/or speed statement is fed by means of an output lead 67 to a printer, indicator or plotter (not illustrated). The delivery unit 66 receives the speed and/or time data from the interrogation unit 61 by way of additional leads 68 and 69, respectively.

It will be understood that the arrangements according to Figs. 3 and 4 are operated in a synchronised manner, as is generally known from data processing procedures.

It will be understood that the invention is not limited to the constructional examples illustrated, but that deviations therefrom are possible without the scope of the invention being exceeded. In particular individual features of the invention may be employed by themselves or in a combination of a plurality thereof. Thus, for example, it is possible to detect at constant time intervals the distance travelled during each such interval of time and to store the resultant data together with time data. In this case a distance detector is used which feeds pulses to a counter which is set to O at the start of each time interval. The counter counts the pulses and at the end of the time interval the result of the count is read out, transferred together with the time data into the store, and the counter is reset to O again.

Claims (21)

1. A method of detecting and storing data concerning the movement of a vehicle to enable the data relating to a certain instant of time to be recovered, in which at least two of the variables time, speed and distance are measured to provide measurement data and the measurement data is stored, characterised in that the measurement date is continuously entered into a store the contents of which can be cancelled, when the store is full the stored data which is oldest in time is selectively removed, and fresh measurement data is entered into the store in place of the removed data.

2. A method according to claim 1, characterised in that the measurement data is stored in a magnetic store, for example an endless tape or a disc, or in a digital store, for example a FIFO shift register.

3. A method according to claim 1 or 2, characterised in that the stored measurement data is reproduced as a diagram or a table in the form of speed distance measurement pairs.

4. A method according to any one of claims 1 to 3, in which digital measurement data is processed in synchronism, characterised in that the unit of time is derived from the distance travelled or the speed.

5. A method according to any one of the preceding claims, characterised in that the older measurement data is removed from the store in a manner such that intermediate data is initially removed and thereby the chronological spacing of the stored older values one from the other is increased.

6. A method according to any one of the preceding claims, characterised in that newly derived measurement data is compared with the newest measurement data in the store and the newly derived measurement data is entered into the store only if it differs from the newest measurement data in the store.

7. A method according to any one of claims 1 to 5, characterised in that the course of travel is recorded by video camera and the output signals of the video camera are stored in a digital store or on an endless magnetic tape.

8. An apparatus for detecting and storing data concerning the movement of a vehicle to enable the data relating to a certain instant of time to be recovered, comprising means for sensing at least two of the variables time, speed and distance and a store for storing measurement data derived from the sensing means, characterised in that means are provided for entering measurement data continuously into the store, for detecting when the store is full and thereafter removing the stored data which is oldest in time, and for entering fresh measurement data into the store in place of the removed data.

9. An apparatus according to claim 8, characterised in that it comprises a clock for providing date and time signals, and in that the store has individually deletable storage locations each of which comprises storage for mutually associated time-distance or timespeed data.

1 0. An apparatus according to claim 9, characterised in that it comprises a digital store, for example a FIFO shift register, and a clockpulse generator.

11. An apparatus according to claim 8 or 9, characterised in that a video camera is provided to which the store is connected, the video camera being in addition to or comprising said sensing means.

12. An apparatus according to claim 10, comprising a distance sensor providing pulses related to the distance travelled, characterised in that a pulse former and optionally a divider stage is connected to the output of the distance sensor, and that an input of the clockpulse generator is connected to the output of the pulse former or the distance sensor, the store being connected to the output of the clockpulse generator.

1 3. An apparatus according to claim 12, characterised in that the output of the clockpulse generator is connected to at least one divider stage the sub-divided output of which releases the transfer of the stored measurement data from one part of the store to a subsequent part of the store.

14. An apparatus according to any one of claims 8 to 13, characterised in that the output of the sensing means is connected to a comparator which is arranged on detection of a zero output from the sensing means to block a gate connected to the input of the store, the gate being positioned such that when blocked measurement data cannot be entered into the store.

1 5. An apparatus according to any one of claims 8 to 14, characterised in that the output of the sensing means is connected to two successive intermediate stores each with one storage location, the two storage locations being connected to a comparator the output of which, when its inputs are not the same, opens a gate in the transmission path between the two intermediate stores, and a clockpulse gate to cause the transfer of measurement data from the second intermediate store to the store.

16. An apparatus according to any one of claims 8 to 15, characterised in that the contents of the storage locations of the store may be interrogated by an evaluation unit, the evaluation unit comprising a calculator unit for forming the quotient of distance and time difference between consecutive distance-time measurement data and/or a calculator unit for forming the product of speed and time difference between consecutive speed-time measurement data.

1 7. An apparatus according to claim 16, characterised in that an indicator, a printer or a plotter is connectable to the output of the calculator unit via a delivery unit.

1 8. An apparatus according to claim 17, characterised in that the calculator unit is associated with a unit for the purpose of associating the time value of measurement data with the product of that data, or the distance value of measurement data with the quotient of that data.

1 9. An apparatus according to claim 18, characterised in that the printer or plotter receives the associated values as parameters of the quotient or the product, respectively.

20. A method of detecting and storing data concerning the movement of a vehicle substantially as hereinbefore described with reference to the accompanying drawings.

21. An apparatus for detecting and storing data concerning the movement of a vehicle substantially as hereinbefore described with reference to the accompanying drawings.