This invention relates generally to a method and system for mapping traffic congestion and in particular to a method for improving the accuracy of said mapping when a relatively small percentage of vehicles are used as traffic probes.
Traffic congestion is an increasingly serious problem in cities.
One way to identify and map such congestion in real time (the first step to relieving it) is to identify and map the positions of vehicles that are stopped or moving slowly. Such systems are often referred to traffic control and car navigation in the field of Intelligent Transport Systems (ITS).
PCT publication WO 96/14586, published May 17, 1996, the disclosure of which is incorporated herein by reference, describes, inter alia, a system for mapping of vehicles in congestion.
In one embodiment described in the above publication, a central station broadcasts a call to the vehicles which requests those vehicles which are stopped or which have an average velocity below a given value to broadcast a signal indicative of their position. Such signals are broadcast in slots, each of which represent one bit (yes or no) which relates to a position. Preferably, only one logical slot (that may be represented by more than one actual slot) is used to define the related position. Such signals are then used to generate a map of those regions for which traffic is delayed or otherwise moving slowly.
Preferably, an additional call is sent to the vehicles requesting transmission of indication signals which locate the slow moving or delayed vehicles at a higher resolution than that of the first call. Further calls may be made to allow for transmission of additional information on the status of the vehicles and/or to provide further characterization of the delays.
FIG. 1 shows an initial map generated by such a method, wherein the area represented by a pixel (slot) may, for example, be of the order of 250 to 1000 meters square.
In a preferred embodiment of the invention described, the system then determines, based, inter alia, on the extent of the various contiguous areas which shows positive responses, a smaller area or areas for further study. Preferably, the system broadcasts a further query requesting those vehicles within the smaller area that have at least a given delay (which may be the same as or different from that used in the first query) to broadcast in slots, each representing a position, using a finer resolution, for example, 100 to 250 meters square. Based on the responses to this query a second map such as that shown in FIG. 2 is generated. As can be seen from FIG. 2, various branches of a road network radiating from an intersection, designated as A-F in FIG. 2, can be identified. To improve the usefulness of the display, a background map, such as a road map, may be displayed underlying the displays of any of FIGS. 1, 2 or 4 (described infra).
In the event that additional information relating to the delay is desired, further queries can be made. For example, vehicles which are traveling toward the intersection can be requested to broadcast in a slot which corresponds to the slot they are in and to their velocity toward the intersection. This allows for generation of the graph shown in the lower portion of FIG. 3. Additional slots may be used for the acquisition of other information regarding the responding vehicles. Such information may also be graphed as shown in the upper portion of FIG. 3.
Alternatively or additionally, a map which shows the average velocity of the vehicles toward the intersection as a function of the position can be generated. Such a map is shown in FIG. 4. To acquire the information needed for generating such a map, a number of queries maybe made, each requesting an indication from all vehicles within the area of interest having a given average velocity toward the intersection. The responding vehicles would broadcast their indication signals in slots corresponding to their position. In the map of FIG. 4, the velocity for a given pixel is determined, for example, as the average velocity of the reporting slots for that position. In a display of the map of FIG. 4, the velocity or delay toward the intersection can, for example, be displayed as a gray scale value or as a color, with for example red being the highest velocity or delay and blue being a minimum displayed velocity or delay.
FIG. 5, is a generalized block diagram for a system useful for performing the ITS function described above (and which is also useful for the method of the present invention). FIG. 5 shows a base station or control center 91 having a control center transmitter 79 which broadcasts queries and optionally other signals to vehicles on command from a control computer 80. A remote vehicle 85 (only one vehicle is shown for simplicity) receives the query at a vehicle receiver 84 and transmits commands to a microprocessor 86, based on the queries it receives from the control center.
Microprocessor 86 also receives information regarding the status of the vehicle from one or more information generators and sensors indicated by reference numeral 88. This information may be sent by the sensors on a regular basis or may be sent on command from the microprocessor.
Microprocessor 86 is then operative to command vehicle transmitter 90 to transmit indication signals (or if required, information bearing signals) in a suitable slot in accordance with the information received by microprocessor 86.
The indication (or other) signals are received by a control center receiver 92 and processed by receiver 92 and computer 80. While the operation and construction of the apparatus designated by reference numerals 82, 84, 86 and 90 is straightforward and needs no further explanation, the operation of receiver 92 is usefully expanded upon with reference to FIG. 6.
Generally speaking, the RF signals transmitted by the vehicle may be at any frequency slot. It is to be expected that there will a certain amount of frequency diversity caused by the imperfect accuracy and stability of the vehicle transmitters 90. The slots are wide enough to accommodate this diversity.
Furthermore, often the system utilizes very large numbers of vehicles. If too many of these vehicles (in some particular situation) transmit in the same slot, then the total power transmitted may exceed authorized ERP or dynamic range restrictions. To overcome this problem longer, lower power, pulses may be used for indication signals. Furthermore, if a single receiver is used for receiving signals for all of the slots, internodulation effects may cause spurious signals to appear in slots for which no actual signals have been received.
These problems as well as near-end to far-end transmission problems are substantially solved by the system shown in FIG. 6 and by certain constraints placed on the system which are not shown in FIG. 6. The problems and constraints but are described in the above referenced PCT publication, which should be consulted for a more complete exposition of the method and apparatus shown in FIGS. 1-6.
FIG. 6 shows a receiver system corresponding generally to reference number 92 and to a portion of computer 80 of FIG. 5. While the system of FIG. 6 is suitable for the prior art ITS system of the PCT publication, it is also suitable for use with the ITS system of the present invention.
An antenna 94 (or an array of antennas) receives signals from a plurality of vehicles simultaneously and passes them to a receiver and (optionally) AGC 96. Receiver and AGC 96, which may be of conventional design, downconverts the received signals from RF to IF frequencies. The threshold levels of the detection process may be dependent on the AGC process. Alternatively, the system is operated in a closed loop mode in which the power radiated by the vehicles is dependent on the power received by the base station.
The IF signal is digitized by an A/D system 98 and further down converted by a downconverter 100 to base band. It should be understood that this receiver/downconverter system does not demodulate the incoming signals, but only downconverts the RF so that the same relative frequency differences of the signals is present at the output of converter 100 as in the incoming signals, except that the absolute frequency has been reduced to a low frequency from the RF frequency of the transmitted signal. At these lower frequencies digital systems can be used to analyze and detect the signals.
The low frequency band signals are fed to a series of correlation filters 102 (correlation-type receiver), each of which has a very narrow bandwidth which is related to the correlation time of the correlation filter. Preferably, the frequency bandwidths of adjacent receivers 102 overlap so that the entire bandwidth of each of the slots is covered by one set of receivers 102. The output of each of the receivers is compared to a threshold 104 to determine if a signal is present at the frequency of the respective receiver 102 and the outputs of all of threshold detectors for a given slot are OR gated (or the best signal is selected) to determine if any signal is present in the slot.
In an alternative preferred embodiment of the embodiment disclosed, the strongest output of the set of correlation receivers is chosen for comparison with a threshold, with or without post-detection integration.
Use of a plurality of overlapping narrow band receivers in this manner also reduces the extent of side lobes of the detection process outside the band of the slot. This allows for closer frequency spacing of the slots since interference between slots having adjacent frequencies is reduced.
One set of receivers 102, threshold detectors 104 and an OR gate is provided for each slot and is referred to herein as a slot detector unit. Slot detector units for all of the slots feed a data processor 108 which, together with computer 80 processes the data as described above. When large numbers of vehicles are used in the system and intermodulation becomes a problem (or if AGC is used, and low level signals are lost), it may be necessary to provide a plurality of front end portions of receiver 92 (the front end being defined as receiver 96, converter 98 and converter 100), where each front end receives signals from only a portion of the entire frequency band including one or many of the slots. The function of correlation receivers 102 may also be implemented, for example, using set of DFT""s or an FFT (for CW signals), matched filters or other correlation receiver methods or other optimum receiver methods, depending on the transmitted signals. Other methods such as energy detectors (e.g., radiometers) with or without tracking may also be used, however, they will give less optimal results, because of practical limitations on input band-pass filter designs.
It should be understood that using a plurality of correlation receivers for the same slot may increase the false alarm probability and hence the threshold for positive detection may be adjusted to provide a desired low false alarm probability.
The system may also be provided with a display 110 for displaying the data, and with a user interface 112 which is used by an operator to control both the operation of the system. The user interface also preferably controls the display and the memory to allow for the operator to review the maps previously generated or to generated new displays based on information previously received.
This system works well. However, there is a need for improved accuracy of mapping and/or utilizing a relatively small percentage of participating vehicles.
The present invention provides a system and method for mapping parameters of a traffic congestion, e.g., a road congestion, relative to a focus. Mapping of the road congestion may include determination of an average length of the road congestion over a time interval, motion rate in the road congestion and arrival rate to the road congestion. These parameters, in turn, may be used to determine an expected delay in traveling throughout the road congestion as well as trends (i.e., changes with time) in the road congestion.
The mapping is performed relative to a mapping focus, typically the front end of a road congestion. The mapping focus is preferably identified using the system and method described in the above-mentioned PCT Publication WO 96/14586, or it may be identified using any other suitable method known in the art, for example, by simple polling of predesignated target vehicles. Alternatively, the mapping focus may be provided from an external source, for example, based on reports regarding a problematic intersection or suspect intersections that are to be continuously monitored.
In an embodiment of the present invention, the mapping system constructs snapshots of mapping samples received from a small percentage of predesignated probes, e.g., a small percentage of vehicles equipped with an appropriate receiver and transmitter. The mapping samples are preferably received in response to predefined broadcast queries sent from the mapping system. In an embodiment of the invention, the determination of the average length of a road congestion may be based on a direct approach, obviating the need to estimate discrete lengths of the road congestion, in dynamic conditions that may include variations in the arrival rate of vehicles to the road congestion and the departure rate of vehicles from the congestion over time. In preferred embodiments of the invention, the average motion rate within the road congestion may be determined without the need to increase the bandwidth of the radio spectrum that is used by the probe vehicles. The determination of motion rate in addition to the length of the congestion enables to estimate the expected time delay for a vehicle that is about to enter the road congestion. The method of determining motion rate may isolate reporter vehicles, whereby of reporting may be utilized to improve the method of the invention, e.g., to increase the accuracy at which the length of the road congestion may be determined and/or to provide information about trends in the road congestion, i.e., expansion or contraction of the congestion. Such isolated mapping enables the system, for example, to concatenate non-overlapping segments in the mapping samples and, thereby, to estimate the average arrival rate to the congested road. This technique maybe used in conjunction with an estimated departure rate to provide trends in the average length over time, whereby a preferred path chosen by a vehicle may be selected based on a current time delay as well as on the trend in the road congestion.
The concatenation of non-overlapping segments of mapping samples, in accordance with a preferred embodiment of the invention, may also be useful for estimating the percentage probe vehicles within the road congestion. Based on this estimation, in conjunction with a calculation of the estimated arrival rate and the estimated motion rate, the average length may be determined more accurately. This means that the number of mapping samples can be optimized to provide an accurate determination of the average length of the road congestion based on the parameters described above. Pre-stored data which may be generated by computer simulation of different road congestion conditions may be used in determining the optimum number of mapping samples for average length determination.
In accordance with the present invention, as described herein, concatenated mapping samples may be used to estimate the arrival rate and the percentage of probes. In traffic situations where two or more road congestions are correlated, several such concatenations from several different road congestion may be combined to improve parameter estimation. For example, to estimate the percentage of probes based on Maximum Likelihood estimator for Binomial distribution, the concatenation of more than one concatenated mapping samples from several correlated roads may be used by a statistical estimator. This can be used to improve estimates from short concatenated sample at an early stage of mapping a road congestion.
The motion rate within the road congestion, which may be detected based on two mapping samples, may also ne used for determining a minimum required rate for taking snapshots of mapping samples according to a desired accuracy in determining the average congestion length. In embodiments of the present invention, the level of accuracy in determining average length based on motion rate may be estimated by computer simulation and provided as pres-stored data to determine an appropriate mapping sample rate. As mentioned above, motion rate can be detected by two mapping samples. At an initial stage of a mapping process, when the average arrival rate of vehicles to the road congestion and the probability of an arriving vehicle being a probe cannot be correctly calculated, prior statistical data may be used to initiate the estimation process. Refinement of these initial values may be performed during the sampling process by constructing concatenated segments of non-overlapping mapping sample segments and determining the average arrival rate as well as the percentage of probes, thereby enabling to determine the probability of an arriving vehicle being a probe. A similar approach may be used for determining the number of mapping samples according to the pre-stored data.
The pre-stored data may be based on computer simulation to provide minimum error in the determination of the average length or a modified average length. The modified average length may take into account predetermined parameters, e.g., giving more weight to later mapping samples than to earlier mapping samples or any other desired criteria that may result in a more accurate estimation process. As traffic condition are being mapped, statistical data is collected relating to average arrival rates and distribution of probe vehicles, whereby the system converges to realistic values at relatively early stages of the mapping, even before one would expect to have sufficient mapping samples to estimate these parameters.
In case of traffic light control, the sampling rate may be adjusted in accordance with the rate of change of the traffic lights, e.g., the timing of the green light activations. The timing of light changes may be provided by probe reports according to their reaction time to green light setting calibrated to distance from the traffic light. According to this embodiment, the times may be provided by a report from a probe which has been isolated for the purpose of motion rate estimation and other estimations, as described above. It should be noted that the average road congestion length may be determined with minimal error when the departure rate in each mapping sample is substantially equivalent to the average arrival rate.
When the average departure rate is not equal to the average arrival rate, the departure rate may be artificially adjusted to increase or decrease the length of the mapping samples, thereby to adapt the average departure rate to the average arrival rate. This may assist in determining the length of road congestion. Once the road congestion length is determined, based on the artificial adjustment, a readjustment stage may be applied to compensate for the artificial adjustment. The compensation maybe based on a new weighted average which takes into account a trend in the length of road congestion. At any given time, the average length determination may be based on the latest mapping samples according to the number of mapping samples that will best determine the average length of the road congestion. Successive average length values may fed through an appropriate filter, as is know in the art, to remove large, random changes in value.
The present invention is comprised in a number of improvements on the prior art system which improve the position related accuracy of the system.
As in the prior art system described above, preferred embodiments of the present invention may utilize the position related data transmission system of the above referenced PCT publication. In addition, the present invention may utilize the general structure of the transmitter and receiver as described in that publication and in the Background of the present invention. It should be noted that, because the present invention may utilize a communication platform and related technology similar to those described in the above mentioned publications, many features of the methods, devices and systems described in that publication are also applicable to the present invention.
According to some aspects of some preferred embodiments of the invention, mapping of congestion is based on identification of the starting point of traffic congestion and a determination of a distance of vehicles from a congestion start point or focus. The length of the congestion is estimated from the distance of the vehicle farthest from the congestion whose velocity is below a given velocity, preferably for some minimal time period.
Preferably, the vehicle positions are not determined for individual vehicles. Rather the vehicle report according to their positions, that correspond to a pre-determined sub-area, if they are stopped or if their velocity is below some value.
Preferably, vehicle positions over a time period are combined to form a congestion map. Preferably, the positions that are combined are determined at the same position resolution. Alternatively, they do not.
According to an aspect of some preferred embodiment of the invention, the position of a vehicle is reported based on a distance to a known focus of a congestion.
In a preferred embodiment of the invention, the location of a potential congestion is determined by vehicles that are stopped or moving slowly reporting their positions at a low resolution, for example using a rectangular grid for two dimensional mapping. Once a potential congestion is identified, the position of the vehicles is reported based on their distance from a focus of congestion.
There is thus provided, in accordance with a preferred embodiment of the invention, a method of estimating the position, in an ITS system, of the length of congestion at a focus of a slowdown, the method comprising:
determining the positions of one or more vehicles farthest from the focus as a function of time; and
estimating the length of the congestion based on the function.
Preferably, the position is estimated as the position of a vehicle farthest from the focus.
Preferably, the position is estimated as the position of a vehicle furthest from the focus during a given preceding time period.
There is further provided, in accordance with a preferred embodiment of the invention a method of improving the reliability of an ITS system, comprising:
determining the position of a plurality of vehicles;
determining an indication of a traffic stoppage if more than one vehicle is stopped along a line of vehicles.
Throughout this disclosure, where applicable, the terms and phrases listed below may be defined as follows:
Mapping Focus
A position in a mapped road that defines the front end of the mapping range towards traffic moves usually refers to the front end of a road congestion.
Probe
A vehicle equipped with a transmitter connected to a computer both comprising an intelligent transmitter wherein the computer is provided with timing and positioning peripherals that according to a predetermined procedure can identify congested conditions and motion cycles parameters in a congested road, preferably equipped also with a receiver that enables a mapping system to control the activity of the reports preferably including levels of congestion to be experienced by the probe fore a report, resolution of position report, actual report time of a characteristic value of its position, disabling transmission of probes that are closer than a certain position to the mapping focus and re-enabling the transmission, and according to a predetermined protocol reports will preferably include, but not limited to, one or more of the following:
arrival time to a congested road preferably in a short form such as elapsed time within a mapping cycle, indication on out of mapping range, time related to passing a position such as mapping focus, expected time of green light turn on when a road controlled by traffic light based on predetermined estimate for the delay in response of vehicle to departure according to its position in a waiting line preferably in a short form such as elapsed time within a cycle such as cycle of mapping samples or cycle of traffic light control (several of such different reports can be averaged by the mapping system); reports will preferably use a method of transmission that reports characteristic values by transmitting a signal in slot that best represents its characteristic value.
Characteristic Value of Position
A value that a probe provides according to a predetermined protocol regarding its position, or an indication on its position, such as its distance from a mapping focus along a road or otherwise along a path determined by the protocol.
Mapping System
A system comprising a receiver that receives reports from probes and a computer that constructs mapping samples from received reports and processes the mapping samples to provide characteristics of the road congestion including, but not limited to, one or more of the following reports: departure length from the road congestion between mapping samples and preferably its varying characteristics; arrival length to the congested road between mapping samples and preferably its varying characteristics; estimated length of road congestion; estimated average waiting time in a congested road; trend in the length of the congested road; estimated length of a congested road at a certain time and possibly interpreted values of length in a congested road to number of vehicles based on expected average occupation length of a vehicle such as in a stoppage.
The system will preferably be equipped also with a transmitter that according to a predetermined protocol controls the transmission of probes preferably including, but not limited to, one or more of the following: required criteria of traffic conditions that enables a report; resolution of reports; and preferably the time of the transmission of a characteristic value of a position that relates to earlier time than the transmitted time, disabling transmission of probes that are closer than a certain position to the mapping focus and re-enabling the transmission.
The system will preferably allocate slots to the probes that according to a predetermined protocol slots divide a range of positions or time interval to smaller segments so that each range will be represented by a different slot.
Mapping Sample
One or more time correlated characteristic values of position usually relates to time constraints that provide a snapshot of probe positions in a congested road.
Range Characteristic Value
A value that represents one or more characteristic values such as positions within a range of reports in a mapping sample. Range characteristic values can provide an average of positions or average distance from the mapping focus or a weighting average that consider parameters that affect inaccuracy in reports. Range characteristic value can also average several reported values about a common estimate made by probes, for example, estimate of green light time setting reported by more than one probe in a waiting line according to distance form the traffic light and reaction time to the traffic light. Such reports can use differential updates referred to a common time reference.
Occupation Length of Vehicle
Average segment along a road equivalent to the length between front of one vehicle in front or behind of it.
Mapping Range
A range respective with the mapped part of the road usually covers the congestion starting from the mapping focus.
Using the above terminology, in according with preferred embodiments of the invention, there is thus provided a method of estimating the length of a road congestion, based on probe vehicles reporting characteristic values of their position to a receiver of a mapping system which processes the reports, the method including:
(a) constructing a predetermined number of mapping samples,
(b) determining in each mapping sample a position that relates to the position of a probe relatively far from a mapping focus, preferably a position close to the farthest probe position; and
(c) selecting from the positions determined in step (b) the position which is the farthest from the mapping focus, thereby to determine an indication of the length of the road congestion.
In a preferred embodiment, the position determined in step (b) is the position of the farthest probe from the mapping focus.
According to an embodiment of the invention, after construction of a mapping sample a response is transmitted to the reporters that disables transmitters that did not transmit a report within a preselected range in the constructed mapping sample, to prevent the disabled transmitters from continuing to report. The selected range may include the position of the farthest probe.
Additionally, in accordance with preferred embodiments of the invention, there is provided a method of determining traffic motion and length of road congestion, in a system wherein probes, in response to a predetermined protocol, report characteristic values of their position to a receiver of the mapping system which processes the reports, the method including:
(a) constructing a mapping sample that includes at least one of said reports,
(b) selecting a range of said position characteristic values in which the farthest reporter from a mapping focus is identified in a mapping sample constructed in (a),
(c) transmitting to reporters a response that according to a predetermined procedure disables transmitters that did not transmit a report within the selected range from continuing to report,
(d) receiving further reports and constructing a subsequent mapping sample,
(e) repeating steps (a) to (d) according to a predetermined procedure,
(f) selecting from the ranges selected in step (b) the farthest selected range to be indicative of the length of the road congestion; and
(g) determining motion length toward a mapping focus by calculating a range characteristic value for a range in a mapping sample, subsequent to the first mapping sample, which includes the position characteristic value indicative of the closest position to the mapping focus and calculating the difference between the said range characteristic value and the range characteristic value of a corresponding selected range in an earlier mapping sample.
In an embodiment of the present invention, the range selected in step (b) is substantially the characteristic value of position of the farthest probe.
In an embodiment of the present invention, the indication of the length of the road congestion is determined substantially based on the farthest selected position in the constructed mapping samples.
In an embodiment of the present invention, the resolution of the position reports acquired from the probes is determined according to an occupation length of vehicle in a congested road in different traffic conditions.
In an embodiment of the present invention, at least two different reports from at least one probe are required to determine length of road congestion.
In an embodiment of the present invention, the number of mapping samples is 3 to 6 for expected average percentage of probes in the range of 3 to 5 percent and wherein the time interval In an embodiment of the present invention, the number of mapping samples is determined based on pre-stored data relating to an average motion over time period, estimated probability of probe arrival, and estimated arrival rate of vehicles. In some embodiments of the invention, the mapping system estimates the probability of probe arrival according to a predetermined procedure based on the percentage of probes among vehicles arriving at the congestion, and the method may further include:
concatenating a plurality of non-overlapping segments of consecutive mapping samples according to motion between mapping samples,
determining the number of vehicles in the concatenated segments by the ratio of the length of the concatenated segments to expected occupation length of vehicles in the road congestion, and
determining by a statistical estimator the percentage of probes based on the distribution of the accumulated probes identified over the period relevant to mapping samples of concatenated segments.
In an embodiment of the present invention, the statistical estimator is chosen according to assumed Binomial distribution of probes in the concatenated mapping samples.
In an embodiment of the present invention, the number of concatenated mapping samples is substantially limited to elapsed time interval wherein the expected probability of probe arrival to the road congestion is stationary.
In an embodiment of the present invention, the pre-stored data is optimized to provide the number of mapping samples to produce substantially the minimum expected error in the determined indication of the length of the congestion compared to the real average length of the congestion according to the respective mapping samples.
In an embodiment of the present invention, the optimization criterion is the minimum difference between the cumulative distribution function of errors that indicates that the estimates are too long and the cumulative distribution function that indicates that the estimates are too short.
In an embodiment of the present invention, the pre-stored data is prepared based on a simulation that shows the number of mapping samples required to provide minimum expected error for various conditions of congestion including motion rate, arrival rate and percentage of probes.
In an embodiment of the present invention, at a time prior to determining the indication on the length of the road congestion, mapping samples are adjusted according to a predetermined procedure that virtually adjusts the position of the mapping focus in the mapping samples in order to remove differences between motion rate and average arrival rate.
In an embodiment of the present invention, at a time after determining indication of the length of the road congestion, the determined indication is adjusted by a value which is indicative of the prior adjustments that were made to the mapping samples in order to remove the effect of prior adjustments on the mapping samples.
In an embodiment of the present invention, successive new indications of the length of the road congestion are determined according to a procedure that include successive newest mapping samples.
In an embodiment of the present invention, a one dimensional median filter is applied to the successive indications on length of road congestion.
In an embodiment of the present invention, time correlated mapping samples are collected according to a required resolution of the road congestion length determination, based on departure rate of vehicles from the road congestion.
In an embodiment of the present invention, the number of vehicles in a lane segment of the road congestion is determined according to estimated occupation length of vehicles.
In an embodiment of the present invention, the number of vehicles in a lane segment of road congestion is determined according to the estimated occupation length of vehicles.
Further, in accordance with a preferred embodiment of the present invention, there is provided a method of creating conditions which enable assessment of traffic motion rate toward a mapping focus in a congested road and of the road congestion length at a certain time, wherein according to a predetermined protocol probes report characteristic values of their position to a receiver of a mapping system which processes the reports, the method including:
(a) constructing a first mapping sample that includes at least one of said reports,
(b) determining a range of said position characteristic values in which at least one of said reports was identified in the first mapping sample, and
(c) transmitting to reporters a response that according to a predetermined procedure disables transmitters that did not transmit a report within the selected range of the first mapping sample from continuing to report.
In an embodiment of the present invention, the characteristic value of a position is an indication of the distance of a reporter from the mapping focus.
In an embodiment of the present invention, the mapping sample constructed by reports is transmitted to the mapping system within a response time synchronized to the mapping system and to the reporters.
In an embodiment of the present invention, a report of a position characteristic value is a signal transmitted by at least one probe in a slot of the response time that is indicative on a range of positions.
In an embodiment of the present invention, the time of the position related reports is determined by a broadcast query to the probes.
In an embodiment of the present invention, the selected range in which reporters are not disabled includes the position of the farthest reporter from the mapping focus.
In an embodiment of the present invention, the transmitted response to disable transmitters is a message including mapping sample that according to predetermined procedure reporters disable their transmitters from continuing to report if they had not transmitted a report within a range in the mapping sample selected according to the predetermined procedure.
In an embodiment of the present invention, after disabling a transmitter, the probe enables its transmitter by predetermined procedure at a time after it passes the mapping focus.
In an embodiment of the present invention, a non-disabled reporter reports a time indication representing its arrival at the reported position.
In an embodiment of the present invention, the time indication report is a signal transmitted by reporter in a slot that is indicative of a time interval in the mapping sample time constraints.
In an embodiment of the present invention, after a disabling response the mapping system receives the new reports and constructs a second mapping sample including new reports.
In an embodiment of the present invention, a reporter from the selected range reports an indication that it is out of mapping range if it passed the mapping focus at a time prior to time constraints of the second mapping sample. Preferably, in an embodiment of the present invention, the indication of out of mapping range determines no change in motion rate.
In an embodiment of the present invention, the indication of out of mapping range is a signal transmitted in a slot that a transmission in it is indicative on such condition.
In an embodiment of the present invention, the mapping system determines length of motion toward mapping focus by calculating a range characteristic value for a range in a mapping sample subsequent to a disabling response, which includes the position characteristic value indicative of the closest position to the mapping focus and calculating the difference between the range characteristic value and the range characteristic value of the corresponding selected range in an earlier mapping sample.
In an embodiment of the present invention, the mapping system determines the departure rate from the mapping focus in a congested road in units of length of a congested road segment per unit of time according to the determined length of motion towards mapping focus.
In an embodiment of the present invention, according to motion towards mapping focus, the mapping system determines a non-occupied space expected between vehicles along the road congestion.
In an embodiment of the present invention, the determined motion length towards the mapping focus related to average occupation length of a vehicle, including its unoccupied space, determines departure rate of vehicle from the congested road.
In an embodiment of the present invention, the time at which the traffic light system changes to a green setting is substantially identified by the mapping system through reports from probes.
In an embodiment of the present invention, according to predetermined procedure, a probe reports to the mapping system a substantial time when the traffic light system changes to the green setting by a predetermined delay taken for a vehicle to react according to its position in a waiting line.
In an embodiment of the present invention, the time of light change is substantially identified based on signal transmitted in a slot representing a time interval that best characterizes the time report.
In some preferred embodiments of the present invention, according to a predetermined procedure, the mapping system estimates arrival rate to the congestion, and the method further includes:
concatenating a plurality of non-overlapping segments of consecutive mapping samples according to the two said position related reports,
determining time intervals between two time of arrival reports corresponding to two position related reports,
determining average arrival rate in terms of segment length occupied by arriving vehicles between successive mapping samples by calculating the ratio of the total length of concatenated segment, in relation to the number of concatenated mapping samples.
In an embodiment of the present invention, the number of concatenated mapping samples is substantially limited to elapsed time interval in which the average arrival rate of probes to the road congestion is expected to be stationary.
In an embodiment of the present invention, the length of motion of non-disabled reporter towards mapping focus between two consecutive mapping samples determines the point between consecutive concatenated segments.