Abstract:
A vehicle detection unit (VDU) is embedded beneath each of a plurality of parking spaces. Each VDU has a sensor which detects magnetic field fluctuations caused by the arrival and departure of a vehicle in the parking space. The VDU runs a vehicle detection algorithm to distinguish magnetic fluctuations caused by vehicles from “exceptional” magnetic fluctuations from other sources. The VDU also stores parameters which define notifiable vehicle space occupancy events. A processor of the VDU processes the sensor signal to determine occupancy status of the vehicle space, and compare the occupancy status of the vehicle space with the parameters in order to determine whether a notifiable event has occurred, such as a vehicle going into violation of parking restrictions. As necessary, the VDU initiates radio communications with a supervisory device, either a transient portable device such as a Parking Officer&#39;s PDA or a fixed radio node mounted within range.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Australian Provisional Patent Application No 2007904549 filed on 23 Aug. 2007, and from Australian Innovation Patent Application No. 2009100796 by the same applicant filed on 21 Aug. 2008 and entitled “Vehicle detection”, the content of each of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to vehicle parking compliance and in particular to automated determination of notifiable parking events. 
     BACKGROUND OF THE INVENTION 
     Vehicular parking is usually subjected to rules and regulations such as absolute prohibitions in areas in which no parking is permitted, and conditional prohibitions such as permit-only parking. Time-limited parking regulations, whether free or metered/ticketed, are also common on public roadways. 
     Enforcement of vehicular parking regulations can be the responsibility of either private or public agencies. Such enforcement is costly and time consuming. Typical enforcement involves a parking inspector manually inspecting all of the restricted spaces periodically, regardless of whether vehicles are actually present. Further, to identify a violation in a free time-limited zone requires that the parking inspector inspect the violating car at least twice, once to establish a time of arrival of the car (for example by chalking the tire of the car), and a second time once the permitted time period for parking has elapsed. Even in paid metered zones, such manual enforcement is inefficient as the parking inspector may fail to note an expired meter or ticket before the violating car departs, reducing the deterrent to infringers and denying fines revenue to the relevant body. It has been estimated that only 1 in 25 parking violations are identified by manual enforcement. Enforcement is made even more difficult when the spaces are distributed over a large area, such as a city block or a large, multi-level parking garage. Manual enforcement is also unable to provide historical data of parking usage, and such data is becoming of increasing importance to the implementation of parking supply and regulation. 
     Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. 
     Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. 
     SUMMARY OF THE INVENTION 
     According to a first aspect the present invention provides a vehicle detection unit comprising:
         a sensor for outputting a sensor signal caused by occupancy of a vehicle space by a vehicle;   a storage device carrying parameters which define notifiable vehicle space occupancy events;   a processor operable to process the sensor signal to determine occupancy status of the vehicle space, and operable to compare the occupancy status of the vehicle space with the parameters in order to determine whether a notifiable event has occurred, and operable to initiate a communication from the vehicle detection unit to a supervisory device upon occurrence of a notifiable event.       

     According to a second aspect the present invention provides a method for vehicle detection by a vehicle detection unit, the method comprising:
         sensing occupancy of a vehicle space by a vehicle and outputting a sensor signal;   retrieving from a storage device parameters which define notifiable vehicle space occupancy events;   processing the sensor signal to determine occupancy status of the vehicle space, comparing the occupancy status of the vehicle space with the parameters in order to determine whether a notifiable event has occurred, and initiating a communication from the vehicle detection unit to a supervisory device upon occurrence of a notifiable event.       

     The present invention thus provides for the vehicle detection unit itself to determine whether a notifiable event has occurred. 
     Preferably, the vehicle detection unit communicates wirelessly directly with a portable supervisory device, for example a personal digital assistant carried by an inspector, in an ad-hoc manner at times when a notifiable event has occurred and when the portable supervisory device is in range. Such a network topology is referred to herein as a transient middle tier. Such embodiments may thus obviate the need for fixed communications infrastructure to connect the or each vehicle detection unit with a head office server, and to connect the head office server with the supervisory device, with such fixed infrastructure being referred to herein as a fixed middle tier. However, it is to be appreciated that the scope of the present invention includes embodiments in which a fixed middle tier is employed. 
     In further preferred embodiments of the invention, the vehicle detection unit is operable to provide information associated with the notifiable event to the supervisory device so that such information may be pre-populated into the supervisory device without requiring manual entry of such information by a parking inspector. Depending on the nature of the notifiable event such pre-population of associated information may save the inspector from manually entering up to 20 or more fields of data. Such data may for example include location information such as street name and parking bay number, vehicle occupancy duration, offence details, applicable regulations or “signage”, an applicable monetary fine, historical usage data, and the like. 
     The notifiable event may comprise occurrence of a parking violation, an imminent parking violation, or any non-violating parking event of interest such as a change in occupancy status. 
     Preferably, a plurality of vehicle detection units in accordance with the first aspect of the present invention communicate with a single supervisory device such as a PDA carried by a parking inspector. The single supervisory device may thus be provided with real time reporting of the occupancy status of each of a plurality of parking spaces, including information such as the presence or absence of a vehicle in each location, a duration of occupancy of each vehicle present, a violation status of each vehicle present such as no violation, violation imminent, and in violation. Notably, due to the ability of each vehicle detection unit to determine whether notifiable events are occurring, the supervisory device is not required to possess the substantial processing power needed to make such determinations for every such vehicle detection unit. 
     In preferred embodiments of the present invention, the vehicle detection unit is embedded in the roadway beneath the associated parking space. 
     The sensor of the vehicle detection unit is preferably a magnetometer able to sense variations in magnetic field caused by the arrival, presence, departure or absence of a vehicle. In particularly preferred embodiments of the invention the magnetometer is a 3-axis magnetometer able to sense components of the magnetic field in x, y and z directions. Preferably, emphasis is placed on variations in the z-axis (vertical axis) of the magnetic field, as such variations are most likely caused by a vehicle directly above the sensor. In such embodiments the vehicle detection unit preferably comprises a vehicle detection algorithm adapted to determine whether detected variations in the sensor signal are caused by a vehicle or by some other event such as the presence of magnetic objects, in order to avoid false positives. 
     In preferred embodiments of the invention the vehicle detection unit is operable to receive updates to the parameters defining notifiable vehicle space occupancy events, for example when the signage associated with the parking space is altered. Such updates are preferably transmitted to the vehicle detection unit wirelessly via the same wireless communications link used by the vehicle detection unit to communicate with the supervisory device. 
     According to a third aspect the present invention provides a method for issuing a parking infringement notice, the method comprising:
         a portable device establishing a transient wireless connection with at least one fixed vehicle detection unit (VDU);   the portable device obtaining from the VDU via the wireless connection data arising from a notifiable parking event detected by the VDU; and   the portable device generating a parking infringement notice using the data obtained from the VDU.       

     Preferably the method of the third aspect further comprises automated pre-population of the data arising from the notifiable parking event into infringement issuing software of the portable device. 
     According to a fourth aspect the present invention provides a system for detecting vehicle parking infringements, the system comprising:
         a plurality of vehicle detection units (VDUs) each associated with a respective vehicle parking space and storing parameters defining notifiable vehicle space occupancy events, each VDU configured to sense occupancy of the associated vehicle space and determine by reference to the parameters whether a notifiable event has occurred, and operable to communicate wirelessly;   a mobile supervisory device configured to wirelessly communicate with each respective VDU when brought within range, to receive information from the VDU arising from a notifiable parking event detected by the VDU, and to generate a parking infringement notice using the information obtained from the VDU.       

     In the fourth aspect of the invention the mobile supervisory device may be a portable digital device carried by a parking inspector or may be vehicle mounted. The supervisory device preferably collates data from a plurality of VDUs for simultaneous display to a parking officer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An example of the invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates a network topology comprising a transient middle tier in accordance with a preferred embodiment of the invention; 
         FIG. 2  illustrates a network topology comprising a fixed middle tier in accordance with an alternative embodiment of the invention; 
         FIGS. 3   a  to  3   i  illustrate cases which influence operation of the vehicle detection algorithm in distinguishing parking events from exceptions; 
         FIG. 4  is a flowchart illustrating operation of the vehicle detection algorithm; 
         FIG. 5  illustrates the generalised network topology of a further embodiment of the present invention; 
         FIG. 6  illustrates operation of a “smart” VDU in accordance with another embodiment of the present invention; 
         FIG. 7  illustrates operation of an “intelligent” VDU in accordance with yet another embodiment of the present invention; and 
         FIG. 8  illustrates operation of a “smart” middle tier together with “smart” VDUs, in accordance with a further embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Some portions of the detailed descriptions which follow are presented in terms of algorithms and symbolic representations of operations on data bits within a computer memory. These algorithmic descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. An algorithm is here, and generally, conceived to be a self-consistent sequence of steps leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. 
     It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussion, it is appreciated that throughout the description, discussions utilizing terms such as “processing” or “computing” or “calculating” or “determining” or “displaying” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
     The present invention also relates to apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, or it may comprise a general purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a computer readable storage medium, such as, but is not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, and magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any type of media suitable for storing electronic instructions, and each coupled to a computer system bus. 
     The algorithms and displays presented herein are not inherently related to any particular computer or other apparatus. Various general purpose systems may be used with programs in accordance with the teachings herein, or it may prove convenient to construct more specialized apparatus to perform the required method steps. The required structure for a variety of these systems will appear from the description below. In addition, the present invention is not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement the teachings of the invention as described herein. 
     A machine-readable medium includes any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium includes read only memory (“ROM”); random access memory (“RAM”); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.); etc. 
     The following acronyms and terms are used throughout the following detailed description:
     Exception An exception is a case where an object other than a vehicle (such as a magnet) is placed within the vicinity of the Sensor. The VDA should detect this event and determine it to be an Exception.   False Negative: The VDU incorrectly determines the space is empty when the space is occupied.   False Positive: The VDU incorrectly determines the space is occupied when the space is empty.   FMT Fixed Middle Tier. An alternative topology to TMT. The FMT is the conduit between the VDU and the VDS IG. The FMT is “fixed” as it is permanently in place in the field. The FMT may comprise wireless network nodes attached to parking meter or light poles or the like.   HHD Hand Held Device, for example running Windows CE, such as an O2 Atom   IIS Infringement Issuing Software   Parking Event: A parking event is the record of a vehicle arriving and departing a bay. The event is recorded as the arrival and the departure time of a vehicle.   PIN Parking infringement notice   PINS Parking infringement notice software   TMT Transient Middle Tier. The TMT is the conduit between the HHD and the VDU(s). The TMT is “transient” as it travels in the field with the Parking Officer and the HHD.   VDA Vehicle Detection Algorithm. A VDU firmware function that detects the presence of a vehicle over the unit.   VDS Vehicle Detection System   VDS IG Vehicle Detection System Internet Gateway.   VDU Vehicle Detection Unit. The unit which will be embedded into the ground. Contains a processor, a magnetic sensor and a communications module.   VIVA Vehicle In Violation Algorithm. This is a VDU firmware function that determines whether a given vehicle has lapsed into violation based on the presence of a vehicle as reported by the VDA.   

     The transient topology utilising a TMT is outlined in  FIG. 1 . Each VDU  110  is battery powered and embedded in the ground. To effect the transient topology shown in  FIG. 1 , each VDU  110  has the ability to detect the arrival and departure of a vehicle  120 , by way of a three-axis magnetic sensor. The magnetic sensor reads the magnetic field at sufficiently regular intervals to detect a magnetic field disturbance created by a vehicle travelling at 20 kph or less. Each reading taken outside thresholds is passed to the VDA for determination of the presence of a vehicle, as discussed further in the following. The sensor utilises a 3-axis sensor, which takes readings in the X, Y, and Z axis of the sensor, and the sensor can be placed into a power saving mode. The VDU  110  runs a vehicle detection algorithm (VDA). The VDA takes readings from the sensor and determines the arrival and departure of the vehicle  120  as discussed further with reference to  FIGS. 3   a  to  3   i . The VDA is run in the firmware of VDU  110 . Upon determination of the arrival of a vehicle  120  by the VDA, the VDU  110  also determines when the vehicle  120  moves into violation of parking restrictions. This is done by a VIVA (Vehicle In Violation Algorithm), which is run in the firmware of the VDU  110 . 
     Upon determination of an actual or impending infringement, the VIVA initiates communications from the VDU  110  to the TMT  132 , to alert the parking officer  130  of the infringement. This is effected by an RF communications component of the VDU  110 . The RF communications component of the VDU  110  “listens when required” for the broadcast beacon from the TMT  132 , and organises communications. 
     Each VDU  110  has onboard memory storage to store Parking Events, enabling the VDU  110  to upload data for pre-population into the HHD  134  as required. Such data includes infringement data, parking event data, system health status, offence type (eg no standing), location details (physical), parking restrictions (eg signage), offence details such as arrival time and departure time, offence notes such as arrival time and time overstayed, and firmware details. Once the Parking Event data has been transferred, it is removed from the VDU  110 . Further, the HHD  134  issues a confirmation message indicating that an infringement has been issued, which is received by the VDU  110  and causes the VDA to be reset. 
     As it is unlikely to be economic to replace the battery and/or VDU  110 , each VDU  110  is configured to be highly power efficient on the basis that once the battery has been exhausted the life of the VDU will be over. One measure to provide for longevity is that each VDU  110  is operable to download updated firmware and firmware parameters (such as changed parking restrictions), as required. 
     The TMT  132  is the component associated with the HHD  134  and the Parking Officer  130 . The TMT  132  communicates to the VDUs  110  and interfaces to the HHD  134 . In this embodiment the TMT  132  is provided by wireless networking hardware positioned in the holster of the HHD  134 . The communications between the TMT  132  and the HHD  134  occur via mini USB port. The HHD  134  and the TMT  132  thus have a permanent connection in place whereby communication can occur at any time. The TMT  132  is a “transient” middle tier as it travels in the field with the parking officer  130 , and thus is not always available for communications with any given VDU  110 . 
     The TMT  132  acts as a conduit, whereby it passes data between the HHD  134  and each VDU  110 . No data is stored on the TMT  132 . Data to be transferred to a VDU  110  from the HHD  134  is downloaded into the TMT  132  for transmission as required. Data to be transferred to the HHD  134  from the VDU  110  will be transmitted to the TMT  132  for upload to the HHD  134 . The communication between the TMT  132  and each VDU  110  is by way of an RF link. Due to the transient nature of the TMT  132  and the need to establish a new wireless connection each time it comes into range of a VDU in violation, the TMT  132  issues a broadcast beacon which can be picked up by any VDU  110  within range. If a given VDU  110  responds, the TMT  132  will then schedule communications with the VDU  110  so as to establish the new connection. 
     The HHD  134  executes VDS portable system management console software and/or IIS software. When running the VDS portable system management console software, the HHD  134  has responsibility for management of all of the VDUs  110  in the field. This includes management of the installation of each VDU  110 , including recording of the location details. It also includes routine maintenance of VDU firmware. The HHD  134  is pre-populated with the required database of details of VDU installation location and details. In this embodiment having a transient middle tier, at least at times when firmware updates are required, the HHD  134  is pre-populated with the required firmware and firmware parameters for download to each VDU  110 . 
     When the IIS is being run on the HHD  134 , the HHD  134  is responsible for receiving and processing of data from the TMT  132 , including the storage of Parking Event data and System Health data for each VDU  110 . When infringement details are acquired, the location of the associated VDU  110  and any other details required for the IIS to produce the infringement notice will be retrieved from an internal memory of the HHD  134  based on the details provided by the VDU  110 . For example, the details of the internal memory may include a street-to-suburb mapping to enable the HHD  134  to include the suburb in the infringement notice. When an infringement has been issued the HHD  134  passes a message via the TMT  132  to the VDU  110  to reset the VDA so that a new monitoring period can commence. 
     While the TMT  132  described above and shown in  FIG. 1  is a preferable solution due to the need for little or no fixed middle tier hardware, it is to be understood that the present invention includes within its scope fixed middle tier topologies communicating with vehicle detection units in accordance with the present invention.  FIG. 2  illustrates a system  200  in accordance with one such fixed middle tier (FMT) topology. 
     The VDUs  210  in the system  200  provide most of the functions described above for the VDUs  110  of  FIG. 1 , which are not described again. In addition, each VDU  210  is operable to listen for the beacon signal of the FMT  240 , when required. Upon detecting the beacon signal, the VDU  210  will perform the actions as outlined above as for the TMT  132 . However, as infringement will only be issued by the HHD  234 , the VDU  210  will continue to listen for the beacon signal of the TMT  232  when a vehicle  220  is in violation. 
     A transient middle tier  232  is provided in the embodiment of  FIG. 2 , however only infringement information would be transferred by TMT  232  as the other data would be sent via the FMT  240 . The  234  HHD functions largely as defined above for the transient topology of  FIG. 1 . The fixed middle tier  240  of  FIG. 2  is similar to the TMT  132  in that it communicates to each VDU  210 , however the FMT  240  provides further capability beyond that of the TMT  132 , namely to mesh network the data received from the VDUs  210  back to the VDS IG  250 . The FMT  240  could be battery or mains powered and attached above ground to appropriate existing fixed infrastructure for example. When battery powered, the FMT  240  should be efficient in use of power, however unlike the VDUs  210  the battery of the FMT  240  can be replaced when exhausted. 
     The FMT  240  thus acts as a conduit, whereby it passes data from the VDUs  210  to other FMTs  240  or VDS IGs  250 . No data is stored on the FMT  240 . The FMT  240  is installed such that its broadcast receive range covers a wide range of VDUs  210  such that no single VDU  210  is within range of only one FMT  240 , thus providing connection redundancy. The communication between the FMT  240  and the VDU  210  is effected by an RF wireless link. The FMT  240  issues a broadcast beacon signal which can be picked up by any VDU  210  within range. If a given VDU  210  responds, the FMT  240  schedules communications with that VDU  210 . Once communications have been scheduled the data exchange between the VDU  210  and the FMT  240  includes Parking Infringement details, Parking Event data, Health Status information and Firmware details. The FMT  240  then passes this information over the Mesh network to the VDS IG  250 . Again, the VDUs  210  erase any data that has been successfully transmitted to the TMT  232  and/or FMT  240 , to minimise VDU memory requirements. 
     The VDS IG  250  is an internet gateway that has a connection to the internet  260 . VDU information transferred to the VDS IG  250  via the FMT  240  is sent on to the parking inspector&#39;s dashboard monitor  236  or HHD  234 . The VDS IG  250  is mains powered and communicates with many FMTs  240  at once. The backend  236  is a computer which receives data from the VDS IG  250  in real time. The backend  236  could be the dashboard monitor or other car-based computing device of a parking inspector  230 , and is capable of displaying this information in a graphical nature for visual interpretation and analysis of the data being received. 
     Operation of VDUs  110 ,  210  thus includes communicability with either or both of a TMT  232  or FMT  240 , depending on topology implemented. In preferred embodiments, each deployed VDU  110 ,  210  is capable of operating under either a fixed topology or a transient topology to allow topology changes without requiring physical replacement of the VDUs, such replacement being laborious and involving excavation of the roadway. Further, each VDU  110 ,  210  supports firmware updates and is able to reboot itself when instructed by the TMT or FMT, and can receive and install firmware updates. For example firmware configuration could include updated firmware version and components, configurable transmit intensity to provide compensation for changing wireless channel conditions, and configurable poll time. Each VDU  110 ,  210  further communicates a “known state” on request, and is able to be awoken for routine maintenance by a beacon signal from the TMT  132  and/or FMT  240 . 
     Each VDU  110 ,  210  supports pre-programmed daylight savings time zones and exception days such as public holidays, and is able to communicate as required that it is in a “fault” state, allowing scheduling of maintenance. Each VDU  110 ,  210  is embedded about 2 centimeters below the surface of the parking space, and in this configuration can typically communicate with a TMT  132  and/or FMT node  240  that is 20 meters away. To conserve battery power, each VDU  110 ,  210  only attempts to detect the presence of the beacon signal of the TMT  132  and/or FMT  240  at times when communication is required. Such occasions include when the VIVA indicates a vehicle is in violation, when the predefined Parking Event storage limits have been met, when the system is in a “maintenance required” state, and when listening for a routine maintenance request. 
     A preferred feature enabling use of a transient middle tier in particular is that each VDU  110 ,  210  also transmits data required by the HHD  134  for the infringement in question, such as parking event data. As the VDU  110 ,  210  stores data representing both the parking space restrictions and the nature of the violation detected, this enables the HHD  134  to be provided with all necessary data to issue an infringement notice without requiring communications with a central server at the back end of the system. 
     Each VDU  110 ,  210  is further configured to establish communications with only one TMT  132  or FMT  240  at any given time, even when more than one TMT or FMT is in range. 
     Each VDU  110 ,  210  runs a vehicle detection algorithm (VDA) which is illustrated in  FIG. 4 . The VDA is remotely updateable as part of a general firmware update. Any object that creates a disturbance to the magnetic field far greater than that created by a vehicle (such as a very strong magnet within the vicinity of the sensor) is to be treated as an exception, as discussed with reference to  FIGS. 3   a  to  3   i . Other situations which may trip the VDA into a false positive are also recorded as exceptions. Exceptions are reported as “space unoccupied” by default. The VDA enables the VDU to determine the arrival of a vehicle over the sensor, and determine the departure of a vehicle from over the sensor. The process of  FIG. 4  is biased towards avoiding false positives, so as to minimise the chance of wrongly reporting a vehicle as being in violation when no violation actually occurred. 
     The VIVA algorithm operates on multiple time based triggers, supporting a minimum of 10 time changes in any operational period. The VIVA algorithm is updateable over the RF link, and being a firmware algorithm can be updated with a full firmware update. The VIVA configuration will be configurable via parameters over the RF link. Updating these parameters will not require a full firmware update, so that the VIVA configuration may for example implement a parameter driven daylight savings time period allowing simple parameter updates to effect changed daylight savings conditions. Similarly the VIVA configuration may implement parameter driven exception days, for example for parking limitations which apply except on certain days, thus providing for simple parameter updates to effect any changes in exception days. 
     The VIVA works in conjunction with the VDA to determine whether a vehicle is in violation. When required the VIVA initiates communication with the TMT  132  and/or FMT  240 . Determinations made by the VIVA constitute parking event data, which is written to the memory of the VDU. The VIVA is further able to support pending violations, for example to indicate that a violation is imminent. The VIVA and/or system further provides for a configurable grace period, which may be afforded to a vehicle operator before issuance of an infringement notice is initiated. 
     In the embodiments described the parking officer is not able to print an infringement notice until the infringement signature information has been transmitted from the relevant VDU  110  to the HHD  134 . However, in alternative embodiments this requirement may be omitted. 
     The systems of  FIGS. 1 and 2  enable generation of data such as estimated average bay movements, including the number of expected infringements in a given day, how long until the car leaves (at which time the VDU can stop “listening”), how long until the infringement notice is issued (after which the VDU can stop listening), and how often the enforcement officer comes past any given VDU. 
     Each state outlined in the following falls into different sections of the state machine defined for the VDU by  FIG. 4 . Each case has a description and an indication of where it fits into the state machine flow chart. 
     Case 1 
     This case is illustrated in  FIG. 3   a  and occurs when a steady state has previously been reached, the VDA run and a car has been identified as occupying the bay. 
                                                       Pre conditions   Post conditions   Effect on VDA                                Previous State   Steady   Value recorded for   VDA is not run       Car Present   TRUE   adaptive steady state       Current State   Steady                    
Case 2
 
     This case is illustrated in  FIG. 3   b  and occurs when a car first arrives, the steady state is realised after a transient phase, once the VDA identifies a car is present the arrival time and arrival signal are stored. 
                                                       Pre conditions   Post conditions   Effect on VDA                                Previous State   Transient   Arrival Buffer is   VDA is run and       Car Present   FALSE   stored   returns TRUE       Current State   Steady   Arrival Time is               recorded                    
Case 2—Alternative
 
     This case is illustrated in  FIG. 3   c  and has the same preconditions as case 2 but when the VDA is run a result of no car present is returned. This means the steady state reached, or the arrival signal cannot possibly be a car. In this case the system transitions to an exception state. 
                                                       Pre conditions   Post conditions   Effect on VDA                                Previous State   Transient   EXCEPTION state is   VDA is run and       Car Present   False   realised   returns FALSE       Current State   Steady               NOTE:       The VDA returns false for the steady state reached            
Case 3
 
     This case is illustrated in  FIG. 3   d  and occurs when a car has previously been identified by the VDA and an external factor interrupts the field, when a steady state is again realised the VDA is run and determines the same car is present. In debug mode this intermediate signal can be stored. 
                                                       Pre conditions   Post conditions   Effect on VDA                                Previous State   Transient   Intermediate buffer is   VDA is run and       Car Present   TRUE   stored   returns TRUE       Current State   Steady                    
Case 3—Alternative
 
     This case is illustrated in  FIG. 3   e  and is similar to case 3 but when the VDA is run after reaching steady state from a transient state a result of no car present is returned. In this case the VDA has determined that the steady state reached cannot be either the same car or a different car. In this case the system switches to exception state. 
                                                       Pre conditions   Post conditions   Effect on VDA                                Previous State   Transient   EXCEPTION state is   VDA is run and       Car Present   TRUE   realised   returns FALSE       Current State   Steady               NOTE:       New Steady State is outside recorded steady state value when VDA returned TRUE            
Case 3—Alternative #2 (VDA Returns True Due to Different Car Being Present)
 
     This case is illustrated in  FIG. 3   f  and is similar to case 3 but when the VDA is run after reaching steady state from a transient state the result denotes that a different car is present. In this case the VDA has determined that the previous car has left and a new car has arrived. Therefore the departure time must be noted, the arrival signal of the new car stored and the arrival time noted. 
                                                       Pre conditions   Post conditions   Effect on VDA                                Previous State   Transient   Previous car   VDA is run and       Car Present   TRUE   departure time   returns       Current State   Steady   stored   DIFFERENT_CAR               transientBuffer               stored as arrival               buffer               Arrival time stored.               NOTE:       New Steady State is outside recorded steady state value when VDA returned TRUE            
Case 4
 
     This case is illustrated in  FIG. 3   g  and is the case where the car has departed and the steady state reached is the baseline state. The departure time is stored and the departure signal may be stored if in debug mode. 
                                                       Pre conditions   Post conditions   Effect on VDA                                Previous State   Transient   Departure buffer is   VDA is not run       Car Present   TRUE   stored       Current State   Baseline   Departure Time is               stored                    
Case 5
 
     This case is illustrated in  FIG. 3   h  and is the case when the transient timeout expires, which means that a steady or baseline state has not been reached fast enough. In this case the system switches to exception state. 
                                                       Pre conditions   Post conditions   Effect on VDA                                Previous State   Transient   EXCEPTION state is   VDA is not run       Car Present   TRUE/FALSE   realised       Current State   Transient               NOTE:       Max Transient Duration expired            
Case 5—Alternative
 
     This case is illustrated in  FIG. 3   i  and is the case when the transient timeout expires, which means that a steady or baseline state has not been reached fast enough. In this case the system switches to exception state. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 Pre conditions 
                 Post conditions 
                 Effect on VDA 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Previous State 
                 Transient 
                 EXCEPTION state is 
                 VDA is not run 
               
               
                 Car Present 
                 TRUE/FALSE 
                 realised 
               
               
                 Current State 
                 Transient 
               
               
                   
               
               
                 NOTE: 
               
               
                 Reading does not stabilise within timeout 
               
             
          
         
       
     
     Each VDU  110 ,  210  has several registers, which allow the run time state of the VDU to be maintained. A Car Present Flag denotes whether the VDA has determined if a car is present or not, and is used when determining which transition case has occurred. A Network Status Register allows the VDU to keep a record of its connectivity to the network when coming out of a sleep cycle. The flags can be checked to determine what network state the VDU was in before sleeping. A NETWORK ASSOCIATED bit represents whether the VDU has successfully joined a network. A NETWORK_JOIN_REQUEST bit is used to signify whether the VDU wishes to establish a wireless connection, and when set, the VDU will try and join a network. 
     A network message request register of the VDU is used to store the type of message the VDU wishes to exchange with a TMT. Multiple bits can be set indicating more than one type of data exchange is desired. Upon being asked by a TMT the type of exchange required this register is used as a return data. 
     An ADMIN bit indicates whether the VDU wishes to exchange Admin based commands, and a VIOLATION bit indicates the VDU wishes to inform a TMT a violation has occurred. A PENDING_VIOLATION indicates the VDU wishes to inform the TMT of a pending violation, and a SYSTEM-HEALTH bit indicates the VDU wishes to inform the TMT of its system health status, such as battery level. A PARKING_EVENTS bit indicates whether the VDU wishes to transfer its parking event data to a TMT, while an EXCEPTION bit indicates whether the VDU is in exception state and needs to inform a TMT of this. 
     A sensor state register of the VDU is used to store the current state of the VDU. The enabled bit will always be set when sensor readings are required, and this state can be used in conjunction with any other state. Any other combinations of state are prohibited. 
     An ENABLED bit indicates whether the VDU should read the sensor in its main loop. A BASELINE bit indicates whether the VDU is in a baseline state, while a TRANSIENT bit indicates whether the VDU is in a transient state. A STEADY bit indicates whether the VDU is in a steady state, and an EXCEPTION bit indicates whether the VDU is in an exception state. A SUSPENDED_SLEEP bit indicates whether the VDU is in a suspended sleep state, used to make the VDU sleep for long periods such as overnight or on weekends. 
     A previous state runtime variable of the VDU is used to store the last state the VDU was in. The previous state may only ever be a single state, multiple previous states are prohibited. The previous state is used when determining the type of transition that has occurred and any subsequent action that should follow. 
     A BASELINE bit indicates whether the VDU was previously in baseline, while a TRANSIENT bit indicates whether the VDU was previously in transient state. A STEADY bit indicates whether the VDU was previously in steady state, and an EXCEPTION bit indicates the VDU was previously in exception state. A SUSPENDED_SLEEP bit indicates whether the VDU was previously in suspended sleep state. 
       FIG. 5  illustrates the generalised network topology of a further embodiment of the present invention, and outlines the various components required for the system of this embodiment to operate. These components and options for structure and intelligence of each are first outlined, and then subsequently examined in greater detail. The backend system  1  provides for configuration, processing and storing of data. In particular, the backend system  1  provides a conduit for the entry of street names and for the identification and recording of the location of each VDU. For example, when a detector is placed in the ground, its location is to be recorded. This location will then be used to identify the details required in order to issue an infringement notice to an offending vehicle. The backend system  1  further provides backend processing of all PINs, and also stores all signage information so as to be able to determine if a vehicle has committed an offence. 
     In this embodiment the backend  1  is a dashboard device and must also thus provide for dashboard functionality such as aggregating data to facilitate a dashboard overview of data across multiples VDUs and multiple HHDs. The back end system  1  also communicates with VDUs  4  (via the wireless communication channel) and, as required, upgrades software running in the VDU, determines if a given VDU is operating as expected, downloads data from the VDUs for aggregation, and communicates with the handheld device  3  for example to inform it of the presence of a vehicle. 
     The middle tier  2  is the layer between the VDUs  4  in the ground on the one hand, and the backend system  1  and/or HHD  3  on the other hand. That is, the middle tier  2  communicates with both the VDUs  4  and the backend system  1  and/or HHD  3 . Depending on the end architecture and design, the middle tier  2  may be required to communicate information to the hand held devices  3 , store details and data being received from the VDUs  4 , aggregate data and send it to the backend system  1 , and communicate with the VDUs  4  to determine device health. 
     The hand held device (HHD)  3  is carried by a parking officer, and receives communications regarding the details of an infringement (or pending infringement) and is provided with “pre populated” details which are automatically passed into the PINS for processing by the officer. The data fields provided for automated pre-population include Offence Type, Offence Date and Time, Offence Location (e.g Street No., Street Name, Suburb), Parking Bay Identifier, Vehicle Arrival Time, Vehicle Stay Duration, Location Information (e.g. Side of Street, Between Streets, Near &lt;Descriptor&gt;), Parking Sign Information (e.g 1P Mon-Fri, 8:00 am to 6:00 pm, No Exemptions), Parking Restriction and Officer Notes. Such communications could arrive from the back-end system  1 , from a middle tier  2  separate to the HHD, or directly from the VDUs  4 . 
     The Vehicle Detection Units (VDUs)  4  are placed into the ground beneath each vehicle parking space. The readings taken by a sensor of each unit are used by the vehicle detection algorithm (VDA) run by the VDU to determine the presence of a vehicle. Depending on the end architecture and design, each VDU  4  may be required to send a signal identifying itself, send a signal defining the magnetic field reading, send a signal defining the presence of a vehicle over the detector, send a signal defining the history of vehicle movements within the bay, and/or be able to download new firmware as required. 
     In using the readings of the sensor, the VDU  4  has the capability to calculate and store an averaged baseline for the “background reading” for the detector, and in determining this background reading the VDU has the capability to take into account variations in the temperature and other factors that may affect the detector. Each VDU  4  stores the temperature for a “reasonable period” and the background reading for a “reasonable period”, and determines the affect of the background reading on temperature, and applies this to the calculation. Each VDU  4  holds signage information and goes into a power saving mode at times when restrictions do not apply. 
     The VDUs  4  are also able to download new firmware algorithms as required, reboot themselves, receive a signal instructing a change in the frequency of the sensor readings, receive a signal instructing a change in the frequency at which a signal is sent to the listening devices  2 ,  3  indicating the presence of a vehicle, and upload any stored data. An advantage of a VDU having such wireless connectivity is that the topologies of  FIGS. 1 ,  2  and  5  do not specifically require a 3 rd  party wireless network. Further, the ability of the VDU to execute the VDA allows the VDU to communicate to the listening devices  2 ,  3  that a vehicle is present and for how long it has been present. 
       FIG. 7  illustrates operation of an “intelligent” VDU  710  in accordance with yet another embodiment of the present invention. This topology is in a number of ways the same as the “smart” VDU described with reference to  FIG. 5 , except as set out below. In this embodiment the VDUs  710  store each reading, such as the readings of  FIG. 3 , for a period of time (say 1 month). The VDU  710  determines if a vehicle has infringed by referring to signage details of the bay stored by the VDU  710 . Such stored signage details include definition of date and time details, and for each uniquely designated period, the allowed length of stay, the start time and the end time. An advantage of the intelligent VDU is that the VDU is able to communicate to the listening device the fact that the vehicle is in violation, and is not required to continuously broadcast the “vehicle present” signal. 
     A “dumb” middle tier may be employed in some embodiments. In this scenario the middle tier acts like a router and passes all data received from the VDUs on to the backend system supplying only secure communications and ensuring that all signals received are passed on and subsequently received at the other end. This system could be used in conjunction with either Intelligent or Smart VDUs. The VDUs behave as outlined above, the dumb middle tier will receive signals from all VDUs that it is configured to receive data from, and then securely transmits this data to the backend system. The backend system communicates this data to any HHD that requests or requires the information. This communication could be done via the middle tier&#39;s wireless network already in place, or alternatively via GPRS or other wireless network to the HHDs in the field The data includes street details and infringement details for issuing of a PIN. 
     Advantages of the dumb middle tier include that the risk of over engineering is small with this solution, the cost impost of providing this “router” is less than having a complete computer system deployed into the field, and there is one central redundant backend system which is fully tunable and upgradeable as opposed to many deployed remote smart middle tiers or deployed remote smart VDU&#39;s. 
     In still further embodiments a smart middle tier (with smart VDUs) may be deployed. In this scenario the middle tier provides the sophisticated functionality of the backend head office system, including receiving signals from the VDUs and storing this data in its own database. The topology of this system is illustrated in  FIG. 8 . This system would work as follows. The Smart VDUs behave as outlined above, the smart middle tier will receive signals from all VDU&#39;s that it is configured to receive data from. The smart middle tier is constantly broadcasting wake-up messages on the designated frequency to all VDU&#39;s that are within range. Once a VDU comes within range of the smart middle tier, it responds to the connect message back to the smart middle Tier. The smart middle tier then handshakes with the VDU and requests data indicating how long the vehicle has been above the VDU (0 mins for no vehicle present), the MAC address of the VDU, and the vehicle movement in the bay since the VDU last spoke to the smart middle tier 
     The middle Tier would then subsequently be responsible for determining if the vehicle in that bay is in violation of parking, communicate to a HHD which bay has a vehicle over it, and pass street details and infringement details to the handheld device for issuing of a PIN. Advantages if the smart middle tier include that by using smart VDU&#39;s, a permanent connection to the smart middle tier is not required in order to detect the presence of a vehicle. Subsequently the infrastructure costs are reduced as it is not required to affix the middle tiers to infrastructure every 100 meters or so. Further, this topology does not require a 3 rd  party permanent wireless network to communicate to the hand held devices. This would be done in a piece meal basis, by the smart middle tier as violations are detected. Also, HHDs communicate via the middle tier (and not the VDU&#39;s), and a smaller set of communications is required for both the VDU and hand held. The handheld receives infringement data and the street details, and is not required to have this data stored and determine the bay details from its internal database. This topology also lowers battery requirements due to the VDU only communicating when the smart middle tier comes within range. 
     The topology of  FIG. 8  does not require a permanent connection to any backend system. Moreover, use of a smart VDU means the smart middle tier is only required to communicate to the back end and the VDUs in a piecemeal fashion, for example upon docking at the end of a working day. The topology of  FIG. 8  allows the smart middle tier to be mobile as it is not always required to be calculating the presence of a vehicle in any particular space. Further, any firmware upgrade of the VDUs can be done via the smart middle tier, and the VDUs are required to only store the details of vehicle movements since the last time that the smart middle tier communicated. The VDUs only have to communicate when asked which extends VDU battery life. The backend system is only required to know about the signage, which is transferred to the smart middle Tier, and only what is required to be printed for the PIN is passed to the HHD. 
     It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.