Patent Publication Number: US-2007118273-A1

Title: Method and system for obtaining traffic information using transponders

Description:
FIELD OF THE INVENTION  
      The present invention relates to traffic management and, in particular, to a method and system for obtaining traffic information using transponders.  
     BACKGROUND OF THE INVENTION  
      Electronic toll collection systems conduct toll transactions electronically using RF communications between a vehicle-mounted transponder (a “tag”) and a stationary toll plaza transceiver (a “reader”). An example of an electronic toll collection system is described in U.S. Pat. No. 6,661,352 issued Dec. 9, 2003 to Tiernay et al., and owned in common with the present application. The contents of U.S. Pat. No. 6,661,352 are hereby incorporated by reference.  
      In a typical electronic toll collection (ETC) system, the reader broadcasts a wakeup or trigger RF signal. A tag on a vehicle passing through the broadcast area or zone detects the wakeup or trigger signal and responds with its own RF signal. The tag responds by sending a response signal containing information stored in memory in the transponder, such as the transponder ID number. The reader receives the response signal and may conduct an electronic toll transaction, such as by debiting a user account associated with the transponder ID number. The reader may then broadcast a programming RF signal to the tag. The programming signal provides the tag with updated information for storage in its memory. It may, for example, provide the tag with a new account balance.  
      Traffic management and transportation authorities currently attempt to alert vehicle occupants to upcoming traffic conditions using overhead or roadside display signs. For example, a display sign may indicate the traffic conditions in the road ahead. By way of example, a display sign may indicate the travel time between the current location and a location some distance down the road, such as the next exit, the next toll plaza, the next town, etc. Vehicle occupants may then use this information about travel conditions to determine whether or not to travel on the roadway.  
      Unfortunately, the information given is sometimes inaccurate. The displayed information may come from human estimations based on observed travel conditions, although this is labour intensive. In some cases, measurements are made of traffic conditions to determine the approximate travel time.  
      Some traffic information systems attempt to capitalize on the presence of ETC transponders in the roadway in order to measure the traffic conditions. For example, a system may (a) read a vehicle transponder at an upstream reader and send time and transponder ID information to a back office location, (b) read the vehicle transponder at a downstream reader and send time and transponder ID information to the back office location. The back office location then attempts to match transponder IDs and determine the travel time based upon the time stamps from the upstream and downstream locations. After processing information from a sufficient number of transponders, an average travel time is calculated and displayed on a display sign by the roadway.  
      A problem associated with existing systems that rely on ETC transponders is a privacy concern with the communication of transponder IDs. The traffic information system is often implemented and/or managed by a different entity than the ETC system. In the existing systems, the transponder IDs are sent wirelessly over a unsecured cellular network or other wireless network from the readers to the back office location.  
      Another problem associated with the existing systems is the high cost and overhead associated with communicating from both readers to a back office for every transponder. These communications are often made using the cellular telephone network, which can be expensive.  
      It would be advantageous to have an improved method and system for obtaining traffic information using transponders.  
     SUMMARY OF THE INVENTION  
      The present invention provides a method and system for obtaining traffic information regarding the travel time between two points using vehicle-mounted transponders. The system includes an upstream reader and a downstream reader. The upstream reader inserts a time stamp into the memory of a passing transponder. The downstream reader reads the time stamp and determines the travel time between the two readers. The downstream reader may accumulate a number of travel times and calculate an average travel time. The downstream reader may then send the calculated travel time to a display sign for display to motorists, or may send information based upon the calculated travel time. The displayed travel time may include a quantitative value or a qualitative assessment of the travel conditions.  
      In one aspect, the present invention provides a method of obtaining traffic information regarding a roadway using a vehicle equipped with a transponder having a memory. The roadway includes a downstream reader having a downstream communications zone over a first portion of the roadway and an upstream reader having an upstream communication zone over a second portion of the roadway. The upstream communication zone is separated from the downstream communication zone by a distance of the roadway. The roadway includes a display sign. The method includes the steps of sending a time stamp to the transponder for storage in the memory of the transponder while the vehicle traverses the upstream communication zone and reading the time stamp from the memory of the transponder when the vehicle is in the downstream communication zone. It further includes steps of determining a travel time from the upstream communication zone to the downstream communication zone based upon the time stamp, and determining travel time information based upon the travel time.  
      In another aspect, the present invention provides a system for obtaining traffic information regarding a roadway upon which vehicles equipped with transponders travel. The transponders each have a memory. The system includes a downstream reader having a downstream communications zone over a first portion of the roadway and an upstream reader having an upstream communication zone over a second portion of the roadway. The upstream communication zone is separated from the downstream communication zone by a distance of the roadway. The upstream reader has a time stamp transceiver for sending a time stamp to the transponder for storage in the memory of the transponder while one of the vehicles traverses the upstream communication zone. The downstream reader includes an interrogation transceiver for reading the time stamp from the memory of the transponder when the vehicle is in the downstream communication zone. It also includes a travel time module for determining a travel time from the upstream communication zone to the downstream communication zone based upon the time stamp and determining travel time information based upon the travel time.  
      In yet another aspect, the present invention provides a downstream reader for use in a system for obtaining traffic information regarding a roadway upon which vehicles equipped with transponders travel. The transponders each have a memory. The system includes an upstream reader having an upstream communication zone. The upstream reader stores a time stamp in the memory of a transponder while a vehicle traverses the upstream communication zone. The downstream reader includes a downstream antenna having a downstream communication zone over a portion of the roadway and an interrogation transceiver connected to the downstream antenna for sending an interrogation signal to the transponder and receiving a response signal. The response signal includes the time stamp. The downstream reader includes a travel time module for determining a travel time from the upstream communication zone to the downstream communication zone based upon the time stamp and determining travel time information based upon the travel time.  
      Other aspects and features of the present invention will be apparent to those of ordinary skill in the art from a review of the following detailed description when considered in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Reference will now be made, by way of example, to the accompanying drawings which show an embodiment of the present invention, and in which:  
       FIG. 1  shows, in block diagram form, an embodiment of a traffic information system associated with an extent of roadway for vehicular traffic;  
       FIG. 2  shows a table detailing a transponder information data structure according to a pre-defined ETC protocol;  
       FIG. 3  shows a block diagram of an alternative embodiment of the traffic information system;  
       FIG. 4  shows a block diagram of an embodiment of an upstream reader for the traffic information system;  
       FIG. 5  shows a block diagram of an embodiment of a downstream reader for the traffic information system; and  
       FIG. 6  shows, in flowchart form, an embodiment of a method of obtaining traffic information. 
    
    
      Similar reference numerals are used in different figures to denote similar components.  
     DESCRIPTION OF SPECIFIC EMBODIMENTS  
      The present application describes a system and method for obtaining traffic information that relies upon existing transponders to carry time stamp information from an upstream reader to a downstream reader. The upstream reader may provide the transponder with a time stamp and a location ID, which the transponder stores in its memory. The downstream reader extracts this information from the transponder memory and uses it to determine a travel time between the two locations. By averaging the travel times of a number of transponders, the downstream reader can determine an average travel time between the two points, which may then be displayed for motorists on a display sign near or over the roadway.  
      Some of the embodiments described below suggest the use of electronic toll collection (ETC) transponders. The systems and methods described below may be incorporated into an ETC system; however, they may be implemented separately even when using ETC-enabled transponders. In other words, the readers used for obtaining traffic information may or may not be involved in conducting ETC transactions. In some embodiments, non-ETC transponders may be used. The transponders may be related to commercial vehicle operations (CVO) systems, for example, or may have other primary uses. In some embodiments, the transponders may be related to manufacturer-installed on-board-diagnostics systems. In another example, the transponders may be dedicated to the traffic management systems and methods described herein.  
      It will also be appreciated that the described methods and systems may be implemented on toll roads or on non-toll roads. A non-toll road may carry a sufficient number of vehicles equipped with transponders to support the system and methods described below. It will be understood that the described system obtains an average or approximate travel time between two points in a roadway and, as such, it does not need to determine the travel time of every vehicle in the roadway; only a representative sample of the traffic is needed. Accordingly, not all vehicles programmed at the upstream reader need to be read at the downstream reader.  
      Reference is first made to  FIG. 1 , which shows, in block diagram form, an embodiment of a traffic information system  10  associated with an extent of roadway  24  for vehicular traffic. For simplicity, the roadway  24  is illustrated as having a single lane for traffic, although it will be appreciated that the roadway  24  may have, and usually does have, multiple lanes. A vehicle is indicated by reference numeral  20 , and the direction of travel is indicated by an arrow. The vehicle  20  is equipped with a transponder  22 . The transponder  22  may be active or passive, provided that the transponder  22  includes a memory having a least a portion for storing data in accordance with instructions from a remote reader.  
      The traffic information system  10  includes a display sign  26 , an upstream reader  12 , and a downstream reader  16 , wherein “upstream” and “downstream” refer to the direction of vehicular travel in the roadway  24 . The upstream reader  12  is located in the general vicinity of the display sign  26 . In the embodiment illustrated in  FIG. 1 , the upstream reader  12  is located downstream from the display sign  26 , although in other embodiments it may be upstream or at the same location. The downstream reader  16  is located a distance d down the roadway  24  from the upstream reader  12 . The distance d may be any distance over which traffic information is to be gathered, but is typically in the range of 1 to 10 kilometers.  
      The display sign  26  is positioned over or next to the roadway  24  such that drivers of vehicles  20  coming towards the display sign  26  may read the information shown on the display sign  26 . The display sign  26  may be an electronic message board that is controlled by a display processor  28  to display a selected message. The message may be varied by the display processor  28  from time-to-time. By way of example only, the displayed message may be used to warn of congestion in the roadway  24 , advise of upcoming construction or lane-closures, suggest good driving habits, and/or display time or temperature information, among other things. In particular, the display sign  26  may indicate the travel time from the location of the display sign  26  to a location further down the roadway  24 . For example, it may indicate the travel time to reach the next exit, next toll plaza, next highway interchange, next municipality, etc.  
      The upstream reader  12  includes an upstream antenna  14  having a coverage zone that includes at least a portion of the roadway  24 . The upstream reader  12  and antenna  14  may communicate with transponders  22  that enter the coverage zone. In particular, the upstream reader  12  may excite the upstream antenna  14  so as to propagate a trigger or wake-up signal within the coverage zone. If a transponder  22  is located within the coverage zone, it responds by sending a response signal, which is received by the upstream antenna  14  and processed by the upstream reader  12 . The upstream reader  12  then provides the transponder  22  with a time stamp. The upstream reader  12  may also provide the transponder  22  with a location ID. The time stamp and location ID may be contained in a programming signal broadcast by the upstream antenna  14  to the transponder  22 . The time stamp and location ID are stored in memory by the transponder  22 .  
      The downstream reader  16  includes a downstream antenna  18 . The downstream antenna  18  has a coverage area over at least a portion of the roadway  24 , such that it is capable of communicating with transponders  22  that pass through the coverage area. The downstream reader  16  causes the downstream antenna  18  to broadcast an interrogation signal and process any response signals. A transponder  22  in the coverage area responds to the interrogation signal by sending a response signal containing transponder information, such as the time stamp and location ID stored in the transponder memory.  
      On obtaining the location ID and time stamp, the downstream reader  16  compares the present time with the time stamp to obtain a travel time between the two readers  12  and  16  over the distance d. Travel times may be accumulated by the downstream reader  16  and may be used to determine an average travel time for the distance d of the roadway  24 . It will be appreciated that the downstream reader  16  includes a timer or clock that provides a source for the present time.  
      The downstream reader  16  includes a communication antenna  30 . The communication antenna  30  enables RF communication with the display processor  28 , which includes a corresponding communication antenna  32 . The RF communication between the downstream reader  16  and the display processor  28  may be a direct wireless transmission, such as a microwave signal. In another embodiment, as described further below, a wireless network like a cellular network, may be used for communications between the downstream reader  16  and the display processor  28 .  
      The downstream reader  16  may send average travel time information to the display processor  28  for display on the display sign  26 . In one embodiment, the downstream reader  16  sends the average travel time information periodically. In another embodiment, the downstream reader  16  sends average travel time information only when an updated average travel time differs from the current average travel time by more than a threshold amount, such as, for example, 5% or 10%. In one embodiment, the downstream reader  16  may be configured to send updated information, if required, in fifteen minute intervals. Other variations in reporting procedure and/or frequency will be understood by those of ordinary skill in the art.  
      It will be appreciated that the display sign  26  may display information other than the average travel time in connection with the traffic information gathered by the system  10 . For example, knowing the distance d the system  10  may determine an average traffic speed for the roadway  24 , which may then be displayed on the display sign  26 . In another example, the average travel time may be used to provide a qualitative assessment of the traffic, such a “slow”, “very slow”, “normal”, etc., for display on the display sign  26 . Other possibilities will be appreciated by those of ordinary skill in the art.  
      The protocol for communication between transponders  22  and the readers  12  and  16  is a two-way radio frequency (RF) communications protocol. In one embodiment, the RF carrier signals used are about 915 MHz and have a data bit rate of about 500 kbps.  
      In another embodiment, the display sign  26  may be located at or near the downstream reader  16 , in which case the downstream reader  16  may not include a communication antenna  30  for sending average travel time information to the display processor  28 . The display processor  28  may, in one embodiment, be incorporated within the downstream reader  16 . In such embodiments, the distance between the upstream reader  12  and the downstream reader  16  may be relatively small, such as, for example, about 50 meters.  
      In yet another embodiment, the downstream reader  16  may send travel time information other than the average travel time to the display sign  26 . For example, a qualitative description of the traffic conditions or a code corresponding to such a qualitative description may be determined at the downstream reader  16 , and this description or code may be transmitted to the display sign  26  via RF communications. Other variations will be appreciated by those skilled in the art.  
      In yet a further embodiment, the downstream reader  16  may send the travel time information to a remote location other than the display sign  26 . For example, the downstream reader  16  may send the travel time information to a central office or computer for processing and/or storage. The central office or computer may accumulate the travel time information from various locations to perform traffic analysis, planning, management, etc. In some embodiments, the central office or computer may send travel time information to the display sign  26  for display.  
      Reference is now made to  FIG. 2 , which shows a table  100  detailing a transponder information data structure according to a pre-defined ETC protocol. The pre-defined ETC protocol specifies a data structure (also referred to as a data string) containing a plurality of fields. The data structure or string may be stored in memory within the transponder.  
      The data string contains three types of fields, as indicated under the table heading “Field Type”: Factory fields, Agency fields, and Reader fields. The field type identifies the entity that is permitted to alter the contents of the particular field. The Factory fields are set by the manufacturer of the transponder and tend to relate to the characteristics of the transponder, i.e. the data in the Factory fields is read-only data. The Agency fields are fields that may be set by the agency deploying the transponders, and thus relate to the vehicle or customer characteristics. The Reader fields are fields that may be altered by individual readers. In one embodiment, these are fields that are used to track the movement of the transponder within an electronic toll collection system. For example, the Reader fields include fields for recording entry and exit points and the time and dates of entry or exit. In other embodiments, the Reader fields may include account information which a reader verifies and then debits in an automatic parking system, automated drive-through retail outlet, or other mobile commerce system.  
      The table  100  shown in  FIG. 2  contains a number of Factory fields, including a Header field  101 , a Tag Type field  102 , an Application ID  104 , a Group ID  106 , an Agency ID  108 , and a serial number field  110 . Because the Factory fields are set by the manufacturer, the content of the fields is well defined and predictable. For example, within the Tag Type field  102  and the Application ID field  104 , there may be certain bit combinations that have not been used for known tags or applications. These bit combinations may be reserved for future use, as the categories of tags and applications develop and expand. The Tag Type field  102  may be used by the reader to distinguish between two or more different classes of transponders.  
      The Agency fields include a Vehicle Type field  112 , a Vehicle Axles field  114 , a Vehicle Weight field  116 , a Revenue Type field  118 , a Mounting location field  120 , and an Agency Data field  122 .  
      The Reader fields may include a set of timing fields  124  and a set of toll collection fields  126  that may be altered by a roadside reader. Within the toll collection fields  126 , there may be an agency data field  130  provided as a ‘scratch pad’. The last field in the data string illustrated by the table  100  is a Reader-class Error Check field  128  containing a 16-bit cyclic redundancy code (CRC). The Error Check field  128  is used by the reader to verify that data from the transponder has been received correctly.  
      From the table  100  in  FIG. 2 , it will be observed that the pre-defined protocol defines a data string containing 247 bits, including 47 bits in Factory fields, 47 bits for Agency fields, and 153 bits for Reader fields.  
      In an ETC operation, when a reader interrogates (i.e. triggers) a transponder, the transponder responds by transmitting a response signal containing the data string stored in local memory. In other words, the transponder sends the contents of its local memory to the reader. The reader then makes any modifications necessary to the Reader fields and transmits the modified data string back to the transponder. In one embodiment, the reader may not transmit the whole string, since many fields cannot be altered. The transponder receives a program signal containing the data string (or a portion of the data string) that includes the modified information. The transponder then stores the Reader fields from the program signal in place of the previous Reader fields in its local memory. The reader may then transmit a further trigger signal to re-read the transponder memory to verify that the transponder information has been updated in accordance with the modifications made by the reader.  
      In one embodiment of the present invention, wherein the traffic information system  10  is a part of an ETC system, the traffic information system  10  relies upon the timing fields  124  and/or toll collection fields  126  for time stamp information and location IDs in connection with the upstream reader.  
      In some embodiments, the time stamp and location ID from the upstream reader may be stored in the agency data field  130 , i.e. the scratch pad. This may especially be the case where the traffic information system  10  is not a part of the ETC system and/or is implemented on a non-toll roadway. In such embodiments, the traffic information system  10  may have sufficient permission to write data to the scratch pad, but may not have sufficient authorization to alter the other fields in the transponder data structure.  
      Reference is now made to  FIG. 3 , which shows a block diagram of an alternative embodiment of the traffic information system  10 . In the embodiment shown in  FIG. 3 , the display processor  28  ( FIG. 1 ) and upstream reader  12  are integrated at a common location. The upstream antenna  14  is a directional antenna having a coverage zone  40  having an approximately elliptical shape and covering a portion of the roadway  24 . The downstream reader  16  features two downstream antennas  18   a  and  18   b  having adjacent coverage areas  42   a  and  42   b  spanning the roadway  24 . It will be understood that although the coverage zones  40 ,  42   a , and  42   b  are illustrated as having elliptical shapes, in reality the actual shapes of the coverage zones  40 ,  42   a , and  42   b  will typically not be elliptical, but will have a shape that is dependent upon a number of factors, including RF reflections or interference caused by nearby structures, the antenna pattern and mounting orientation. The upstream reader  12  may include more than one antenna in other embodiments. The downstream reader  16  may include fewer than two or more than two antennas in other embodiments.  
      The downstream communication antenna  30  and upstream communication antenna  32  communicate via a wireless network  46 . The wireless network  46  includes an upstream access point  44   a  and a downstream access point  44   b . In one embodiment, the upstream and downstream access points  44   a ,  44   b  may be the same, although in many cases they will be different. The wireless network  46  may include a wireless communications network or combination of interconnected networks, including, without limitation, Mobiltex™, DataTAC™, AMPS, TDMA, CDMA, GSM/GPRS, PCS, EDGE, UMTS or CDPD. The wireless network  46  may further include the public switch telephone network (PSTN). Communications with the wireless network by the upstream and downstream readers  12 ,  16  use wireless protocols familiar to those of ordinary skill in the art. In one embodiment, the communications between the readers  12 ,  16  through the wireless network  46  are made by way of short message service (SMS).  
      Reference is now made to  FIGS. 4 and 5 , which show block diagrams of embodiments of the upstream reader  12  and the downstream reader  16 , respectively. It will be appreciated that the upstream reader  12  shown in  FIG. 4  integrates the functions of operating the display sign  26  ( FIG. 1 ). In some embodiments, these functions may be embodied in a separate device/system.  
      The upstream reader  12  includes a processor  50  operating under program control. The processor  50  may be implemented as a general purpose processor configured to perform the operations and functions described herein by way of stored program control. Suitable programming of a general purpose microprocessor or microcontroller will be within the skill of a person of ordinary skill in the art. Program instructions may be stored in firmware and/or temporary memory. In other embodiments, the processor  50  may be implemented through a configurable logic device, such as a field programmable gate array. In other embodiments, the processor  50  may be implemented by way of an application-specific integrated circuit. In some embodiments, the processor  50  may include a combination of integrated and discrete components. The range of implementation possibilities will be understood by those skilled in the art.  
      The processor  50  interacts with a time stamp antenna transceiver  52 . The time stamp antenna transceiver  52  generates the RF excitation signals for output to the upstream antenna  14  ( FIG. 3 ) for interrogating transponders  22  ( FIG. 3 ) in the upstream coverage zone  40  ( FIG. 3 ). It also receives incoming RF signals from the upstream antenna  14 , where the incoming RF signals are electric signals induced in the upstream antenna  14  by RF transmission from the transponders  22 . The time stamp antenna transceiver  52  performs the protocols and functions for engaging in interrogation and response communications with the transponder  22  through the upstream antenna  14 .  
      The time stamp antenna transceiver  52  also excites the upstream antenna  14  to send a programming signal to the transponder  22  instructing the transponder  22  to store a time stamp in memory. The time stamp may be obtained by the time stamp antenna transceiver  52  from the processor  50  or other timing source. Upon detecting the presence of a transponder  22  in the coverage zone  40 , the time stamp antenna transceiver  52  sends the programming signal containing the time stamp to the transponder  22 . In one embodiment, the programming signal further includes a location ID corresponding to the upstream reader  12 .  
      The upstream reader  12  may further include the communication antenna transceiver  54  and the display driver  56 . The display driver  56  outputs a display signal  57  to the display sign  26  ( FIG. 1 ) to control what is displayed on the display sign  26 . The display signal  57  may take one of many formats, depending on the nature of the display sign  26 , as will be appreciated by those or ordinary skill in the art.  
      The content to be displayed on the display sign  26  may be obtained, at least in part, by the display driver  56  from the processor  50 , the communication antenna transceiver  54  and/or a memory (not shown). The obtained content may include content received through the communication antenna transceiver  54  from the downstream reader  16 , such as the average travel time between the upstream reader  12  and downstream reader  16 .  
      The communication antenna transceiver  54  performs the RF signalling and protocols through the upstream communication antenna  32  necessary to receive data communications from the downstream reader  16 , either directly or through the wireless network  46  ( FIG. 3 ). In one embodiment, the communication antenna transceiver  54  enables the reception of SMS messages through the wireless network  46  ( FIG. 3 ).  
      It will be appreciated that in some other embodiments, the upstream reader  12  may have only the processor  50  and the time stamp antenna transceiver  52 , and that the display driver  56  and communication antenna transceiver  54  may be embodied in a physically separate device or system.  
      The downstream reader  16  includes a processor  60  and an interrogation antenna transceiver  62 . The interrogation antenna transceiver  62  performs the RF signalling and protocols necessary to excite the downstream antenna(e)  18  and interrogate any transponders  22  ( FIG. 3 ) in the downstream coverage zone(s)  42  ( FIG. 3 ). Response signals from the transponders received at the antenna(e)  18  induce an incoming signal that is received by the interrogation antenna transceiver  62 . The response signal, and thus the incoming signal, include information stored in memory in the transponder, including the time stamp and location ID inserted at the upstream reader  12  if the transponder  22  passed through the upstream coverage zone  40 .  
      The processor  60  or the interrogation antenna transceiver  62  may perform suitable demodulation and decoding of the incoming signal to extract the transponder information and, in particular, the timestamp and location ID. The processor  62  may filter out information that does not contain the correct location ID or, if monitoring the travel time between various points, it may group or associate the time stamp information according to the location ID.  
      The downstream reader  16  includes a time travel determination module  66  and a memory  68 . The time travel determination module  66  accumulates time information in the memory  68  and makes a determination as to the approximate travel time between the upstream reader  12  and the downstream reader  16 . The determination may be based upon an average of the travel times associated with a number of transponders. It may also be a weighted average or a mean value. Upon detecting a transponder  22  and obtaining its time stamp information, the travel time determination module  66  may calculate the travel time for the particular transponder  22 . This calculation involves comparing the time stamp with a current time. The current time may be obtained from the processor  60  or another timing source. It will be appreciated that the timing sources in the upstream reader  12  and the downstream reader  16  are to be synchronized when initialized.  
      When the travel time determination module  66  calculates a travel time  70  (shown individually as  70   a ,  70   b , . . . ,  70   n ) for a particular transponder  22  it stores the travel time  70  in the memory  68 . Once it has accumulated a sufficient number of travel times  70 , the travel time determination module  66  may determine an approximate or average travel time. The average travel time may be sent to the upstream reader  12  by way of a communication antenna transceiver  64 . The communication antenna transceiver  64  performs the necessary RF signalling and protocols to transmit the average travel time to the upstream reader  12  and/or the wireless network  46  ( FIG. 3 ). In one embodiment, the communication antenna transceiver  64  uses SMS messaging to send the average travel time. In other embodiments, the communication antenna transceiver  64  may send travel time information other than the calculated average travel time. For example, the communication antenna transceiver  64  may send a quantitative travel conditions description or a code relating thereto.  
      It will also be appreciated that, in some embodiments, the communication antenna transceiver  64  may send the travel time information to a location other than the display sign  26  ( FIG. 1 ) or the upstream reader  12 . As noted above in connection with  FIG. 1 , the travel time information may be sent to a central office or computer for storage, analysis, or retransmission.  
      Reference is now made to  FIG. 6 , which shows, in flowchart form, an embodiment of a method  200  of obtaining traffic information. As described above, the method  200  involves a roadway having an upstream reader and a downstream reader. The roadway also has a display sign located in the vicinity of the upstream reader for displaying a message regarding the traffic conditions regarding the section of the roadway between the upstream reader and the downstream reader. The method  200  relates to the operations performed by the downstream reader.  
      The upstream reader includes a coverage zone spanning a portion of the roadway and it instructs one or more vehicle-mounted transponders passing through the coverage zone to store a time stamp and a location ID. It will be appreciated that it is not necessary that every transponder in the roadway be programmed to store a time stamp. Accordingly, the coverage zone need not span the entire roadway, although in some embodiments it may. If the upstream reader and its coverage zone are placed on an open-road section of the roadway, rather than at a gated section (like a toll plaza), then the upstream reader and its coverage zone are configured so as to be able to communicate with transponders travelling at highway speeds.  
      The downstream reader has a coverage zone that spans a portion of the roadway. Again, it is not necessary that the entire roadway be covered, although in some embodiments it may be entirely covered.  
      The method  200  begins in step  202  with the detection of a vehicle in the downstream coverage zone. This may involve transmission of a trigger or wake-up signal from the downstream antenna into the coverage zone and the subsequent reception of a response signal at the downstream antenna from a transponder in the zone. In step  204 , the downstream reader obtains transponder information from the transponder memory. In some embodiments, the transponder response signal contains a data string having the contents of its memory, including any information stored in its “scratch pad” section of memory. If the transponder was programmed at the upstream reader, then the transponder information includes the time stamp and location ID.  
      In step  206 , the downstream reader determines whether the transponder information indicates that the transponder was programmed with a time stamp at the upstream reader. It may do this on the basis of the location ID. If the location ID does not correspond to the known location ID of the upstream reader, then the transponder does not contain a relevant time stamp. If the transponder was not programmed at the upstream reader, then the method  200  returns to step  202  to await detection of another transponder. Otherwise, the method  200  continues to step  208 , wherein the transponder time stamp is compared to the current time to determine a travel time between the upstream reader and the downstream reader.  
      Once the travel time is determined in step  208  it may be stored in memory in the downstream reader in step  210 . In step  212 , the downstream reader determines whether to calculate/update the average travel time. This determination may be based upon accumulating a sufficient number of travel times. It may also or alternatively be based upon predetermined periodic updating. For example, the reader may determine an updated travel time every fifteen minutes. Various combinations and/or modifications will be appreciated by those skilled in the art. Howsoever the downstream reader determines whether or not an update is warranted, if it decides not to update, then the method  200  returns to step  202  to await detection of another transponder. If an updated average travel time is required, then in step  214  the average travel time is calculated. In one embodiment, the average travel time is calculated as the average of all travel times stored in the memory. The average may be based only upon travel times received a preceding time period, such as fifteen minutes, to render the information as current as may be required by the circumstances. In other embodiments, a mean travel time may be determined. In yet other embodiments, a weighted average may be used to determine the average travel time, with greater weight being given to more recent travel times and diminishing importance given to older travel times. Other variations will be understood by those skilled in the art.  
      After calculating the updated average travel time in step  214 , the downstream reader determines whether to send the updated travel time to the display sign. This determination may be based, for example, upon whether the updated travel time varies from the previously calculated travel time by more than a threshold amount. If the variation is not deemed to be significant enough, then the update may not be sent and the method  200  may return to step  202 . If the variation exceeds the threshold (for example, a variation of more than 5 or 10%), then in step  218  the downstream reader transmits the updated average travel time to the display sign. This may, for example, include sending a digital electronic message, such as an SMS message, an e-mail, an instant message, or other such communication, through a wireless network connection. Various other mechanisms for communicating the updated travel time from the downstream reader to the display sign will be apparent to those skilled in the art.  
      References herein to calculation or determination of an “average travel time” will be understood to be broader than mere averaging. In some embodiments, the calculation or determination may involve determining a mean value, a weighted average, or other approximations, to arrive at a travel time for the roadway.  
      It will also be appreciated that any reference herein to sending a “travel time” or “average travel time” or “travel time information” from the downstream reader to the display sign or display processor are broad enough to include sending quantitative measures or approximate travel time and/or sending qualitative assessments of approximate travel time or traffic conditions.  
      Referring again to  FIG. 3 , in another embodiment, the system  10  may include a lane position determination component. Appropriate methods and systems for determining the lane position of a transponder are described in U.S. Pat. No. 6,219,613, issued Apr. 17, 2001, and U.S. patent application Ser. No. 11/176,758, filed Jul. 7, 2005, and owned in common herewith, the contents of which are hereby incorporated by reference. Using such a lane-position determination component, the system  10  may track travel times on a lane-by-lane basis. In other words, the travel times determined, stored, and averaged by the downstream reader  16  may be grouped on a lane-by-lane basis, so that an average travel time may be determined for each lane. This aspect may allow the system  10  to recognize lane-specific slowdowns or problems. It may then display content on the display sign  26  that specifically warns of the lane-specific problem. For example, it may be able to deduce a blockage or accident in a particular lane from the lane-based travel time information.  
      The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Certain adaptations and modifications of the invention will be obvious to those skilled in the art. Therefore, the above discussed embodiments are considered to be illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.