Patent Publication Number: US-6662099-B2

Title: Wireless roadway monitoring system

Description:
FIELD OF THE INVENTION 
     The invention relates generally to roadway monitoring systems and more specifically to in-road, wireless roadway monitoring systems. 
     BACKGROUND OF THE INVENTION 
     The level of traffic congestion on roadways is a serious problem imposing excessive burdens upon commuters in terms of commute time, stress, fuel consumption and vehicle wear and tear. Reports suggest that the amount of congestion-induced delay experienced by the average commuter in a large city such as Los Angeles or Boston has more than doubled over a span of less than two decades. 
     Given the practicalities of driving habits and limited capital resources, the most realistic near-term approaches to reducing road congestion involve improvements to current roadways. For example, an initiative underway at the National Intelligent Transportation Systems (ITS) utilizes information technology to make better use of existing roads. One particularly compelling system envisioned by ITS workers is the Automated Traveler Information System (ATIS). Before embarking on a trip, drivers could consult a Web page to obtain accurate trip time estimates for various departure times and modes of transportation. Upon embarking, a dynamic route guidance system would provide them with turn-by-turn directions based on up-to-the minute information about roadway speeds and congestion levels. 
     At the very least, this type of system would allow drivers to make better route decisions, to be confident that they were taking the most efficient route, and to plan their activities around traffic delays. One of the largest obstacles to the implementation of this type of system is the shortage of accurate, real-time traffic data. Currently available traffic sensor systems (e.g., video, sonar, radar, inductive, magnetic, capacitive, polyvinylidine fluoride (PVDF) wire, pneumatic treadle) use significant electrical power, so each sensor must be connected to a power distribution network. For sensors that are installed on electrical poles (video, sonar, radar), the installation cost per sensor can be several hundred dollars. For cabled sensors that are installed in the roadway receiving power and/or communicating via cables, (inductive, magnetic, PVDF wire, capacitive, pneumatic treadle) the installation cost per sensor can be several thousand dollars. Inroad sensors are currently utilized in certain “trouble spots” because they are very accurate, provide direct information with very little ambiguity, can monitor road conditions (e.g., presence of ice), and do not require a human operator. But their high cost discourages the widespread deployment that would be necessary for large-scale monitoring networks. 
     SUMMARY OF THE INVENTION 
     In general, the present invention provides a low-power, wireless, in-road traffic sensor system using sensors that are small, low-cost, and rugged. The sensors may be capable of measuring the speed of passing vehicles, in addition to measuring information about roadway conditions, e.g., wet or icy. Each sensor may be configured to consume so little power that it can operate from a small internal battery for up to 10 years. The low cost and ease of installation allows communities to outfit entire roadway systems, thus providing a viable near-term solution for managing roadway traffic congestion. 
     Accordingly, in a first aspect, the invention comprises a wireless roadway sensor configured for installation beneath a roadway surface. The sensor includes a sensing element capable of sensing roadway conditions, such as the presence of a vehicle on the roadway, an average speed of vehicles on the roadway, types of vehicles on the roadway, and water and/or ice on the roadway. The sensor also includes a wireless transmitter for periodically broadcasting sensed information to a remote destination. 
     In one embodiment, the sensor includes a magnetic sensing element for sensing vehicles on the roadway through perturbations in the ambient magnetic field. In another embodiment, the sensor includes a capacitive sensor element for sensing precipitation on the roadway through the electrical measures, such as the dielectric constant and the conductivity at the roadway surface. In yet another embodiment, the sensor includes a temperature sensor element for sensing the temperature of the roadway and, in conjunction with the precipitation sensor, inferring the presence of road-surface ice. 
     In another aspect, the invention comprises a wireless roadway sensor that includes a sensing element for sensing a roadway condition and a wireless transmitter for transmitting the sensed information to a remote destination. The wireless transmitter communicates with the sensor and periodically broadcasts the sensed information on a communication channel using a randomized multiplexing scheme. The randomized multiplexing scheme allows the channel to be shared with other sensors broadcasting in accordance with the scheme. 
     In one embodiment, the transmitter is a narrowband radio-frequency (RF) transmitter. In another embodiment, the transmitter is configured to modulate a RF carrier signal using frequency-shift-keying modulation. In yet another embodiment, the sensor is configured to use a receiverless protocol, further reducing its power consumption. 
     In yet another aspect, the invention comprises a wireless roadway sensing and information-integration system. This system includes multiple sensors distributed across a roadway system. The sensors are organized into sets each including one or more sensors. Each of the sensors includes a sensing circuit for sensing at least one roadway condition and a wireless transmitter for periodically broadcasting the sensed information. The system also includes a number of concentrators for receiving the sensor broadcasts, whereby each concentrator receives broadcasts from the sensors of one of the sets. The system also includes a computer in communication with the concentrators. The computer is configured to accumulate and organize the sensed information obtained by the sensors. 
     In one embodiment the computer determines traffic volume through vehicle counts reported by the sensors. In another embodiment, the computer determines alternate routes responsive to traffic congestion being sensed along an initially-planned route. In yet another embodiment, the computer includes a Web server communicating over the Internet for providing the sensed roadway information responsive to Web client requests. 
     In yet another aspect, the invention comprises a method for controlling traffic whereby a sensor is installed beneath a roadway surface for sensing a roadway condition. The sensor, in turn, transmits information relevant to the sensed condition through periodic wireless broadcasts to a remote receiver for actuating a traffic-controlling device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is pointed out with particularity in the appended claims. The advantages of the invention may be better understood by referring to the following description taken in conjunction with the accompanying drawing in which: 
     FIG. 1 is a block diagram depicting an embodiment of the invention; 
     FIG. 2 is a more detailed block diagram depicting the embodiment of the invention shown in FIG. 1; 
     FIG. 3 is a block diagram depicting the transmitter of the embodiment shown in FIG. 1; 
     FIG. 4 is a flow chart of an embodiment of a method in accordance with the invention; 
     FIG. 5 is a block diagram depicting the operational states of the embodiment of the invention shown in FIG. 1; 
     FIG. 6 is a block diagram depicting a traffic monitoring and reporting system embodiment of the invention; 
     FIG. 7 is a flow chart of an embodiment of a method in accordance with the invention shown in FIG. 6; and 
     FIG. 8 is a block diagram depicting a traffic monitoring and control embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     1. Roadway Sensor 
     Referring to FIG. 1, one embodiment of an in-road traffic sensor  10  includes a vehicle sensor  24 , a transmitter  30  and an antenna  32 . In some embodiments, the in-road traffic sensor  10  includes additional sensors shown in phantom, such as a water sensor  22  for sensing the presence of precipitation on the roadway, a temperature sensor  26  for sensing the roadway temperature and a vibrational sensor  28  for sensing the vibrations of passing vehicles on the roadway. The temperature sensor  26  may be used in conjunction with the water sensor  22  to detect the presence of ice on the roadway, while the vibrational sensor  28  may be used to categorize passing vehicles through their vibration signatures (e.g., differentiating between automobiles, motorcycles and trucks). 
     Each of the sensors  22 ,  24 ,  26 ,  28  (generally  20 ) is in electrical communication with the transmitter  30 , and each provides an output signal relating to the respective sensed information. Generally, the transmitter  30  transforms the information received from the sensors  20  into a form suitable for wireless communication via the antenna  32 , and broadcasts the transformed information to a remote destination through wireless transmissions. The sensor information is typically available as baseband electrical signals, such as voltage or current levels, or sequences of binary digits, or bits, of information. 
     In general, the antenna  32  may be any transducer capable of converting electrical into wireless broadcast signals. Examples of transducers include antennas, such as those typically used in wireless radio frequency (RF) communications; electrical-optical converters, such as light emitting diodes, lasers, photodiodes; and acoustic devices, such as piezoelectric transducers. In a preferred embodiment, the antenna  32  is an electrical antenna  32 , designed for operation in the frequency range between 30 MHz and 3,000 MHz, generally known as the ultrahigh frequency (UHF) band. The UHF frequency band is particularly well suited to the in-road sensor  10  application because UHF circuits and components are relatively small in size and consume relatively low power. For example, physical limitations in antenna construction typically result in antennas being scaled to approximately one-half the wavelength of operation. The half-wavelength ranges from 5 meters to 5 cm in the UHF band. 
     In a particularly preferred embodiment, the antenna  32  is a microstrip patch antenna  32  operating within the frequency range of 902 MHz to 928 MHz. Microstrip patch antennas  32  are relatively small compared with other resonant antennas, such as dipole antennas, operating over the same frequency range. Microstrip patch antennas  32  are also rugged, easily designed and fabricated and relatively inexpensive. Although it may be desirable to operate at even higher frequencies, other considerations, such as government regulation, may stand in the way. For example, transmitting RF signals within certain frequency bands may be prohibited altogether, while use of other frequency bands may be restricted to special users, such as airlines or the military. Operation within the 902 MHz to 928 MHz frequency band is largely available for industrial, science and medical applications. 
     The in-road traffic sensor  10  may be configured for installation beneath a roadway. The sensor  10  is particularly well suited to such an installation because of its compact size and its ability to operate without external interconnects, e.g., connections to the electrical power grid or to a receiver. Furthermore, the sensor  10  may be configured in a single, self-contained and environmentally-sealed package. The sensor  10  may be installed completely beneath the roadway surface or partially beneath the roadway surface, with some portion of the sensor  10  (e.g., the antenna  32 ) exposed to the road surface. The sensor  10  may be installed during the initial surfacing of a roadway, or through a retrofit of an existing roadway surface. With currently available components, a sensor  10  may be configured to have a volume of less than one cubic inch. Installation of such a sensor  10  requires minimal disturbance to an existing roadway. Other embodiments are possible, e.g., in which the sensor is installed on top of the roadway, similar to roadway reflectors and lane markers in multi-lane roads; but surface installations may not be advisable where the roadways are cleared by snow plows. 
     In more detail, referring to FIG. 2, one embodiment of an in-road traffic sensor  10  includes a controller  40  in communication with each of the sensors  20  and with the transmitter  30 . The controller  40 , the sensors  20  and the transmitter are also connected to a power source (not shown) such as an internal or parasitic electrical power source. Interconnections to the power source may be established through one or more power control devices  44 ,  44 ′,  44 ″ (generally  44 ) offering the advantage of controlling and sharing power in an efficient manner. In one embodiment, the vehicle sensor  24  includes a vehicle sensing element  42  (“sensor A”) and a signal conditioning circuit  43  receiving signals from the sensing element  42 . The vehicle sensing elements may also require a calibration device  45  to provide a bias, or offset, or to perform a calibration function for the sensing element  42 . The vehicle sensor  24  may also include a second vehicle sensing element  42 ′ (“sensor B”), shown in phantom, to provide improved reliability through redundancy or, more typically, to support additional sensing capabilities, such as sensing the direction and average speed of vehicles passing the sensor  10 . 
     The controller  40  typically performs central control functions for the in-road traffic sensor  10 . The controller  40  may also perform other overhead functions, such as input/output (I/O) communications control, data formatting, power management, timing and synchronization. 
     In one embodiment, the signal conditioning circuit  43  includes an instrumentation amplifier having a low-voltage supply requirement and having a fast settling time; a suitable device is the INA155 component (Burr-Brown device number) manufactured by Texas Instruments Inc., Dallas, Tex. For embodiments where the sensor  42  generates a differential signal, the instrumentation amplifier also converts it to a single-ended signal. In some embodiments, the output from the instrumentation amplifier is amplified further by an operational amplifier, such as device number OP162, manufactured by Analog Devices, Norwood, Mass. 
     As previously mentioned, the vehicle sensor  24  receives power from the local electrical power source through the power control device  44 . One power control device  44  may provide power to both the amplifier circuit  43  and the vehicle sensing element  42 , or separate power control devices  44  may be used. The vehicle sensing element  42  receives electrical power and senses a roadway condition that varies in relation to the presence of a vehicle on the roadway, providing an electrical output signal relating to the sensed information. In some embodiments, the output signal from the vehicle sensing element  42  may require conditioning, such as amplification, filtration, or conversion, such as analog to digital (A/D) conversion. Where signal conditioning is required, the vehicle sensing element output signal may be input into the amplifier circuit  43 . The controller  40  receives the conditioned vehicle sensing signal and may perform processing thereon. Signal processing may include determining the presence of a vehicle, counting the numbers of sensed vehicles and buffering any information to be broadcast. In one embodiment, the controller  40  provides an output signal corresponding to the vehicle sensor output signal to the transmitter  30 . The controller  40  may also provide timing, monitoring, and control information to the transmitter  30  to frequency tune the transmitter, to control the periods of broadcast, and the like. The transmitter  30  broadcasts the information provided by the controller  40 , under the control of the controller  40 , to a remote destination. The transmitter may also receive electrical power through a controllable power device  44 ″. The transmitter  30  may be configured to transmit information periodically, such as when an event is sensed, e.g., a vehicle passing the sensor  10 , or periodically after some time delay where sensed information is buffered within the sensor  10 . 
     Vehicle sensing elements  42  may require the application of an external signal for calibration or to establish an offset bias. These functions are provided by the calibration device  45 , which is in communication with the vehicle sensing element  42  and the controller  40 . The calibration device  45  receives an input signal from the controller  40  and in response applies an output signal to the vehicle sensor element  42  in accordance with the needed calibration or offset function. 
     In one embodiment, the electrical power source for the sensor  10  is a battery (not shown) capable of powering the entire sensor  10 . In one embodiment, the electrical power is applied to the sensors  20  and to the transmitter  30  through the power control devices  44 . In a preferred embodiment, the battery is compact and capable of storing a substantial charge for a relatively long time, e.g., several years. In a preferred embodiment, the battery is a lithium battery such as a lithium thionyl-chloride battery. 
     The power control devices  44  receive input power from the power source, provide power to a load through an output, and are capable of being operated to control the amount of power delivered to the load. In some embodiments, the power control device  44  is a transistor. In a preferred embodiment, the power control device is a P-channel enhancement mode, metal-oxide semiconductor field effect transistor (MOSFET), such as device number Si2301 manufactured by Siliconix Inc., Santa Clara, Calif. The power control device  44  may be controlled by the controller  40  through a control port. It is advantageous to control the power to the different elements of the sensor  10  in order to limit the overall power consumption. In particular, dynamically redistributing power to the different elements of the sensor  10  preserves the limited available power from the power source. Indeed, an in-road traffic sensor  10  of the kind described herein might be capable of operating for up to ten years with a single, compact battery source. For example, where the transmitter transmits periodically, power is required during periods of transmission and not during idle periods. 
     In some embodiments, the in-road traffic sensor  10  is equipped with a second vehicle sensing element  42 ′, a second amplifier circuit  43 ′and a second power control device  44 ′. The second vehicle sensing element  42 ′and related components  43 ′,  44 ′are configured similarly to the first vehicle sensing element  42 . The second vehicle sensing element may be included to improve reliability by providing redundancy, or to allow for the computation of vehicle direction and average speed through two independent, spatially separated measurements. The other optional sensors  22 ,  26 ,  28  are shown in phantom and may be interconnected to the power source, to the controller  40  and to the transmitter  30  in a similar manner as the vehicle sensor  24 . 
     In operation, referring to FIG. 4, the sensors  20  senses a roadway condition, such as the presence of a vehicle, and/or the presence of water or ice on the road surface (step  100 ). Optionally, the sensors  20  may process the sensed information, or provide the sensed information directly to the controller  40  for processing, or processing may occur at both the sensors  20  and at the controller  40  (step  110 ). Processing may include signal conditioning, such as amplification, attenuation, or filtering; or signal conversion, such as A/D conversion. Processing may also include manipulation of the sensed information to determine other roadway conditions. For example, where the sensor is equipped with two vehicle sensing elements  42 ,  42 ′, processing may be used to determine the direction of traffic depending on which sensing element  42 ,  42 ′ first reports the presence of the vehicle. Processing may also be used to determine the average speed of a passing vehicle by dividing the baseline separation of the two sensors  42 ,  42 ′ by the time difference that the vehicle is sensed by each sensor  42 ,  42 ′. Additional processing may be used to determine the presence of surface water, ice or snow through capacitive measurements of the water sensor  22  and temperature measurements of the temperature sensor  26 . For example, ice will be detected if the water detector  22  detects the presence of surface water while the temperature sensor detects that the surface temperature is below the freezing point of water. Additionally, processing may include the characterization of vibrations sensed by the vibrational sensor  28  into vehicle classifications. 
     In an application where the sensor  10  periodically transmits information to a remote destination, the sensed and processed information may be temporarily buffered. At any instant of time, the transmitter may be either actively transmitting or not transmitting, or silent. During periods of transmission, the transmitter transmits some or all of the information from the buffer (step  130 ). Periodic transmissions are well adapted to applications where relatively small amounts of data are transferred and offer the advantages of both power conservation and efficient utilization of limited frequency bandwidth. In one embodiment, the transmitter uses a sparse time division multiple access (TDMA) multiplexing protocol to support multiple sensors  10  each sensor  10  transmitting sensed information to a remote destination on the same frequency (step  140 ). 
     1-a. Vehicle Sensing 
     In one embodiment, the vehicle sensing element  42  senses the presence of vehicles on the roadway by sensing perturbations to the ambient magnetic field. In a preferred embodiment, the vehicle sensing element  42  is an anisotropic magnetoresistive sensing element, such as device number HMC1021S, manufactured by Honeywell, Plymouth, Minn. Magnetoresistive sensing elements, when immersed in a magnetic field, convert the magnetic field into a voltage output, such as a differential output voltage. Typically, magnetoresistive sensing elements are relatively small (e.g., standard, 8-pin dual-inline package and smaller), low cost, highly reliable and capable of sensing low-level magnetic fields (e.g., 30 micro-gauss). Anisotropic magnetoresistive sensors are typically made from a thin film of nickel-iron (PERMALLOY) patterned onto a silicon wafer as a resistive strip. The HMC1021S device includes a Wheatstone bridge with one leg of the bridge having such a strip. When a potential of 3.0 volts is applied to the bridge, and the on-axis magnetic field strength can be read across the bridge as a voltage of 3.0 millivolts/gauss. Other suitable vehicle sensors include inductive sensors, pressure sensors, vibration sensors, optical sensors, and other active sensors communicating with the passing vehicles. 
     1-b. Environmental Sensing 
     Roadway environmental conditions amenable to detection in accordance with the present invention may include, for example, precipitation, ice, salinity, and vibration. Referring to FIG. 1, precipitation may be sensed with the water sensor  22 , whereas ice may be sensed with the water sensor  22  in conjunction with the temperature sensor  26 . The temperature sensor  26  senses the temperature of the roadway and provides an output signal to the transmitter corresponding to the sensed temperature value. In one embodiment the temperature sensor  26  is a calibrated thermocouple device. The thermocouple, when suitably biased, provides an output voltage that corresponds to the temperature of the thermocouple junction. In a preferred embodiment, the temperature sensor  26  is a precision analog output complementary metal-oxide semiconductor (CMOS) integrated-circuit temperature sensor, such as device number LM20 manufactured by National Semiconductor Corp. Santa Clara, Calif. In one embodiment, power may be provided to the temperature sensor  26  through the controller  40 . The output of the temperature sensor may be low-pass filtered and received by the controller  40 , which may convert the signal into digital form through an A/D converter. 
     In one embodiment, the water sensor  22  uses a capacitive element to infer the dielectric or conductive properties of the material above the sensor. This approach is well known to those skilled in the art and offers distinct advantages of detecting water reliably at low cost and without consuming a significant amount of power. The capacitance may be measured through a minimally-complicated circuit, such a circuit measuring high-to-low and low-to-high voltage transition times between the assertion of a signal on a microcontroller pin and the corresponding voltage transition at an associated sensor plate connected to the microcontroller pin across a high impedance (e.g., several MΩ). Other well-known capacitive measuring techniques may also be used, such as switched capacitor techniques, relaxation oscillator techniques, heterodyning techniques, transmit-receive coupling techniques, etc. 
     Additional information as to the condition of a roadway may be determined through a sensor configured to measure the conductivity at the roadway surface. In one embodiment, exposed capacitive leads are placed in contact with the road surface and may be used to sense the road-surface conductivity. Determination of the road-surface conductivity through such a contact method facilitates the inference of road-surface conditions, such as the presence of precipitation and/or whether the roadway has been treated, such as with an ice inhibitor (e.g., salt). In other embodiments, the roadway surface sensor  10  may be configured to measure the complex impedance of material on the roadway, e.g., through alternating current (AC) measurements, RF measurements or switched capacitor techniques, such as the QPROX sensor system manufactured by Quantum Research Group Limited, Pittsburgh, Penn. Time-varying measurement techniques such as these would preclude any need to expose conductive electrodes directly to the environment. 
     An vibrational sensor  28  may include a piezoelectric transducer sensing element converting pressure variations into electrical signals. The electrical signal may be amplified and conditioned, in a manner similar to that already described for the vehicle sensor  24 . Different categories of vehicle typically impart different vibrations to the roadway surface depending on such factors as the weight of the vehicle, the type of motor and wheels, etc. The output signal of the vibrational sensor  28  may be related to categories of vehicle based on, for example, peak or average amplitude values, the amplitude profile, the duration, and spectral content. Ranges of these parameters associated with different types of vehicle may be stored within sensor  28  in the form of a database, which is addressed when signals are detected. In some embodiments the vibrational sensor  28  may include an in-air or contact microphone, such as an electret microphone (e.g., the model EM9765-422 manufactured by Horn Industrial Co. Ltd., Shenzhen, Guangdong, China, or the model WM-54B, manufactured by Panasonic Industrial Company, Secaucus, N.J.). In other embodiments, accelerometers may be used to detect vibrations, such as the model ADXL202 dual-axis, low power, low voltage, digital output accelerometer, manufactured by Analog Devices. Other components and implementational details are described in Knaian,  A Wireless Sensor Network for Smart Roadbeds and Intelligent Transportation Systems  (graduate thesis on file at Massachusetts Institute of Technology), the entirety of which is hereby incorporated by reference. 
     In some embodiments, the vibrational sensor  28  may include a low power, or even passive (i.e., consuming virtually no power) acoustic or acceleration sensing element. The vibrational sensor  28  may be used to enhance the power conservation features of the in-road traffic sensor  10 . In such an application, the sensor  10  may operate in a default low-power operational mode, or inactive mode, where elements of the sensor, including the magnetic field sensing element, are normally inactive. When the vibrational sensor  28  senses through roadway vibrations that a vehicle may be approaching, the vibrational sensor  28  transmits a signal to other elements of the sensor  10 , e.g., to the microcontroller  40 , to activate the other elements of the sensor  10 . In this way, vibrations resulting from an approaching vehicle cause a suitably configured sensor  10  to activate and operate as previously described (e.g., sensing the vehicle through perturbations to the ambient magnetic field). The vibrational sensor  28  may also be configured to transmit a signal to the microcontroller  40  after some predetermined period of inactivity to resume low-power operation (e.g., return to a “sleep mode”). 
     1-c. Transmitter 
     Referring to FIG. 3, the transmitter  30  includes a buffer  50  for receiving and storing information from the sensors  20 . Alternatively, a buffer may be included within the controller  40  shown in FIG.  2 . The transmitter  30  also includes a modulator  51  for modulating a carrier signal with information derived from the sensors  20 . The transmitter  30  also includes a mixer  52  for translating the modulated signal to a desired RF frequency of operation, an amplifier  54  amplifying the transmitted signal to a sufficient signal strength to support wireless communications with the remote destination, a local oscillator  56  for supplying a reference signal, and a controller  58  for controlling the overall operation of the transmitter  30 . Alternatively, the functions of the controller  58  may be performed by the sensor controller  40  shown in FIG.  2 . 
     The buffer  50  receives sensed information from the controller  40 , and provides the sensed information as an output signal to the modulator  51 . The modulator  51 , in turn, is in communication with the RF amplifier  54  through the mixer  52 , and may be in electrical communication with the modulator  51  and the local oscillator  56  (interconnections shown in phantom). 
     The information received by the buffer  50  originates with the sensors  20 . The buffer  50  temporarily stores the received sensor information until the transmitter broadcasts the information. The modulator  51  receives a first signal containing baseband data received from the buffer  50 . The modulator  51  impresses the received baseband data of the first signal onto a second signal, which may be an intermediate signal having a dominant frequency component other than the baseband signal or the RF signal; the intermediate signal is transformed to an RF broadcast signal before exiting the transmitter  30 . Alternatively, the second signal may be the broadcast signal itself. For example, in an RF transmitter  30 , the baseband signal may be a relatively low-frequency signal, e.g., 2400 bits per second (bps). This signal is provided to the modulator  51  and the modulator, in turn, changes some aspect of an intermediate signal, such as an audio-frequency (10,000 Hz) tone, or the broadcast signal, such as a 928 MHz RF signal. The modulator  51  may change the amplitude, the frequency, or the phase of the intermediate signal according to the baseband data. 
     In a preferred embodiment, the transmitter  30  is a frequency shift keying (FSK) transmitter. The FSK transmitter  30  modulates a tone between two or more frequencies according to the value of the baseband data. For example, a baseband input of a binary “0” into the modulator  51  may result in an intermediate 10,000 Hz signal output. Likewise, a baseband input of a binary “1” into the modulator  51  may result in an intermediate 20,000 Hz signal. The modulator output is a signal having an instantaneous frequency of either 10,000 Hz or 20,000 Hz, depending on whether the output corresponds to a binary “0” or a binary “1”, respectively. Preferably the amplitude of the envelope of the modulator output signal is also substantially constant. The modulated intermediate signal at the output of the modulator  51  is translated to an RF broadcast signal suitable for broadcast through the antenna  32 . In some embodiments, the transmitter may be frequency agile, while in other embodiments, the transmitter may be a spread-spectrum transmitter, using such techniques as frequency hopping or code division multiple access (CDMA). 
     The mixer  52  has three ports: an intermediate frequency (IF) input port, a local oscillator (LO) input port, and an RF output port. The IF port of the mixer  52  receives the modulated intermediate signal from the modulator  51 . The LO port of the mixer  52  receives an RF reference signal from the local oscillator  56 . The mixer  52  produces an output substantially corresponding to the sum and difference of the signals at the IF port and the LO port (i.e., the local output signal frequency of the oscillator  56  and the intermediate signal frequency). 
     The amplifier  54  amplifies the RF broadcast signal to an amplitude suitable for wireless transmission to an intended external destination through the antenna  32 . The amplifier may be a standard RF amplifier and may include a filtration stage to filter any unwanted output products of the mixer  52 . For example, where the intermediate frequency is 10,000 Hz and the local oscillator  56  frequency is 928 MHz, the output of the mixer  52  would be 928.010 MHz and 927.990 MHz. The amplifier  54  filtration stage may attenuate the unwanted of the two mixer output signals (e.g., 927.990 MHz) while amplifying the other (e.g., 928.010 MHz). 
     Generally, operating multiple sensors  10  within the same general proximity may result in unwanted interference. For example, if two sensors  10  communicating with the same remote destination broadcast information at the same time and on the same frequency, neither signal may be discernable and the transmissions will be lost. Interference may be avoided by using multiplexing techniques, such as assigned frequencies or assigned broadcast intervals for individual sensors  10 . In one embodiment, the transmitter  30  is configured to operate according to a sparse-TDMA transmission protocol. The sparse-TDMA protocol includes a master time interval (e.g., 60 seconds) that is arbitrarily divided up into a number of time slots (e.g., 7693 time slots, each of 7.8 milliseconds duration). In one embodiment, each sensor  10  may randomly select a time slot and broadcast its information in that slot. With each transmitter  30  operating according to such a protocol, the probability of interference can be maintained at a sufficiently manageable level. 
     The transmitter  30  may be configured to inhibit a transmission responsive to the vehicle sensor  24  during the time that a vehicle is directly over the sensor  10 , since overhead vehicles can reduce the probability of reception of a wireless transmission at a remote destination. In some embodiments, the vehicle sensor  24  may transmit a signal to the transmitter  30 , or to the microcontroller  40 , indicating that a vehicle may be located on the roadway above the sensor  10 . The transmitter  30 , or the microcontroller  40  having received such a signal, may in turn respond by inhibiting normal transmissions. The inhibited transmissions may be stored and transmitted at a later time. 
     1-d. Receiver 
     In some embodiments, the in-road traffic sensor  10  includes a wireless receive capability. A suitably configured receiver receives wireless signals through the antenna  32  and converts the wireless signals into electrical signals. Such a receive capability is particularly useful for performing remote diagnostics or remote repair (e.g., receiving updated system firmware). Since the receive capability represents another power dissipation source, the receive capability may be configured to operate periodically. For example, the receiver may routinely operate only during a predetermined duration of time and according to a predetermined period (e.g., the receiver operates for five minutes each day at 12 o&#39;clock). Occasionally, any extended periods of operation that may be required, such as during a firmware upgrade, could be negotiated during the routinely occurring operational periods. 
     1-e. Vehicle Counting Algorithm 
     Referring to FIG. 5, in one embodiment, the in-road traffic sensor  10  includes a state machine for counting passing vehicles. The state machine may be driven by the variation in the vehicle sensor output signal with respect to a baseline value. Generally, the magnetic field will vary in a similar fashion for a vehicle passing over the sensor, increasing from a baseline value to a maximum excursion in one direction (e.g., positive), followed by an excursion to a similar maximum value, but to the opposite side of the baseline (e.g., negative). In one embodiment, the state machine begins in an untriggered state. When the signal deviates by more than a first threshold (“S TH     —     LHIGH ”) from the baseline, the state machine progresses to a half-triggered state. If the signal deviates by more than the same threshold, but on the opposite side of the baseline, the state machine progresses to the count state, and a counter may be advanced indicating that a vehicle has passed the sensor. Before the state machine can count another vehicle, it must be first returned to either the untriggered state or again to the half-triggered state. When the signal comes within a second threshold (“S TH     —     LOW ”), smaller than the first threshold, the state machine transitions to the untriggered state and available to repeat the process when the next vehicle passes. If the state machine is in the half-triggered state and the signal reduces below the second threshold for a period of time greater than a predetermined minimum, e.g., 500 milliseconds, without reaching the first threshold in the opposite side of the baseline, the state machine is returned to the untriggered state. The state machine may also return to the half-triggered state directly from the count state, if the signal deviates again to the opposite extreme. 
     In one embodiment, the baseline value is established during initial power on over a period of time, e.g., 10 seconds. When the state machine is untriggered, the measurement baseline is continuously adjusted to compensate for changes in the ambient magnetic field and to maintain measurement fidelity. For example, the measurement baseline may be adjusted upward by some amount, e.g., {fraction (1/10)} of a count per sample, if the signal is above the baseline and downward by some amount, e.g., {fraction (1/10)} of a count per sample, if the signal is below the baseline. When the state machine is in any state other than the untriggered state, the baseline may be adjusted in a similar manner, but using a smaller increment, e.g., {fraction (1/100)} of a count per sample. 
     2. Roadway Sensing System 
     Referring to FIG. 6, the in-road traffic sensors  10  may be used to monitor several roadway segments, or an entire roadway system. In a roadway sensing system, the sensors  10  provide information relating to traffic and roadway conditions to a central location where the data may be processed, stored and made available to serve several traffic management objectives. In one embodiment, groups of sensors indicated at  10   1 , . . .  10   n  are organized into sets (of n sensors each, for simplicity, it being understood that different sets may have different numbers of sensors) and installed across a roadway system. Each set contains one or more sensors  10 , and the sensor(s)  10   1 , . . .  10   n  of a set of sensors broadcasts sensed information to a common concentrator  60 . Generally, each of the concentrators serves one set of sensors  10 . Suppose, for example, that the system includes seven sets “a” through “g.” A concentrator  60   a  receives signals from sensor set a, i.e., sensors a, through an, while the last concentrator  60   g  receives signals from sensor set g, i.e., sensors g 1  through g n . The sensors  10  communicate with the concentrators  60  through wireless communications, allowing the concentrators  60  to be located remotely from the sensors  10 . The concentrators  60  may, for example, be located at an elevated vantage point such as on a telephone pole, or traffic signal pole. Placing the concentrators  60  at such convenient locations allows them to be powered remotely, e.g., by means of electrical power lines, rather than imposing an internal power requirement. 
     Each of the concentrators  60 , in turn, may communicate with a centrally located control center  62 . Communications between the concentrators  60  and the control center  62  may also be established with available telephone lines, dedicated communications lines, cellular telephone communications, or radio communications. The control center  62  may combine information from the various concentrators  60  into an overall picture of roadway conditions and delays for the covered region. Roadway sensor information may also be made available to a larger audience by placing the sensed information on a communications network, such as through a Web application hosted on the Internet  64 . Having the roadway information available on the Web allows Web clients  66   1 , . . . ,  66   x  (generally  66 ) to access up-to-date roadway information on demand. 
     2-a. Roadway Monitoring System 
     In operation, referring to FIG. 7, each of the roadway sensors  10  senses roadway information as previously discussed (step  200 ). Each of the sensors  10 , assigned to one of the sensor sets, may further process the sensed information (step  205 ) and broadcast the information to a concentrator  60  corresponding to its sensor set (step  210 ). The concentrators  60 , in turn, send the received information from the sensors  10  to the control center  62  (step  220 ). At the control center  62 , further processing may be performed (step  230 ). Control center processing may include, for example, estimating travel time for particular routes, identifying alternate routes to both avoid and manage traffic congestion, generating traffic signal control signals, and determining roadway surface conditions. 
     2-b. Web Server 
     As already mentioned, the sensor information and processed sensor information may be made available on the Web through a Web server application. In one embodiment, a Web application may be provided offering access to roadway sensed information as processed by the control center  62 . Alternatively, the concentrators  60  may be interconnected directly to the Internet  64 , facilitating Web-based access thereto. This may serve as the basis upon which the control center  62  communicates with the concentrator  60 , or may allow Web clients to obtain information directly from the concentrators  60 . 
     The control center  62  may respond to Web client requests for traffic service in the form of a traffic report, travel route time estimate, or travel route planning to avoid traffic congestion, preparing the requested product and serve it to the requesting Web client  66 . The control center  62  may make use of information routinely collected from the sensors  10 , serving a Web client request with the latest available information. Alternatively, the control center  62  may request updates from the concentrators  60  relevant to the Web client request. 
     3. Traffic Control System 
     Referring now to FIG. 8, the in-road traffic sensors  10  may be configured to control traffic. A set of sensors,  10   1 , . . .  10   n  (generally  10 ) are placed at strategic locations around a segment of roadway. The sensors  10  sense passing vehicles as previously described and broadcast information to the concentrator  60  associated with the respective set of sensors  10 . The concentrator  60 , in response to the received vehicle information from the sensors  10 , controls one or more traffic control mechanisms  70   1  . . .  70   n  (generally  70 ). The traffic control mechanism  70  may, as illustrated, include traffic lights. For example, at a roadway intersection, one or more sensors  10  may be placed in each lane approaching the intersection. As vehicles approach the intersection, the sensors  10  detect the passing vehicles and broadcast related information to the common concentrator  60 . The concentrator may be located on a light pole or telephone pole as previously indicated, typically in the general vicinity of the intersection. Alternatively, the concentrator may be located at a more remote distance from the sensors  10  limited only by the restrictions of the wireless communications link from the sensors  10  to the concentrator  60 . 
     In this application, it is advantageous for each of the sensors  10  provide some form of identification allowing the concentrator  60  to distinguish which sensor  10  is reporting a passing vehicle. Identification means may include broadcasting a unique address tone, or bit sequence, broadcasting in a pre-assigned time slot, or broadcasting on an allocated frequency. The concentrator  60 , being able to identify the reporting sensor  10 , is thereby apprised of which portion of the roadway segment (e.g., which lane) contains the approaching vehicle and can control the traffic lights  70  accordingly. Because the wireless communications link distances may be greater than one kilometer, it is possible to have a single concentrator controlling traffic flow at a number of different roadway segments. Integrating information from contiguous chains of segments can facilitate the control of overall traffic flow over relatively large metropolitan areas to avoid gridlock. 
     Having shown the preferred embodiments, one skilled in the art will realize that many variations are possible within the scope and spirit of the claimed invention. It is therefor the intention to limit the invention only by the scope of the claims.