SYSTEM AND METHOD FOR LEVEL OF VISIBILITY DETERMINATION

AND VEHICLE COUNTING

FIELD OF THE DISCLOSED TECHNIQUE The disclosed technique relates to road safety, in general, and to a system and method for the determination of visibility level, traffic flow and traffic density in a motorway environment, in particular.

BACKGROUND OF THE DISCLOSED TECHNIQUE Poor visibility is a key factor in numerous traffic accidents.

When a driver has a limited view of the road ahead, his decision making period is reduced, and the likelihood of an accident occurring increases. Poor road visibility is generally the result of inclement weather, particularly fog or mist, and precipitation such as rain, snow, sleet, dew, and hail. Other conditions, such as sandstorms, dust storms, swarms of locusts, grasshoppers, or other flying insects, ashes of volcanic eruptions, smoke, smog and the presence of air pollutants, also restrict visibility in the surrounding environment.

A particularly common type of traffic accident during poor visibility conditions is a multi-vehicle collision. Due to the poor visibility, drivers tend to drive too close to the vehicle immediately ahead and do not maintain a sufficient distance apart. If one vehicle should suddenly stop or decelerate, the following vehicle may not have sufficient time to reach a

complete stop, resulting in a rear-end collision. Subsequent vehicles may succumb to the same predicament, since the drivers are unable to recognize stopped vehicles on the road before they can react, further increasing the number of vehicles in the collision. There have been cases where over a hundred vehicles were involved in such collisions. Along with causing serious bodily injuries, and even fatalities, traffic accidents are a source of significant property and economic loss, both direct and indirect, as well as a severe inconvenience for other commuters.

Another factor which further aggravates driving hazards is the presence of very dense localized fog. A driver may be driving along a stretch of road with relatively good visibility, and suddenly encounters an area with very high fogginess. Unaware of the reduced visibility until it is too late, the driver may be unable to change his driving habits accordingly, which can quickly lead to a traffic accident. Certain regions are more susceptible to particular weather conditions, such as fog, and are therefore more susceptible to such incidents.

Various road warning systems indicate to drivers the presence of fog or similar conditions leading to poor road visibility in an upcoming region. These systems may detect the presence and level of fog in the region using different types of fog detection sensors, which are known in the art. One technique for detecting fog involves measuring the amount of scattering of light. Such road warning systems may further adjust traffic rules in the particular region in accordance with the weather conditions, in

order to enhance safety, for example by lowering the speed limit within the region. The adjusted traffic rules may also take into account additional factors which influence driving conditions.

One type of speed limit adjustment system relies on the number of oncoming road markers that the driver is able to perceive. For example, if the driver can clearly view the next three road markers on the side of the road, then the speed limit is set to 90 km/h; if the driver can clearly view the next two road markers, then the speed limit is set to 60 km/h; while if the driver can clearly view only one road marker, then the speed limit is set to 30 km/h. Such a system however is difficult to enforce, and also somewhat distracts the driver and prevents full concentration on the road.

US Patent No. 5,592,157 to Metz et al, entitled "Relative visibility measuring process and device", is directed to an apparatus and process for measuring relative visibility through the detection and indication of weather conditions such as fog, rain, snow, smoke, and the like. A rangefinder transmits a measurement pulse to a reflector located at a measurement site (e.g., a road boundary post). The reference measurement is carried out during ideal conditions (i.e., with good visibility and with clean outer surfaces on the rangefinder and the reflector). The measurement pulse is reflected from the reflector back to the rangefinder over a reference range. The propagation time of the measurement pulse is designated as the reference propagation time, and the amplitude value of the measurement pulse is designated as the reference amplitude.

During actual conditions, the rangefinder transmits an actual signal toward the reflector. The amplitude of the time element of the actual signal having a propagation time equal to the reference propagation time is designated as the actual amplitude. This time element represents the part of the actual signal reflected from the reflector. The ratio between the actual amplitude and the reference amplitude is determined, and the ratio is compared to a threshold value that represents the measure of signal attenuation at the reference range for establishing the visibility state. If the ratio is below the threshold value, the condition for a visibility undershoot (i.e., poor visibility conditions) is satisfied.

The actual signal is further investigated, to determine if the visibility determination was influenced by a measurement disturbance. The amplitude response of the actual signal at time elements less than the reference propagation time is examined. If this amplitude response exhibits characteristics specific to soft targets (e.g., fog droplets, snowflakes, etc), then the indication of a visibility undershoot is confirmed. If however this amplitude response exhibits characteristics which differ from those of soft targets, then a measurement disturbance is indicated. If for example this amplitude response exhibits characteristics specific to hard targets (e.g., wild animals, passers by, abandoned vehicles), then the presence of an obstruction is indicated. If for example all the time elements of the actual signal having propagation times less than or equal to the reference propagation time are equal to a previously defined null

signal, than loss of the reflector (e.g., the road boundary post has broken) is indicated. If for example all the time elements of the actual signal having propagation times less than the reference propagation time are equal to a previously defined null signal, than a contamination of the outer surface of the rangefinder or of the reflector is indicated. After a confirmed indication of a visibility undershoot, appropriate actions can be undertaken. For example, a speed limit corresponding to the determined visibility can be activated, an electronic fog warning sign can be switched on for road traffic, or a fog or snow warning can be initiated at an airfield. US Patent No. 5,787,385 to Tognazzini, entitled "Computer controlled laser fog tracking", is directed to a system for detecting fog along a section of highway. A scanning transmissometer is located on one side of the highway at an elevation from which a large stretch of road can be observed. Reflectors are mounted on poles adjacent to the opposite side of the highway. Initially, the scanning transmissometer identifies and locates all the existing roadside reflectors, by directing the laser to a certain point and moving over a deterministic search pattern until a reflection is detected, and then storing the azimuth and elevation coordinates for each reflector in a look up table. Prior to operation, the reflectors are warmed during a prescan period to prevent condensation on the reflective surface. The reflectors may be arranged in a heat conductive configuration, such that heat is absorbed from the transmitted laser during the prescan, warming the reflector sufficiently to prevent

condensation. During operation mode, the scanning transmissometer transmits a laser beam to each of the reflectors sequentially. The scanning transmissometer first checks the look up table for the location of the first reflector, and adjusts its azimuth and elevation coordinates so that the directed laser will match the azimuth and elevation of the first reflector. The scanning transmissometer turns on the laser, receives the reflection from the reflector, and determines the transmissivity, or the amount of attenuation of the laser light. The scanning transmissometer proceeds in the same manner to obtain reflections from each of the other reflectors listed in the look up table. The amount of attenuation over the path to and a given reflector and back, provides an indication of the presence of fog or other conditions limiting visibility in the region. The scanning transmissometers and their associated reflectors may be arranged in an overlapping and interlaced configuration, to provide redundancy. For example, the reflectors may be positioned such that the reflectors scanned by one transmissometer are interleaved with the reflectors scanned by another transmissometer.

US Patent No. 6,411 ,221 to Hόrber, entitled "Device and method to detect an object in a given area, especially vehicles, for the purpose of traffic control", is directed to a device and method for detecting an object in a predetermined spatial region, such as the detection of vehicles for traffic monitoring. A transmitter, mounted above a roadway, emits laser pulses toward the roadway. A receiver receives the radiation backscattered or

reflected from the object. The receiver emits a time-dependent detection signal for each individually received backscattered pulse, to a plurality of instantaneous voltage measuring devices. Each voltage measuring device measures an instantaneous voltage value of the detection signal within a measurement time interval. The intervals for all the devices cover the entire time range, where each interval can have a different width and overlap in time. Every received pulse is capable of detection in at least two measurement intervals offset from each other in time. The set of instantaneous voltage values are digitized and sent to an evaluation device. The evaluation device determines an instantaneous voltage pattern (e.g., a set of quotients of adjacent instantaneous voltage values) for each transmitted laser pulse, and compares the instantaneous voltage pattern with experimentally determined instantaneous voltage patterns stored in a calibration table. The distance between the object and the measuring device is then determined based on the comparison. If for example the determined distance differs from the fixed distance between the measuring device and the road, then a vehicle is present. Profiles of moving vehicles may also be determined based on the distance varying between successive pulses. US Patent No. 6,493,123 to Mansell et al, entitled

"Modulated-retroref lector based optical identification system", is directed to a system and method for identifying a vehicle, such as in a military environment. The operator (e.g., a soldier) locates a target mounted on a

vehicle using a handheld sighting unit. The sighting unit includes a laser, which is co-aligned with the ocular of the sighting unit. The laser transmits a laser beam toward the target. The target includes a reflector which reflects the transmitted laser beam precisely back in the direction of transmission. For example, the reflector is a corner cube having three mirrored surfaces each at 90° to each other. The target also includes a shutter disposed in the optical path of the laser beam, between the laser and the reflector. The shutter modulates the polarization of the reflected beam with a unique code or pattern. The code corresponds to identification information of the vehicle on which the target is mounted, such as a binary representation of a number or name of the vehicle. The reflected beam returns to the sighting unit and is focused onto a photodetector. A polarizer and optical filter are disposed on the sighting unit in the path of the reflected beam, and serve to only pass through a reflected beam having the prescribed polarization and wavelength, respectively. The identification information of the reflected beam is then decoded and presented to the operator, either visually (e.g., on a display) or as an audio message (e.g., through audio headsets).

US Patent No. 6,693,555 to Colmenarez et al, entitled "Automatic setting of variable speed limit", is directed to a system and method for setting a variable speed limit on a roadway in accordance with weather and traffic conditions. A plurality of sensors is arranged at predetermined intervals along a road. The sensors are capable of

monitoring various weather conditions, such as fog, ice, sleet, snow, wind and humidity, using techniques known in the art. For example, one of the sensors may check visibility across the road, using a mirror on the opposite end the road which reflects an optical signal. A regulating unit receives the detected weather conditions from the sensors. The regulating unit accesses a database, which contains tables of predetermined speed limits for each road location, based on the present weather conditions, along with additional factors. These factors may include the day, date, time, severity of previous accidents, severity of weather conditions, type of road structure, etc. The predetermined speed limits may be based on interpolations of previous accident data for similar roads and recommendations of traffic experts. The regulating unit determines the appropriate speed limit using the database, and transmits the speed limit for display at a speed limit indicator at the road. The regulating unit may also receive weather alerts from a weather forecasting service (e.g., foggy conditions expected some distance away), and reported traffic alerts (e.g., stopped vehicles or accidents in the area). The weather alerts or traffic alerts are then taken into account when determining the appropriate speed limit. US Patent No. 6,825,778 to Bergan et al, entitled "Variable speed limit system", is directed to a system for managing traffic speed approaching and in a construction work zone. The system includes a plurality of traffic monitoring and signing stations, installed at various

intervals within and approaching the construction zone. Each station includes a sensor and a display, and at least one of the stations includes a controller. Each sensor obtains information related to lane occupancy, as well as other factors such as: road conditions, vehicle speed, vehicle presence, and weather conditions. The sensor provides the information to the controller, which derives a speed limit for the region adjacent to the station based on the received information. Determination of the speed limit may take into account criteria such as lane occupancy, vehicle speed, average speed of a series of vehicles, work zone characteristics, road conditions, and user configured parameters (e.g., maximum/minimum speed, maximum speed increment, maximum time before speed increment). For example, the speed limit is set inversely with the lane occupancy information. The controller may also utilize a series of profiles to derive the speed limit. One of the profiles is designated as an active profile, which takes into consideration factors such as the time of day or the day of the week. The controller transmits the derived speed limit to the display, which is displayed to the drivers of vehicles approaching the station. The display may also provide a text message in addition to the speed limit, such as an indication that road conditions ahead are impaired due to ice or rain.

US Patent Application Publication No. 2002/0083630 to Marcon, entitled "Conditionally adjusted speed limit for roads", is directed to a method for increasing the overall vehicular capacity of a road utilizing

conditionally adjusted speed limits. The speed limits are adjusted based upon seasonal parameters and situational parameters. Seasonal parameters include: the season, Daylight Savings Time, and holidays. Situational parameters include: time of day, smog alerts, traffic density, accidents, roadway conditions, weather conditions (e.g., winds, rain, flash floods, fog, fire, ice, hail, sleet and snow), and evacuations. The adjusted speed limit is displayed along a road lane, such as via an electronic speed limit sign having a changeable electronic messaging display. A modified speed limit sign displaying two speed limits may be implemented, to indicate that the road is regulated by at least one conditionally adjusted speed limit. Information relating to the road situation may be fed to the computers controlling the electronic speed limit signs via sensors or devices, such as for vehicular sensing, vehicular counting, vehicular tolling, temperature sensing, wind measuring, precipitation sensing, light sensing, and cameras.

SUMMARY OF THE DISCLOSED TECHNIQUE

In accordance with one aspect of the disclosed technique, there is thus provided a system for determining the visibility level in an outdoor environment. The system includes at least one retroreflective element, a laser transmitter, a visibility level detection receiver, and a processor. The processor is coupled with the visibility level detection receiver. The retroreflective element is located at a fixed site in the outdoor environment. The laser transmitter transmits a plurality of laser pulses toward the retroreflective element. The visibility level detection receiver receives reflections of the laser pulses reflected from the retroreflective element. The processor determines the level of visibility in the outdoor environment based on the number of reflected pulses received by the visibility level detection receiver required to reach an energy threshold, respective of the number of reflected pulses required to reach the threshold during predetermined visibility conditions.

In accordance with another aspect of the disclosed technique, there is further provided a method for determining the visibility level in an outdoor environment. The method includes the procedures of transmitting a plurality of laser pulses toward a retroreflective element located at a fixed site in the outdoor environment, and receiving reflections of the laser pulses reflected from the retroreflective element. The method further includes the procedure of determining the level of visibility in the outdoor environment based on the number of reflected pulses required to reach an

energy threshold with respect to the number of reflected pulses required to reach the threshold during predetermined visibility conditions.

In accordance with a further aspect of the disclosed technique, there is further provided a system for determining the traffic flow and traffic density in a motorway environment. The system includes a laser transmitter, a vehicle counting receiver, and a processor. The processor is coupled with the vehicle counting receiver. The laser transmitter transmits a plurality of laser pulses toward retroreflectors on the rear of vehicles traveling along a road in the motorway environment. The vehicle counting receiver receives reflections of the laser pulses reflected from the retroreflectors. The processor determines the traffic flow along the road and the traffic density in a section of the road, based on the number and timing of reflected pulses received by the vehicle counting receiver.

In accordance with yet another aspect of the disclosed technique, there is further provided a method for determining traffic flow and traffic density in a motorway environment. The method includes the procedures of transmitting a plurality of laser pulses toward retroreflectors on the rear of vehicles traveling along a road in the motorway environment, and receiving reflections of the laser pulses reflected from the retroreflectors. The method further includes the procedure of determining the traffic flow along the road and the traffic density in a section of the road based on the number and timing of reflected pulses received.

In accordance with yet a further aspect of the disclosed technique, there is further provided an architecture for determining the visibility level, traffic flow and traffic density in a motorway environment. The architecture includes a plurality of systems disposed throughout the motorway environment. Each system includes at least one retroreflective element, a laser transmitter, a visibility level detection receiver, a vehicle counting receiver and a processor. The processor is coupled with the visibility level detection receiver and with the vehicle counting receiver. The retroreflective element is located at a fixed site at the side of a road in the motorway environment. The laser transmitter transmits a plurality of laser pulses toward the retroreflective element, and toward retroreflectors on the rear of vehicles traveling along the road. The visibility level detection receiver receives reflections of the laser pulses reflected from the retroreflective element. The vehicle counting receiver receives reflections of the laser pulses reflected from the retroreflectors. The processor determines the level of visibility in the environment based on the number of reflected pulses received by the visibility level detection receiver required to reach an energy threshold, respective of the number of reflected pulses required to reach the threshold during predetermined visibility conditions. The processor further determines the traffic flow along the road and the traffic density in a section of the road, based on the number and timing of reflected pulses received by the vehicle counting receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed technique will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: Figure 1 A is a schematic illustration of a top view of a system for joint determination of visibility level, traffic flow and traffic density in a motorway environment, constructed and operative in accordance with an embodiment of the disclosed technique;

Figure 1 B is a schematic illustration of a perspective view of the system of Figure 1 A;

Figure 2A is a schematic illustration of a perspective view of the transceiver unit of the system of Figure 1A;

Figure 2B is a schematic illustration of a top view of the transceiver unit of the system of Figure 1A in operation; Figure 3 is a schematic illustration of an expanded view of the laser transmitter of the transceiver unit of Figure 2A;

Figure 4 is a schematic illustration of an expanded view of the visibility level detection receiver of the transceiver unit of Figure 2A;

Figure 5 is a schematic illustration of an expanded view of the vehicle counting receiver of the transceiver unit of Figure 2A; and

Figure 6 is a schematic illustration of a method for joint determination of visibility level, traffic flow and traffic density in a motorway

environment, operative in accordance with an embodiment of the disclosed technique.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The disclosed technique overcomes the disadvantages of the prior art by providing a novel road warning system and method, which uses a single device for determining the level of visibility in the surrounding environment of the road, along with determining the traffic flow along the road and the traffic density in a road section. The disclosed technique utilizes retroreflectors to reflect transmitted laser light, enabling the light to be transmitted at a low power while receiving a sufficient amount of reflected light for subsequent analysis. The system includes a transceiver unit, which transmits laser pulses toward the road. The transceiver unit receives the light reflected from a retroreflective element located at the side of the road, and determines the level of visibility in the surrounding environment based on the number of reflected pulses required to reach an energy threshold. The transceiver unit takes advantage of the retroreflectors already present on vehicles on the road, and further receives the light reflected from these vehicle retroreflectors. The transceiver unit determines traffic flow along the road and traffic density in the road section based on the number and timing of reflected pulses received. A control unit establishes traffic recommendations for the road, in accordance with the determined level of visibility, traffic flow and traffic density. For example, the control unit establishes an updated speed limit, which is then displayed on a speed limit sign on the road.

Reference is now made to Figures 1A and 1 B. Figure 1A is a schematic illustration of a top view of a system, generally referenced 100, for joint determination of visibility level, traffic flow and traffic density in a motorway environment, constructed and operative in accordance with an embodiment of the disclosed technique. Figure 1 B is a schematic illustration of a perspective view of the system of Figure 1A. System 100 includes a plurality of transceiver units 104 and a plurality of retroreflective elements 108. Each transceiver unit 104 is mounted on a road structure 106 elevated above a road 102, such as a bridge or an overpass spanning the width of road 102. Each retroreflective element 108 is part of a boundary post 109 located adjacent to the side of road 102. Retroreflective element 108 may be an inherent section of post 109, or may be fixedly attached onto post 109, such as via a permanent adhesive. In the embodiment of Figures 1A and 1 B, the distance between successive road structures 106 defines a "road section", referenced 116. It is appreciated that road section 116 may be any portion of road 102. Road structures 106 are spaced apart from each other on road 102 by a fixed distance (e.g., every 1km). Each road section 116 includes at least one retroreflective element 108, located substantially close to a transceiver unit 104 (e.g., 50m away). Road 102 may include any number of lanes, and may run in any direction. A speed limit sign 114 is located in each road section 116, such as at the side of road 102, or on a road structure in the center of road 102. Speed limit sign 114 is an electronic display which

provides a numerical representation of the current speed limit in the upcoming road section 116. Speed limit sign 114 may be a variable message sign (VMS), which is capable of also displaying text or graphic images. An external control unit (not shown) is in communication with each transceiver unit 104 and each speed limit sign 114. Alternatively, system 100 includes a plurality of control units, where each control unit is coupled with a group (e.g., ten) of transceiver units and speed limit signs. The control unit may be a control station with dedicated computing hardware (e.g., processor, memory, database), which may be operated manually (e.g., by a control operator) or automatically. Alternatively, the control unit may be implemented in another transceiver unit.

Transceiver unit 104 transmits laser light toward road 102, in the form of a series of short pulses. The transmitted laser light spans at least the entire width of road 102, encompassing retroreflective element 108 at the side of road 102, as depicted in Figures 1A and 1 B. Transceiver unit 104 receives the light reflected from retroreflective element 108, and also receives the light reflected from retroreflectors 112 on the rear of vehicles 110 on road 102. Retroreflectors 112 may be the retroreflectors embedded in the vehicle taillights, or the retroreflectors embedded in the vehicle license plate. Retroreflectors 112 may be disposed on the rear of the vehicle, such as by attaching a sticker or an adhesive element that includes retroreflectors, by applying paint or other coloring or coating that includes retroreflectors, and the like. Retroreflectors 112 may cover the

entire rear of vehicle 110 or any portion thereof. In the context of the disclosed technique, the term "rear of the vehicle" refers to any surface which reflects light toward transceiver unit 104, and should be broadly interpreted accordingly. A retroreflector directs light back toward the source, regardless of the angle of incidence. A retroreflector may be based on a corner reflector, a transparent sphere and spherical mirror, or other optical elements and techniques known in the art. The conventional taillights of a vehicle are commonly designed with retroreflectors embedded within, so that when illuminated by the headlights of the following vehicle, the light is reflected back toward the driver of that vehicle in its entirety (rather than diffusing in other directions), providing the driver with maximum illumination. It is noted that the inclusion of retroreflectors in the vehicle taillights is a legal requirement in most countries, thereby ensuring that such retroreflectors are present.

Transceiver unit 104 determines the level of visibility in the surrounding region. Transceiver unit 104 further determines the number of vehicles crossing road structure 102 (i.e., traffic flow), and the number of vehicles present in road section 116 (i.e., traffic density). Transceiver unit 104 forwards the determined information to the control unit (not shown). The control unit (not shown) processes the information and determines an appropriate speed limit for road section 116. The control unit (not shown) updates speed limit sign 114 with the new speed limit,

which is displayed. Speed limit sign 114 may also receive additional messages to display, for example to indicate the presence of poor visibility, a traffic jam, an upcoming accident, road construction, and the like, based on information sent to the control unit (not shown) from other sources.

Reference is now made to Figures 2A and 2B. Figure 2A is a schematic illustration of a perspective view of the transceiver unit of the system of Figure 1 A. Figure 2B is a schematic illustration of a top view of the transceiver unit of the system of Figure 1A in operation. Transceiver unit 104 includes a laser transmitter 122, a visibility level detection receiver 124, and a vehicle counting receiver 126. Laser transmitter 122 transmits a series of laser pulses. The transmitted pulses span at least the entire width of road 102, encompassing retroreflective element 108. Visibility level detection receiver 124 receives light reflected from retroreflective element 108. Vehicle counting receiver 126 receives light reflected from retroreflectors 112 on the rear of a vehicle 110.

The field of view of each receiver may be spatially filtered, ensuring that it only receives the intended reflected light. In particular, the field of view of visibility level detection receiver 124 is such that it cannot receive reflected light from retroreflectors 112, while the field of view of vehicle counting receiver 126 is such that it cannot receive light reflected by retroreflective element 108. For example the field of view of each of visibility level detection receiver 124 and vehicle counting receiver 126 is

approximately 6-11 ° azimuth and 0.3-0.6° elevation. Alternatively, segregation between pulses reflected from the different types of retroreflectors may be achieved by using gating techniques, by transmitting a different wavelength or polarization to each retroreflector, or by using filtered retroreflectors that reflect light of a specific wavelength or polarization. For example, transceiver unit 104 may include a second transmitter channel (i.e., in addition to laser transmitter 122), where each transmitter transmits laser pulses at a different wavelength or polarization. It is noted that retroreflectors usually maintain polarization, but those that change polarization are also taken into account.

In an alternative embodiment of the disclosed technique, the laser transmitter, visibility level detection receiver, and vehicle counting receiver are not embedded within a single transceiver unit, but are separate units which may be disposed at separate locations. For example, the laser transmitter, visibility level detection receiver, and vehicle counting receiver may each be disposed at a different section of road structure 106.

Reference is now made to Figure 3, which is a schematic illustration of an expanded view of the laser transmitter of the transceiver unit of Figure 2A. Laser transmitter 122 includes a lens 128, a laser diode 130, a temperature regulator 132, a current switch 134, a current source 136 and a controller 138. Laser diode 130 is aligned in the optical path of lens 128. Temperature regulator 132 is coupled with laser diode 130.

Current switch 134 is coupled with laser diode 130 and with current source 136. Controller 138 is coupled with temperature regulator 132 and with current switch 134.

Current source 136 receives a direct current (DC) voltage from an external voltage source, and generates a stabilized current, at a constant peak power of approximately 5W. Current source 136 is, for example, a pulse-width modulation (PWM) based current source. Current switch 134 provides very short pulses to power laser diode 130, by directing the current either directly to laser diode 130 or to ground. Laser diode 130 emits laser light, preferably at a wavelength of approximately 808nm. For example, each transmitted pulse has a pulse width of approximately 200ns with a duty cycle of 1 :1000, resulting in an average power of approximately 5mW for each pulse. It is noted that the operational wavelength is in the near infrared (NIR) range just outside the visible spectrum (i.e., approximately 400-700nm), and therefore will not distract the driver of the vehicle, and is also eye-safe at the operational power. Alternatively, the operational wavelength is in the spectral range of 800-1600nm. Temperature regulator 132 maintains the temperature of laser diode 130 at a stable level, and prevents overheating of laser diode 130. Controller 138 controls operation of current switch 134 and temperature regulator 132. Controller 138 is coupled with an external control unit (not shown).

Reference is now made to Figure 4, which is a schematic illustration of an expanded view of the visibility level detection receiver of the transceiver unit of Figure 2A. Visibility level detection receiver 124 includes a lens 142, a spectral filter 144, a photodiode 146, a first capacitor 148, an amplifier 150, a band-pass filter 152, a second capacitor 154, a clamping switch 156, a sampling switch 158, an integrator 160, an integrator reset 161 and a processor 162. Spectral filter 144 is aligned in the optical path of lens 142. Photodiode 146 is optically coupled with spectral filter 144. First capacitor 148 is electrically coupled with photodiode 146 and with amplifier 150. Amplifier 150 is further coupled with band-pass filter 152. Second capacitor 154 is electrically coupled with band-pass filter 152, with clamping switch 156, and with sampling switch 158. Processor 162 is coupled with clamping switch 156, with sampling switch 158, with integrator 160, and with integrator reset 161. Integrator 160 is further coupled with sampling switch 158 and with integrator reset 161. It is noted that capacitors 148 and 154 may be implemented by another type of alternating current (AC) coupling element (e.g., a high-pass filter).

The light reflected from retroreflective element 108 (Figure 2B) enters visibility level detection receiver 124 via lens 142. Spectral filter 144 only passes through light in the operational frequency (i.e., approximately 808nm) and filters out light of all other frequencies. Photodiode 146 (e.g., a silicon photodiode) acts as a sensor and detects

the filtered light. First capacitor 148 converts the photocurrent (produced by photodiode 146) to an AC signal. First capacitor 148 also serves to filter out additional photons which are in the operational frequency but result from sources other than the transmitted laser, such as solar radiation. Amplifier 150 amplifies the signal, and band-pass filter 152 only passes through pulses within a specified frequency. Clamping switch 156 and sampling switch 158 serve to pass through only those pulses (or the portion of a pulse) which actually impinged on retroreflective element 108, while blocking out other pulses, according to a technique known as gating. This is achieved by taking into account the distance from retroreflective element 108, the pulse width of the transmitted and received pulses, and the timing of the transmitted and received pulses.

The filtered and gated pulses are collected by integrator 160. Processor 162 determines the level of visibility in the surrounding environment. The visibility level is generally a function of the amount of localized scattering of light. Inclement weather conditions, such as fog, rain, snow, sleet, ice, and smoke, restrict visibility to varying degrees. Visibility level may be quantified by the "visibility range", which is the horizontal distance for which the contrast transmission of the atmosphere in daylight is a certain percentage (e.g., 2%). Processor 162 analyzes the number of pulses in integrator 160 required to reach a predetermined threshold, and compares this number with a benchmark. In particular, visibility level detection receiver 124 is first operated when good visibility is

present in the surrounding environment, during clear weather conditions, ideally when the atmosphere is relatively free from any visibility impairing elements (e.g., a visibility range above 15km). For example, visibility level detection receiver 124 obtains a benchmark measurement representing the number of collected pulses necessary to reach a threshold (i.e., when the maximum energy capacity of integrator 160 is attained). Processor 162 stores this benchmark measurement for use as a reference during all subsequent measurements. When visibility level detection receiver 124 is operated during regular weather conditions, processor 162 compares the number of collected pulses in integrator 160 required to reach the threshold (i.e., the "operational measurement"), with the benchmark measurement. For example, if the benchmark measurement is established at 100 pulses, the operational measurement may be 500 pulses. The visibility level is a function of the difference between the operational measurement and the benchmark measurement. The greater the number of pulses required to reach the benchmark, the lower the visibility level of the surrounding environment.

Integrator reset 161 resets the contents of integrator 160 prior to performing a new measurement. Visibility level detection receiver 124 periodically carries out a new benchmark measurement, when it is known that good visibility conditions exist, and processor 162 resets integrator 160 with the updated benchmark measurement.

It is noted that the various filtering and gating operations taking place within visibility level detection receiver 124 limit the influence of noise and interference. The gating ensures that only the pulses impinging on retroreflective element 108 (Figure 2B) enter visibility level detection receiver 124, while preventing the entry of pulses reflected from other objects in the environment. The filtering blocks out unwanted light, particularly solar radiation, which is the primary source of interference, as well as the dark current produced by photodiode 146, and the light emitted from the headlights of other vehicles. Reference is now made to Figure 5, which is a schematic illustration of an expanded view of the vehicle counting receiver of the transceiver unit of Figure 2A. Vehicle counting receiver 126 includes a lens 164, a spectral filter 166, a photodiode 168, a capacitor 170, an amplifier 172, a band-pass filter 174, a comparator 176, and a processor 178. Spectral filter 166 is aligned in the optical path of lens 164. Photodiode 168 is optically coupled with spectral filter 166. Capacitor 170 is electrically coupled with photodiode 168 and with amplifier 172. Amplifier 172 is further coupled with band-pass filter 174. Comparator 176 is coupled with band-pass filter 174 and with processor 178. It is noted that capacitor 170 may be implemented by another type of AC coupling element (e.g., a high-pass filter).

The light reflected from retroreflectors 112 (Figure 2B) enters vehicle counting receiver 126 via lens 164. Spectral filter 166 filters out

light that is outside the operational frequency (i.e., approximately 808nm). Photodiode 168 (e.g., a silicon photodiode) acts as a sensor and detects the filtered light. Capacitor 170 converts the photocurrent (produced by photodiode 168) to an AC signal, and filters out additional photons which are in the operational frequency but result from sources other than the transmitted laser, such as solar radiation. Amplifier 172 amplifies the signal, and band-pass filter 174 only passes through pulses within a specified frequency.

Comparator 176 analyzes the filtered pulse and determines if the pulse was reflected from retroreflectors 112 (Figure 2B) on the rear of a vehicle 110 (e.g., if the energy is above a threshold). Processor 178 maintains a count of such pulses, where each group of consecutive pulses corresponds to a vehicle passing the road structure 106 (Figure 1A). A delay of sufficient duration during which no such pulses are received, indicates that the next group of consecutive pulses corresponds to a subsequent vehicle. Processor 178 further performs gating, by only examining comparator 176 at the appropriate time, based on when the pulse reflected from retroreflectors 112 is expected to arrive. For example, if the pulse duration is 200ns with a 1 :1000 duty cycle, and the distance between transceiver unit 104 and retroreflective element 108 is 50m, then the comparator is examined after approximately 230-430μs has elapsed following the initial operation of laser transmitter 122 (i.e., corresponding to the time required for the entire pulse to arrive). Processor 178 samples

the pulses periodically. For example, the pulses are sampled every 200 ms, resulting in a total of 90 reflected pulses received from a vehicle traveling at 200km/h (i.e., at a pulse duration of 200ns, such that the vehicle advances a distance of 10μm during a pulse). According to another embodiment of the disclosed technique, the sensitivity of visibility level detection receiver 124 or vehicle counting receiver 126 may be adjusted as a function of range, to compensate for the deterioration of the reflected light. For example, as the expected distance traveled by the reflected pulse grows larger, the sensitivity level of the receiver is increased. Alternatively, the intensity of the pulse transmitted by laser transmitter 122 is increased as the range is increased.

Referring back to Figure 1A, processor 178 determines the traffic flow along road 102 across road structure 106, as well as the traffic density within road section 116. The term "traffic flow" refers to the number of vehicles crossing a certain point along the road, for example the number of vehicles crossing road structure 106 on which transceiver unit 104 is mounted. The term "traffic density" refers to the number of vehicles present within a certain segment of the road at a given point in time; in this case, the number of vehicles present in road section 116. Traffic density may be calculated using the traffic flow data. For example, traffic density may be obtained by referring to the traffic flow measured by one transceiver unit 104 (possibly applying extrapolation assumptions), or to the accumulation of vehicles crossing road structure 106 and entering

road section 116 (whose distance and width are known), and subtracting those leaving road section 116 (crossing the next road structure 106), in a given time period.

Transceiver unit 104 is typically disposed at a height of 2.5-15m above road 102, preferably at approximately 5m. The distance between transceiver unit 104 and retroreflective element 108 may be between 10m and 1km, and is preferably approximately 50m. It is appreciated that as this distance increases, the transmitted pulses are influenced by a greater portion of the atmosphere, thus resulting in a more accurate determination of visibility, but at the cost of greater transmission power required.

It is further appreciated that at substantially large distances, the angle between the transceiver unit and the vehicle becomes very acute, and consequently the line-of-sight to a vehicle may be blocked by the vehicle immediately behind, and the reflected pulse from the taillight of that vehicle would not be received by the transceiver unit, resulting in an inaccurate vehicle count. A formula based on vehicle speed may determine at which angle this situation is problematic. The height at which the transceiver unit is mounted, and the distance from the retroreflective element, may then be set accordingly. Each of visibility level detection receiver 124 and vehicle counting receiver 126 may also be implemented using a sensor device and a digital signal processor (DSP). Controller 138 (Figure 3), processor 162 (Figure 4) and processor 178 (Figure 5) may all be implemented by a

single processing device. Transceiver unit 104 forwards the determined visibility level, traffic flow and traffic density to the control unit (not shown), which determines an appropriate speed limit for road section 116 accordingly. The updated speed limit takes into account the required stopping distance for a vehicle at the current visibility level. For example, if the transmittance over 100m (i.e., the distance from transceiver unit 104 to retroreflective element 108 and back) is 0.451 , for a visibility range of 500m, then the stopping distance is approximately 200m. If the transmittance over 100m is 0.347 for a visibility range of 400m, then the stopping distance is approximately 160m. If the transmittance over 100m is 0.244 for a visibility range of 300m, then the stopping distance is approximately 130m. If the transmittance over 100m is 0.14 for a visibility range of 200m, then the stopping distance is approximately 80m. Generally, a visibility range of under 500m indicates the presence of fog. The control unit (not shown) may also establish other types of traffic recommendations to be implemented, such as the decision to close down a portion of a road, in response to poor visibility, a traffic jam, an upcoming accident, road construction, and the like. This information may also be displayed on speed limit sign 114 (Figure 1A). According to one embodiment of the disclosed technique, traffic information is transmitted to rescue personnel, such as police or ambulance services, in order to provide them with real-time information relating to driving conditions and to help them select an optimal driving route to their destination. The control

unit (not shown) may also utilize the received visibility level, traffic flow and traffic density for statistical purposes. For example, the information may be used as part of the research for the analysis of a potential traffic engineering project or for future traffic planning. The disclosed technique is further applicable to the formation of an architecture for determining the visibility level, traffic flow and traffic density in a motorway environment, where the architecture includes several systems, such as system 100 (Figures 1A and 1 B), disposed throughout several roads, such as road 102, or within different portions of the same road.

The disclosed technique is further applicable to determining only visibility level or alternatively, to determining only traffic flow and traffic density. According to one embodiment of the disclosed technique, system 100 (Figures 1A and 1 B) is disposed in an outdoor environment other than a motorway environment, such as an airport terminal, a naval port, a marina, a military base, a meteorological measurement site, and the like, where the system is operative to determine the visibility level in the particular environment in which it is disposed. In this embodiment, the transceiver unit and the retroreflective element are each disposed at a fixed site in the outdoor environment, which allows for unobstructed transmission and reception of the laser pulses. It is noted that the operational wavelength of laser transmitter 122 (Figure 3) is preferably in the near IR spectral range, as stated above, when system 100 is disposed

in a motorway environment, but may be in the visible spectral range (e.g., a green color, for improved visibility) when system 100 is disposed in a different environment.

Reference is now made to Figure 6, which is a schematic illustration of a method for joint determination of visibility level, traffic flow and traffic density in a motorway environment, operative in accordance with an embodiment of the disclosed technique. In procedure 202, a plurality of laser pulses are transmitted toward a retroreflective element located at the side of a road, and toward retroref lectors on the rear of a vehicle on the road. With reference to Figure 2B, laser transmitter 122 of transceiver unit 104 transmits laser pulses toward retroreflective element 108 at the side of road 102, and toward retroreflectors 112 on the rear of a vehicle 110 on road 102 (e.g., retroreflectors embedded in the vehicle taillights, in the vehicle license plate, in a bumper sticker, or in the vehicle paint). Transceiver unit 104 is mounted on a road structure 106 elevated above road 102.

In procedure 204, reflected pulses reflected from the retroreflective element are received. With reference to Figure 2B, retroreflective element 108 reflects the transmitted laser pulse back toward visibility level detection receiver 124 of transceiver unit 104.

In procedure 206, the level of visibility in the surrounding environment is determined, based on the number of reflected pulses required to reach an energy threshold, with respect to the number of

reflected pulses required to reach the same threshold during predetermined visibility conditions. With reference to Figure 4, processor 162 of visibility level detection receiver 124 determines the level of visibility in the surrounding environment. During good weather conditions, visibility level detection receiver 124 carries out a benchmark measurement, representing the number of collected pulses necessary to reach a threshold (i.e., when the maximum energy capacity of integrator 160 is attained). Processor 162 compares the benchmark measurement with the operational measurement, representing the number of collected pulses required to reach the same threshold, and determines the level of visibility as a function of the difference between the operational measurement and the benchmark measurement.

In procedure 208, reflected pulses reflected from the retroref lectors on the rear of a vehicle are received. With reference to Figure 2B, retroreflecors 112 on the rear of vehicle 110 reflect the transmitted laser pulse back toward vehicle counting receiver 126 of transceiver unit 104.

In procedure 210, the traffic flow along the road and the traffic density in a road section is determined, based on the number and timing of reflected pulses received. With reference to Figures 1A and 5, processor 178 of vehicle counting receiver 126 determines the traffic flow along road 102, and the traffic density in road section 116.

In procedure 212, traffic recommendations are established for the road, in accordance with the determined level of visibility, traffic flow, and traffic density. With reference to Figure 1A, a control unit (not shown) receives the determined visibility level, traffic flow and traffic density, and establishes an updated speed limit (or other type of traffic recommendation) based on this information. The updated speed limit is displayed on speed limit sign 114.

It will be appreciated by persons skilled in the art that the disclosed technique is not limited to what has been particularly shown and described hereinabove. Rather the scope of the disclosed technique is defined only by the claims, which follow.