An effective (and often extensive) traffic surveillance and monitoring system is a pre-requisite for any intelligent traffic control system to keep track of prevailing conditions across the network. A wide range of different sensors are installed in, on and above the roadway for this purpose and to obtain the necessary geographical and critical time coverage. They include inductive loops, non-intrusive traffic detection devices, video cameras and video image processing. Each technology has its own advantages and shortcomings – so the choice of sensor type for any ITS application will depend on what performs well in the prevailing environmental conditions, and its cost.
Traffic detectors (or vehicle presence detectors) are used in many ITS applications for – network monitoring, traffic control, speed measurement and automatic incident detection. Many different types of detection technologies are available. The following are typical technologies that have been developed to measure traffic data in specific locations and zones. (See Vehicle Detection)
Inductive loop detectors are currently the most widely used devices for vehicle detection, although microwave radar detection is also common. Their main uses are at intersections in conjunction with advanced signal traffic control systems, and on freeways for traffic monitoring and incident detection purposes. ILDs typically take the form of one or more turns of insulted wire embedded in the pavement. The loop is connected via lead-in cable to the detector unit, which detects changes in the loop inductance (changes in the in the magnetic field of the sensor) when a vehicle passes over it. ILDs can be used to detect a vehicle’s presence or passage. They can also be used to measure speed (by using two loops a short distance apart) and for classification of vehicle types. The main problem with using ILDs, however, is their reliability. Because ILDs are subject to the stress of traffic, they tend to fail quite frequently. Moreover, their installation and maintenance require lane closure and modifications to the pavement.
Microwave radar detectors are examples of non-intrusive detection devices whose installation and maintenance does NOT require lane closure and pavement modifications. Unlike inductive loops, non-intrusive detection devices are not embedded in the pavement. Instead, they are typically mounted on a structure over, or to the side of, the road such as the radar detection system in the photograph below. Depending on the type of electromagnetic wave used, microwave radar detectors can measure either vehicle presence, or vehicle presence as well as speed. They are also widely used to detect pedestrians waiting at pedestrian crossings.
One of the major advantages of microwave sensors is their ability to function under all weather conditions. Exceptions can be extreme weather such as sand-storms. Given that these sensors are installed above the pavement surface, they are not typically subject to the effects of ice and ploughing activities. Experience shows that microwave sensors function adequately under rain, fog, snow, windy conditions. Their main problem is that they can be obscured by tall sided vehicles – reducing their accuracy when they are installed at the side of the carriageway.
Infrared (IR) sensors are also non-intrusive detection devices. There are two types: passive and active detectors.
Passive IR detectors do not transmit energy – instead, they detect the energy that is emitted or reflected from vehicles, road surfaces and other objects. Passive infrared detectors can measure speed, vehicle length, vehicle counts and occupancy but their accuracy is affected by adverse weather conditions.
Active IR detectors emit a beam of Infrared energy which is reflected back to an IR receiver. They function in a similar way to microwave radar detectors – by directing a narrow beam of energy towards the road surface. The beam is then reflected back to the sensors – and vehicles are detected through changes in the “round-trip” transmission time of the infrared beam. Active infrared detectors supply vehicle passage, presence, speed, and vehicles classification information. They work well in controlled environments such as tunnels – and infrared can be used for safety purposes to detect over-heating vehicles or fire. Their accuracy is affected by weather conditions such as fog and precipitation.
Ultrasonic vehicle detectors function in a similar way to microwave detectors by actively transmitting pressure waves, at frequencies above the human audible range. These detectors can measure volume, occupancy, speed, and classification. Ultrasonic sensors are sensitive to environmental conditions. They require a high level of skill for their maintenance.
Vehicle traffic produces acoustic energy or audible sound from a variety of sources within the vehicle and from the interaction of the vehicle’s tyres with the road surface. Using a system of microphones, acoustic detectors are designed to pick-up these sounds from a specific area within a lane on a roadway. When a vehicle passes through the detection zone, a signal-processing algorithm detects an increase in sound energy and a vehicle presence signal is generated. When the vehicle leaves the detection zone, the sound energy decreases below the detection threshold and the vehicle presence signal ends. Acoustic sensors can be used to measure speed, volume, carriageway occupancy and presence. The advantage of acoustic sensors is that they can function under all lighting conditions and during adverse weather.
Similar to inductive loop detectors (ILD), magnetometers provide for point detection, but they differ from ILD in that they measure changes in the earth’s magnetic field resulting from the presence of vehicles. They can provide information on traffic volume, lane occupancy, speed as well as vehicle length. In general, there are two types of magnetometers:
Micro loops, (like inductive loops), require lane closure and pavement modification, with consequent delays to traffic. In recent years, the use of wireless magnetometers has received increased interest because of advances in battery technology which allow a unit to operate wirelessly for a period of 10 years before needing to be replaced.
AVI can be used to identify vehicles as they pass through a detection zone. Typically, a transponder (or a tag) mounted on the vehicle can be read by a roadside reader as the vehicle passes by. This information can then be transmitted to a central computer. Currently, the most common road transport application of AVI technologies is in combination with Automatic Toll Collection systems (such as EZPass). With these systems, the value of the toll is automatically deducted from a driver’s account each time the driver goes through the toll plaza.
An important method of AVI, in common use, are ANPR (also known as Automatic Licence Plate Recognition - ALPR) systems which use optical character recognition technologies to identify and recognise vehicle registration plates. They typically consist of a specially adapted video camera linked to character recognition software. As a vehicle passes an ANPR/ALPR camera its registration number is read and can be checked against a database of vehicle records. There are two broad types of ANPR/ALPR systems:
With recent advances in computer hardware and software – processing in the field in real-time has become quite feasible (it typically takes less than 250 milliseconds). This avoids the cost associated with the need for large bandwidth to transfer images to a remote server.
Weigh-in-motion sensors are designed to measure and record axle weights and gross vehicle weights while the vehicle is in motion (driving – not stopped). WIM systems are attractive because they avoid the need to stop and weigh every vehicle. They have not eliminated the need for weighbridge sites for accurate weighing of trucks, but WIM acts as a filter and only vehicles which register an excess axle load need to be stopped and checked. The key component of any WIM system is the force sensor – for example quartz crystals produce electric charge when a force is applied along the vertical axis (the weight of the vehicle). WIM systems have several applications in ITS, especially as a part of an electronic pre-clearance system for commercial vehicles, as well as for enforcement applications. (See Enforcement and See Video)
Speed detection is an integral part of speed camera enforcement systems used to detect speed-related violations of traffic rules especially at accident hot spots. (See Speed Management) Regulation of speed is important at work zones where personnel are at increased risk of an accident. It is also a feature of active traffic management schemes on motorways. Some speed enforcement systems automatically link speed cameras to vehicle number-plate (Licence-plate) recognition for issuing enforcement notices. (See Traffic Management and Integrated Strategies) Speed detection can also be used as a safety measure at signalised intersections on fast arterial roads by using microprocessors to extend the green time at traffic signals when a vehicle is approaching at speed.
For measuring speed, the most common device is a radar meter or sensor which uses the Doppler principle. Specifically, the device measures the difference in the emitted and reflected frequency of a radar wave – which is proportional to the speed of the moving object. Other types of vehicle sensors can be adapted in pairs to measure speeds – such as ultrasonic sensors and magnetometers.
Journey time monitoring is related to speed monitoring. Vehicle journey times are significant sources of information for network performance monitoring and advising road users about travel delays in real-time. They are a measure of the level of service on offer. Some road authorities display point-to-point journey times on roadside VMS as a form of real-time information. Journey time data (historic and in real-time) is also a useful resource for journey planning and logistics support. (See Journey Time Monitoring)
Various methods are available to anonymously track vehicles on the network and enable network operators to determine average travel times, point-to-point demand and traffic flow conditions. For example, Automatic Toll Collection (ATC) systems can be used to determine the average travel time on highways between toll collection plazas or specially installed roadside readers. Infra-red (IR) tag-equipped vehicles are used as probes for monitoring traffic flow conditions – which are detected by transponder readers, installed along roadways. Aggregate data on average speeds and travel times can be compiled and this helps support incident and traffic management. To protect travellers’ privacy, these systems scramble the toll tag identifiers and only keep records of trips made by anonymous vehicles.
A number of other techniques are used to provide continuous, non-invasive, point-to-point tracking of individual vehicles to determine travel times and calculate average speeds. They include automatic number-plate recognition cameras (ANPR cameras) to identify vehicle licence plates. A new development is point-to-point monitoring of Bluetooth signatures emitted by equipment present in the vehicle. Bluetooth sensors have been used successfully for point-to-point average speed monitoring as a cheaper alternative to ANPR. Some road authorities use aggregated data (made anonymous) to display on VMS to provide drivers with expected journey times between key points on the network.
Environmental sensors are used in road network monitoring to detect adverse weather conditions such as icy or slippery conditions, high winds or precipitation (snow or rain) or the presence of fog/mist. This information can then be used by operators to alert drivers via variable message signs (VMS). It can also be used by highway maintenance personnel to optimise winter maintenance operations. (See Weather Monitoring)
Environmental sensors can be divided into six types:
Many manufacturers provide complete weather station systems that are capable of monitoring a wide range of environmental and surface conditions. The figure below shows one example of these weather stations.
(Figure 4.7 to be located here – to be supplied by the author)
Weather stations typically include the following types of sensors and capabilities:
Road condition sensors: A critical component of any road weather information system (or RWIS) is a set of road condition sensors that measure surface temperature and moisture, and detect the presence and thickness of snow and ice. Road condition sensors can be embedded in the pavements. They can also be non-intrusive – mounted to side or above the road surface. Non-intrusive road condition sensors typically measure the emitted infrared radiation from the road surface.
Visibility sensors: These sensors are designed to measure visibility along a road section. They typically use the principle of “forward scattering” or diffraction of light to detect changes in visibility resulting from inclement weather conditions such as fog, haze, and smoke. The sensors need to be carefully sited because they can only provide spot measures at a specific location. For example, fog detectors need to be sited as near as possible to the source where mist or fog forms first.
Thermal mapping: Given that temperatures can vary significantly along a roadway segment, thermal (temperature) mapping sensors are typically a critical component of an effective ice detection system. Thermal mapping provides road operators with information on road-surface temperatures to inform decision making on the need to set-up warning messages on VMS or to deploy snow clearance, road salting and gritting services. Examples of thermal imaging sensors include thermal imaging cameras/ video and infrared thermography.
Wind speed sensors: These are an essential component of an environmental sensing station and are installed at high and exposed bridges and windy locations on the road network. They typically measure surface wind speed and direction and can be used to provide warnings to vehicles towing trailers and high-sided vehicles. For safety reasons it is sometimes necessary to close the road in high winds.