Under the framework of the Grenelle de l’environnement (French round table on the environment) the optimisation of the existing network is now a priority. Effective traffic management is vital for this to be achieved. This requires the implementation of active traffic management strategies which require the collection of the most accurate and accessible real-time and non-real-time data.
The electromagnetic loop is the most common sensor but other technologies have appeared. Here is a review of the different sensors available, organised in the following categories:
1. Electromagnetic loop
The electromagnetic loop is the most commonly used sensor in urban and rural environments. This loop consists of 3 spires of insulated wire. It is rectangular in shape, measuring 2 m x 1.5 m. It is similar to a self-inductor capable of detecting the presence of metal objects.
A loop measures traffic flow and occupancy rate. Two loops spaced 1.5 m apart measure the speed of the vehicles and their length.
This sensor provides highly accurate results in terms of traffic flows, vehicle speeds and lengths. It has the advantage of combining heavy-duty construction and state-of-the art technology.
An axle detection loop, associated with 2 conventional electromagnetic loops, can distinguish 14 vehicle silhouettes by means of signal analysis.
- high traffic flow accuracy,
- no meteorological disturbance
- civil engineering,
- operational disturbance during the installation
2. Piezoceramic sensor
This sensor is placed in a bar before being inserted into the roadway: the piezoceramic sensor is coated with epoxy resin injected under pressure, glass fibre, glass fabric and sand.
The bar is then placed in the roadway in a groove measuring 6 X 6 cm. It is bonded to the roadway with the same epoxy resin mixed with sand and glass fibre, then reinforced with glass fabric.
A piezoceramic sensor coupled with 2 electromagnetic loops allows the traffic flow to be classified in terms of axle weight and total weight.
Advantages : the sensor is able to provide precise information on the dynamic weight of vehicles
Drawbacks : civil engineering associated with the loops
3. Detection board
This detector, associated with a single electromagnetic loop, provides:
- The traffic flow,
- The flow classified according to 7 classes of vehicles,
- The occupancy rate,
- The inter-vehicle time,
- The speed of vehicles,
- The length of vehicles.
The expected accuracy for traffic flow is 1%, and 7% for the classified flow rates, with standard deviation of 7% for individual speeds, and standard deviation of 10% for the occupancy rate and lengths.
- a maximum of measurements with a single loop:
- Q, T, V (low), L
- remotely queriable smart card
- real-time transmission of vehicle signatures
- Siredo compatible (Normandy tests)
- less accuracy on V and L
- classification in 7 categories
4. Magnetic sensor
This is a small intrusive sensor that is installed in a loop in the traffic lane. Depending on the manufacturer, two or four sensors are installed to measure the traffic in one lane of traffic. The operating principle is identical to the electromagnetic loop which detects the variations in the magnetic field induced by a passing car.
The possible measurements are:
Traffic flow of all vehicles: maximum deviation in the order of 5%;
Speed: approx. ± 16%;
Length: between -30% and +34%
Occupancy rate: approx. ± 30%.
The measurements are less accurate beyond these flow rates.
- smaller than loops
- stand alone, wireless connection
- some can be connected to Siredo stations
- faster installation
- reduced accuracy for V, L
- for the moment, adjustments are delicate (low sensitivity)
1. Hyperfrequency sensor
This is an overhead sensor (single-channel radar), placed above the lane of traffic. It is installed on a gantry or a bridge. One sensor is required per lane of traffic.
The accuracy of traffic flow and speed measurements is equivalent to that obtained with electromagnetic loops:
Traffic flow of all vehicles: less than 1% of relative deviation;
Speed: between - 5% and + 6%.
For the length, the radar only provides "long vehicle" or "short vehicle" information according to a set threshold. No occupancy rate measurement.
- accuracy comparable to loops (Q, low V)
- not sensitive to the environment (dust, weather)
- Siredo compatible (certain technologies)
- more expensive,
- requires a support (gantry, bridge)
- delicate adjustments (regular maintenance)
2. Multi-technology sensor
This non-intrusive sensor combines the best of three technologies: doppler radar (hyperfrequency) for speed, an ultrasound sensor for counting and an infrared sensor for night vision. The data recovered is merged before transmission.
The accuracies tested are as follows:
- Traffic flow of all vehicles: approx. 7%;
- Speed: approx. 7%;
- Length: 2% of deviation between light-duty and heavy vehicles.
This sensor cannot be used to measure the occupancy rate.
- decent accuracy (Q, V)
- easily accessible and addressable
- GPRS transmission
- Siredo compatible
- no occupancy rate,
- binary L
- minimum refreshing 1 min.
3. Video sensor
The processing algorithms are able to extract the relevant information from a standard image obtained from video surveillance cameras. The video camera, which permanently films the site, is connected to a video acquisition and processing board which detects the presence of vehicles. This system is original as it integrates virtual sensors directly into the image of the site being monitored. This means that the sensors are activated following a modification of the image, due to a passing vehicle for example.
The possible traffic measurements are:
Traffic flow of all vehicles
Performance characteristics are close to those of the loop flow rates, speed, and occupancy rates.
- good Q, occupancy rate, and V accuracy
- (performance similar to that of the loops)
- complex movement measurements
- large zone surveillance (300 m)
- real-time display < 10 s)
- virtual sensor simulation
- masking, shadow, brightness problems
- sensitive to vibrations
- difficult maintenance in tunnels
4. Infrared sensor
This single-channel non-intrusive sensor provides traffic measurements by analysing infrared radiation emitted by vehicles. The sensor features a built-in detector and transmitter, enabling the measurements to be read directly on the output. Sensor adjustments are simple and easily accessible.
Advantages : no maintenance or adjustment required (excluding installation)
- no occupancy rate,
- accuracy to be improved
- sensitive to rain,
- not Siredo compatible
5. Ultrasound sensor
The sensors analyse the noise generated by the traffic. The sound analysis is performed using one or two microphones, implementing specific processing algorithms to measure traffic flow and the speed of vehicles. The sensors are placed on the roadside, and are therefore not intrusive.
The electronic and IT development of these systems is not sufficiently advanced. They lack the measurement accuracy of a presence loop but can provide traffic flow alerts based on traffic threshold settings. The speed measurement significantly correlates to the distance between the sensor and the sound source.
- lateral positioning, stand alone
- low V detection
- remotely queriable smart cards
- performance to be improved (Q, V, L accuracy)
- unsuited to individual measurements
- collection 6 min, data available 50% of the time
6. Optronic sensor :