Automated Vehicle Counting Technologies

Author: Bill Rochat
Last Updated: 04/05/2007

 

Automobiles and Trucks may be detected and counted in a variety of ways. Each technology has its pros and cons. This article will attempt to explain all the technologies being currently offered in the marketplace. With the exception of the very earliest of the Air Hose counters, all of these technologies use some form of electronic data storage for the counts. The very earliest automated air hose counters recorded the air pulses on a punched tape. This site is primarily focused on portable vehicle counters, but all the technologies are discussed and compared. At one time or another companies have offered products that allow normally permanent technologies to be used in portable applications.

 

Acoustic (active)

This technology has been around for a while, but has not been in wide-spread usage. Acoustic sensors operate normally in the Ultra-sonic portion of the band. They send out a "chirp" then measure the time it takes for the return "echo". These sensors can measure short distances very accurately (you see them in the electronic tape measures now on the market). These sensors can detect the presence of a vehicle in a relatively controlled environment. More sophisticated units can detect multiple vehicles in a multi-lane environment. Usually permanent installations where other technologies are not suitable.

Pros: Off roadway installation.

Cons: Suseptability to interfering ultrasonic noise sources. Medium power requirement.

 

Acoustic (passive)

Passive Acoustic is a glorified microphone optimized to "listen" for typical sound patterns caused by passing vehicles. Though microphone technology has been around for a long time, the ability to differentiate the sound of passing vehicles from all other normal sounds has only recently become viable with improving computers and computer programs. Probably will not be used much for portable survey work, but may find uses in permanent or semi-permanent applications.

Pros: Off roadway installation. Medium to low power requirements.

Cons: Suseptability to interfering noise sources.

 

Air Hose (or Pneumatic Hose)

The single most popular technology for portable/temporary vehicle counting. This technology was the first developed for vehicle counting. Basically, an air hose (or hoses) is stretched across a road or highway, either a single lane, or multiple lanes. One end of the hose is plugged off (to keep dirt and water from being sucked into the hose) and the other end is connected to a vehicle counter unit equipped with air switches. Each time a vehicle tire drives over a hose, an air pulse is generated which is then detected by an air switch. This technology counts the vehicle axles, and if two hoses are stretched across the same lane in parallel, this technology can determine vehicle speed and axle spacings, as well as direction of travel. Counters designed to accept the two parallel hoses for the purpose of determining vehicle speeds and axle spacings are called "classifiers". Often, the these counters will classify the vehicles by axles spacings using standards like FHWA, Austroads, and the EURO standards. Most of the new hose counters being offered now time-stamp the readings and the data is analyzed later, after downloading.

The pros for this technology are: Cheap sensing elements (air hose), high accuracy speed and axle spacing measurements possible, relatively quick set-up time, simple set-up equipment, will operate in all types of weather (in snow and ice, they can be inadvertently scrapped up by snowplows though). Properly set up, the counters can approach 100% count accruacy. Very low power consumption.

The cons for this technology are: Hoses are only good for a few months of use, undetected splits in hose can cause erroneous readings, occasionally hoses can break loose from their anchors so counts can be lost, and hoses are open to vandalism, since you can't very well hide them, to set-up the hoses does mean entering and crossing the roadway which presents a safety issue.

Echo's are also a cause of miscounting. Very slow stop and go traffic can cause considerable miscounting. When counting multiple lanes, simulaneous vehicle hits can cause missed counts, and in classifiers can cause mis-classification of vehicles.

 

Contact Closures

In one form or another this technology has been around for a long time. For vehicle counting, making the swiching element rigous enough for constant flexing under high pressures has been the challange. This technology comes in several forms; PET switches which look line rubber air hose, pressure mats which are flat rectangular mats and pressure plates. PET switches consist of two or more parallel wires with a conductive elastamer (plastic) formed between them and the whole thing encapsulated in a rubber insulation, when a tire compresses the rubber housing, it presses the wires against the conductive elastamer which becomes more conductive with more pressure. Pressure mats consist of two woven metal mats with plasic insulation woven in between and a rubber insulated coating on the outside. As pressure is applied to the mats, the metal fibers will eventually touch causing a switch closure. Pressure plates are mechanical plates that tip or deform pressing against a sealed switch of some kind. These are always permanent installations. The other two schemes can be used for portable counting, but usually not.

Pros: Capable of axle counting. Very low power.

Cons: In roadway installation required. Expensive sensors to replace when damaged as likely in portable usage.

 

Fiber Optics

A new technology for vehicle counting, Fiber Optics is similar to the Piezo sensors in application. Fiber optics are basically coaxial cables consisting of an optical fiber down the center of a protective housing. A laser light is sent down the fiber and the reflected light coming back is analyzed by a detector (similar conceptually to Radar). When a tire drives over a portion of the optical fiber, the fiber is bent. This bend causes a change in the reflected laser light coming back to the detector. With proper installation, the degree of bending can be calibrated into an axle weight, which could be useful for weigh-in-motion. In portable applications, a fiber optic cable is stretched across a lane, or multiple lanes. It is usually taped down. Two parallel cables can be laid down to get speed and classifications. Normally, these sensors would be most useful in permanent installations.

Pros: Totally immune to electrical interference of any kind.

Cons: Medium power consumption. Expensive sensor elements. Tricky to install.

 

InfraRed (IR active)

InfraRed technology has been around for quite a while, but mostly in permanent installations. There have been many attempts to adapt this technology to portable applications, but beam alignment presents a serious problem to overcome. Basically, the technology works by sending out an InfraRed beam of light across the lane (or lanes) you wish to count vehicles in, and reading back a reflected signal from a Retro-reflector (like the reflectors on the back of bicycles). Whenever a vehicle breaks the beam, a vehicle is counted. Two sensors can be mounted to detect vehicle speed, length and direction.

Pros: Off roadway installation.

Cons: Medium power consumption. Prone to false reflections from shiny surfaces. Sensitive to headlight and solar light reflections. Requires an accurate alignment of sensor and reflector elements. In portable applications, vandalism (even curiosity) can easily mis-align the units, also the sensor unit itself can be affected by wind. Can fail to work during heavy rain, fog and dust conditions.

 

InfraRed (IR passive)

Passive infrared technology has been around for a long time, but not for vehicle counting. It works on the priniciple of detecting heat from an object. The sensors are glorified versions of the motion sensors used in security (like lights that turn on when you walk nearby). There have been attempts to adapt this technology to counting applications, but getting consistent heat signatures from vehicles and differentiating other sources of heat (such as persons), present serious problems to overcome. This type of sensor can find application for counting in unusual circumstances, like tunnels, or bridges, where pedestrians are not present. These would mostly be used in permanent installations.

Pros: Off roadway installation. Low power.

Cons: Prone to false responses from unwanted heat sources and failures to detect wanted sources. Response times can be slow.

 

Laser

This technology has been around for a few decades, but not applied to traffic counting. Laser sensors can be configured in a large variety of ways. They find use in a lot of applications where long distance sensing is needed or precision distance measurements. Probably most familiar to most people are the laser pointers that are currently very popular. Lasers generate a very narrow beam which can travel long distances without spreading very much. Lasers can be visible like the laser pointers mentioned above, or invisible in the infrared bands. The laser schemes being used for vehicle detection and counting work on the backscatter priniciple. Namely, when the laser beam strikes a surface, some percentage of the beam will be reflected back towards the source. With lasers, the tight and intense beam make this method of detection practicle. Also, unlike the typical infrared sensors mentioned above, no tricky alignment is required. Because of the potential for human eye damage when exposed to laser light, the application must take great care in controling the beam intensity and total delivered energy. This technology would be mostly for permanent installations.

Pros: Off roadway installation. No critical alignment.

Cons: Medium to High power requirements, potential for eye damage, and can fail during heavy rain, fog and dust conditions.

 

Loops

This technology is nearly always a permanent installation. Loops are just big loops of wire buried in the roadway. The loop electronics drives an alternating current through the loop which creates an alternating magneting field around the loop. They detect vehicles by the vehicles influence on this magnetic field which induces a change on the alternating current generated by the electronics. This influence is small, generally 1% to 2% of the total signal strength (or frequency), so electronics detection circuitry is a necessity. You can purchase temporary loops that sit on the surface of the roadway, generally held down by tape or nails, but this practice is not too practical for most counting applications. Though the loop installations are permanent, the counting electronics can be fixed (such as signal light controllers), or portable, where the counting electronics is moved from sight to sight on a weekly basis. Relatively low power consumption.

Pros: High reliability, the loops can last many years before needing replacement. Vandal proof, since all parts of the system are either buried or in robust enclosures. One count per vehicle.

Cons: Susceptability to RF and magnetic interference. High cost of loop replacement. Loops are mounted permanently just below the surface of the roadway, so must be accounted for any time road work is being performed in the area. Do not count well in slow stop and go situations.

 

Magnetometers

A more recent technology, magnetometers, has been developed for the portable vehicle counting industry. The magnetometers developed for vehicle counting detect distortions in the Earths magnetic field as a vehicle passes over the magnetometer. The sensor element and the recording electronics are built as a single unit. This unit is typically mounted in the middle of the lane you want counted, as a result, most units are designed to be very low profile, flat plate-like structures that are nailed or bolted down to the pavement in the middle of the lane. By incorporating two sensors in the same unit with a reasonable spacing, speed, direction and vehicle length can be measured. Some magnetometers are permanent installations. These units are buried in the pavement in the middle of the lane. These self-contained units operate off of batteries for several years. Data is retrieved via radio transmissions (which requires a nearby receiver/data logger unit), or a buried communications cable exiting at a roadside enclosure. Relatively low power consumption.

The pros for this technology are: One count per vehicle, even in very slow or stopped traffic conditions. Portable units can be mounted in roadways that are awkward or impossible for the other technologies, such as on curves or in tunnels.

The cons for this technology are: Units are mounted in the middle of the road, so there is a safety issue during set-up and pull-up. Units are prone to damage and have to be replaced or repaired at a pretty high rate. Counting accuracy is not as good as other technologies, but it is nearly always better than 95%, which is sufficient for the vast majority of counting applications. Cannot classify vehicles by axle distribution.

 

Microwave (Radar)

A recent technology for vehicle counting. Nearly always a permanent installation, though some portability is possible. Works on the classic Radar principles. Namely a Microwave "beam" is sent out from the transmitting unit and a "reflection" signal is detected at the receiving unit. In modern radar units, the tranmitter and receiver are a single unit. The simplest forms of these radars are used in radar guns used by the police to measure vehicle speeds. For counting, a more elaborate system is required, since range information is also needed to be retrieved from the signal. A properly installed unit can detect across multiple lanes. The sensing and counting electronics are built as a single unit.

Pros: Off roadway installation. Multi-lane counting.

Cons: Expensive. High power consumption. The radar unit can be affected by wind.

 

Piezo

Another recent technology for vehicle counting, Piezo sensors. Piezo sensors can come in a variety of shapes and sizes, but for vehicle counting, they generally come in long narrow strips, or in coaxial cable. These sensors are generally used in permanent installations, where they are installed just below the surface of the roadway, however, they can be taped to the surface of the road in a portable application. These sensors have the ability to measure the amplitude of the pressure signal, so find use in weigh-in-motion applications. The Piezo sensor generates a voltage when the sensor element is stressed, either flexed, or compressed. Piezo sensors count axles as they drive over the sensor. These sensors require no power to operate and connect to electronics similar to air hoses. They are normally mounted one sensor (or two in parallel if classifying) per lane.

Pros: Precise per-axle detecting. Nearly noise and interference free. Large detection signals. With two parallel sensors, speed, direction and classification possible. Nearly 100% count accuracy.

Cons: Sensor elements are expensive. In portable use sensor damage can get very expensive.

 

Video

The latest technology being applied to vehicle counting. To date video counting amounts to recording a section of roadway and later sitting down with a video player unit and manually counting the vehicles and vehicle types and directions. Video stop light controls are now being installed which could ultimately be used for counting vehicles. Automated counting is still a long way off for video, but the promise is there. It will take pretty sofisticated artificial intelligence algorithms to determine vehicles, their types, the lanes they are in, all under varying weather and lighting conditions, etc. The first real installations will be permanent, such as the traffic monitors set up all over most major cities, but as the software developes, portable counting may become possible. The other major problems for portable use will be securing the video cameras against vandalism.

Pros: Off roadway installation. Multi-lane, multi-direction detection.

Cons: High power requirements.

 

 

Technology	Usage	Sensor Cost	Counter Cost	Set-up Cost for Portable use	Set-up Cost for Permanent installation	Dominant market
Acoustic active*	Fixed some Portable	Low to High	Low to High	Low to Medium	Medium	Parking garages. Traffic monitoring.
Acoustic passive*	Fixed some Portable	Low	Low to Medium	Low	Medium	Traffic monitoring.
Air Hose	Portable	Very Low	Low to Medium	Low	Not Applicable	Portable surveys
Contact closures	Fixed some Portable	Medium	Low to Medium	Low to Medium	Medium	Parking garages and lots
Fiber Optics	Fixed some Portable	Medium	Low to Medium	Medium to High	Medium to High	Traffic monitoring
InfraRed active**	Fixed some Portable	Low to Medium	Low to Medium	Medium	Low to Medium	Parking garages, toll booths
InfraRed passive*	Fixed some Portable	Low	Low to Medium	Low to Medium	Low to Medium	Parking garages, tunnels
Laser*	Fixed some Portable	High	High	Medium to High	High	Traffic monitoring
Loops	Fixed some Portable	Medium	Medium to High	Medium	High	Signal light control. Traffic monitoring.
Magnetometer*	Portable some Fixed	Low	Low to High	Low	Medium	Portable surveys
Microwave*	Fixed some Portable	High	High	Medium to High	Medium	Traffic monitoring
Piezo	Fixed some Portable	Medium	Low to Medium	Medium	Medium to High	Weigh-in-motion
Video***	Portable and Fixed	Medium	High	Low to Medium	Low to High	Traffic monitoring

* These technologies usually house the sensor and counter electronics in the same assembly.
** IR counters come in two flavors, one where the sensors are incorporated into the counter and the other that uses external IR sensors.
*** Video at this time is not able to do automated counting with any degree of accuracy.