Vehicle Detection: Ten Ways to Count Traffic

There are a surprising number of ways available today to count vehicles, from manual counting to the latest ai-powered computer vision.

ManualComputer Vision (Video)Pneumatic TubesPiezoelectricInductive LoopMagneticAcousticPassive InfraredDopplerRadar

Manual counts

A simple but accurate method of traffic counting comprises people manually counting vehicles.

A person either uses an electronic hand held counter or records data using a tally sheet.

They may stand at the side of the road, or, more commonly watch a video of the road and count from that.

In tests manual vehicle counting was 99% accurate over the counting period.

With manual counts a small sample of data is taken – typically during less than a day – and results are extrapolated for the rest of the year or season. This is where errors are introduced as the small sample is rarely exactly representative of the the entire year or even that week.

Vehicle Detection using Computer Vision

Obviously manual counting is labour intensive. Systems are now available that will automatically analyse video pictures as cars pass cameras, detecting vehicles with a similar accuracy to that of people watching the video.

This vehicle counting method has several advantages over other automatic systems. It is cost-effective as it can count in many directions at once: only one camera is needed for several lanes or exits at a junction.  It is easy to add or modify the zones through which vehicles are counted from a laptop computer.

Local video analytics minimises bandwidth use. Traffic counts are uploaded in real-time wirelessly via the internet, so traffic engineers can view live and historic counts from their web browsers.

Counts are easily verified simply by watching the video and checking the automated counts.

Video traffic counting typically takes place continuously, year round, giving precise figures. This method of vehicle detection is often used in smart cities – the count data is made available over the cloud wherever it is needed. Useful for monitoring congestion, car parking and assessing traffic control initiatives. AI-powered software enables traffic counts and classification – trucks, cars, tractors, bikes etc.

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With intelligent camera counting, systems can track repeat visits without compromising peoples’ privacy and complying with GDPR.

Pneumatic Road tube counting

This has for many years been a popular method of vehicle sensing.

Here one or more rubber hoses are stretched across the road and connected at one end to a data logger. The other end of the tube is sealed. When a pair of wheels hits the tube, air pressure in the squashed tube activates the data logger which records the time of the event.

A pair of tubes can be stretched across several lanes of traffic. The data logger can establish vehicle direction by recording which of a pair of tubes the vehicle first drives over. This has the drawback that if two vehicles cross the tubes at the same time then the direction can’t be accurately determined. Should two cars be very close together when they cross the tubes, the system may see them as one multiaxle vehicle.

Vendors claim an accuracy of 99%. Studies show though, that the absolute error of a typical 15-minute count averaged closer to ten percent. This suggests that the level of inaccuracy is being masked by the positive and negative counting errors cancelling each other out.

The counts need to be physically downloaded onto a computer from the loggers at the roadside. At least one road tube is needed for each direction on every road or junction at which you want to count. Installation requires working within the traffic lane.

Road tubes can work well for short duration counts on lower volume roads. They are not as effective on higher volume, multi-lane highways. They also can’t categorise vehicles into types. First used in 1920.

Piezoelectric Sensor

Piezoelectric sensors collect data by converting mechanical energy into electrical energy. The piezoelectric traffic sensor is mounted in a groove cut into road’s surface.

When a car drives over the piezoelectric sensor, it squeezes it and causes an electric potential – a voltage signal. The size of the signal is proportional to the degree of deformation. When the car moves off, the voltage reverses.

This change in voltage can be used to detect and count vehicles.

The counting device which is connected to the sensors is housed in an enclosure by the side of the road. Data may be collected locally via an Ethernet or RS232 connection to a laptop, or may be transmitted wirelessly.

Piezoelectric traffic counter sensor by the side of the road
Photo by Louis van Senden [CC BY-SA 4.0]
Piezoelectric traffic counter sensor by the side of the road Photo by Louis van Senden [CC BY-SA 4.0]

A drawback of piezoelectric sensors is their sensitivity to vehicle speed and road temperature.

Inductive Loop

An inductive loop is a square of wire embedded into or under the road. The loop utilizes the principle that a magnetic field introduced near an electrical conductor causes an electrical current to be induced. In the case of traffic monitoring, a large metal vehicle acts as the magnetic field and the inductive loop as the electrical conductor. A device at the roadside records the signals generated.

The road has to be closed to install the inductive loop. Although they can perform some automatic classification of vehicles, they do not do well in high congestion

Magnetic Sensor

This detects vehicles by measuring the change in the earth’s magnetic field as the vehicles pass over the detector.

The traffic counter sensor is either buried in the road, or enclosed in a box by the side of the road.

If vehicles are following each other very closely, the magnetic detector may have difficulty discriminating between them.

Acoustic detector

This detects vehicles by the sound created as the vehicle passes.

The sensor is mounted on a pole pointing down towards the traffic. It can collect counts for one or more travel lanes.

Some can communicate their counts wirelessly.

Passive Infrared

Passive infrared devices detect vehicles by measuring the infrared energy radiating from the detection zone. When a vehicle passes the energy radiated changes and the count is increased.

Slow changes in road surface temperature, caused by changing weather conditions, are ignored.

Lane coverage is limited to one to two lanes.

Doppler and Radar Microwave Sensors

Doppler microwave detection devices transmit a continuous signal of low-energy microwave radiation at a target area and then analyze the reflected signal. The detector registers a change in the frequency of waves occurring when the microwave source and the vehicle are in motion relative to one another. This allows the device to detect moving vehicles.

Radar is capable of detecting distant objects and determining their position and speed of movement. With vehicle detection, a device directs high frequency radio waves at the roadway to determine the time delay of the return signal, thereby calculating the distance to the detected vehicle. Not suitable for dense traffic congestion.


1An Investigation on the Manual Traffic Count Accuracy, Procedia – Social and Behavioral Sciences 12/2012; 43:226-231. DOI: 10.1016/j.sbspro.2012.04.095

2Accuracy of Pneumatic Road Tube Counters. McGowen and Sanderson. A report prepared for the 2011 Western District Annual Meeting Institute of Transportation Engineers Anchorage, AK May 2011.

3Traffic Monitoring Guide, 2013, U.S. Department of Transportation

4Vehicle Traffic Counting, Retail Sensing 2023

5Shokravi H et al. A Review on Vehicle Classification and Potential Use of Smart Vehicle-Assisted Techniques. Sensors (Basel). 2020 Jun 8;20(11):3274. doi: 10.3390/s20113274. PMID: 32521806; PMCID: PMC7309154.

Photo credits: Retail Sensing, Louis van Senden [CC BY-SA 4.0]

Windmill Software

Windmill Software specialise in data acquisition and control. They publish a monthly newsletter, called Monitor, giving useful information on sensors and systems -

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