Horses for courses: which parking sensor for which application?

In Australia, we see two major sensor technologies at work in car parks and street side parking: sensors that use infrared and sensors that use magnetometers. These technologies are often used interchangeably, but in truth, they are best suited to different purposes. It’s important to consider the purpose to which sensors will be put before determining what sort of parking sensor technology will be installed.

Parking sensors are installed for a multiplicity of reasons, but there are three major ones:

  1. Way-finding, meaning the use of sensors to help direct motorists to areas of fewer parked vehicles. This can help streamline the parking process, reduce frustration, and make the turn-over of vehicles in a parking area more efficient overall.
  2. Enforcement and infringement issuance. This is the use of vehicle sensors to support parking enforcement, so limited parking spaces can be made equally accessible to a large number of motorists, and to support the issuance and upholding of infringements to incentivise motorist compliance with regulations.
  3. The last major reason we see sensors installed is to facilitate parking data collection—that is, the use of sensors to learn more about motorists’ behaviour and inform local council policy, projects or initiatives.
Infrared sensors

 Infrared refers to light that is just beyond what we can see, past the red end of the visible light spectrum. Active infrared is a useful technology whereby a device emits a beam (usually from from an LED), and the radiation of the beam that is reflected back from an object is detected and measured by a sensor. Applications of infrared technology, like household remote controls, are familiar to us all, but infrared is used in unexpected areas, too—such as astronomy.

In their use in most parking sensors, active infrared devices emit a beam from an LED. The beam is reflected back by the undercarriage of a car, and is detected by a receiver, which then records that an object is above the sensor, and that the car park is therefore in use.

Infrared sensors have specific qualities that help determine their fitness for a purpose:

  • they require limited calibration
  • they can often be installed relatively cheaply

However:

  • the beam of an LED may be easily interrupted by common environmental factors or tampering
  • reliance on active infrared, requiring the use of an LED that pulses often enough to be useful in detecting vehicle presence or absence, creates a battery drain that can decrease the longevity of the sensor
  • the tension between power management and measurement frequency can compromise accuracy, particularly in detecting vehicle turnover in high-turnover areas

Infrared systems can be useful for parking data collection and tracking general behavioural trends, to examine frequency of use or policy or for providing way-finding information to motorists looking for a free space.

They are not regarded as infringement-grade technology—and a peer-reviewed analysis on the efficacy of different sensors concluded that infrared sensors are by and large not accurate enough for enforcement and infringement management purposes.

Magnetometer sensors

Any object with a sufficiency of iron-rich minerals in it creates a measurable change in the Earth’s magnetic field. A magnetometer is any device used to measure those changes. In Australia, we use these for geological surveys by land, air and sea. They’re also used for navigation, to inform ecological management over terrain that is large or difficult, or even to locate shipwrecks.

When used in parking, magnetometer sensors rely on small changes in the Earth’s magnetic field to create a magnetic “map”. The magnetic field is disrupted by the movement of metal objects, and can therefore be used to detect vehicle movements across a specific area.

Magnetometer sensors are also subject to the advantages and limitations of the technology:

  • they use a passive sensing technology that inherently has extremely low power consumption, which makes it possible to achieve longer service life than sensors using active sensing technologies
  • most magnetometers sense at two or more measurements per second, rendering them more accurate in terms of measuring vehicle turnover in a parking environment and less likely to miss a movement

However:

  • they require calibration to ensure that white noise and environmental interference is removed from the data reported

The reliability and accuracy of magnetometer sensors makes their use in infringement issuance and parking enforcement solutions more advisable, and they are significantly more likely to be accepted as viable evidence if it happens that an infringement notice is appealed in court. However, due to the need for calibration and the algorithmic expertise necessary for these sensors to interpret information correctly, they can be a more expensive solution up-front when compared to infrared devices.

If a relatively cheaper solution is required for general data capture or way-finding only, infrared technologies may be the correct solution. However, if an organisation wishes to use sensor data to inform enforcement and infringements issuance, only the infringement-grade magnetometer sensors pay for themselves—appealed or withdrawn notices incur more expenses than correctly issued ones made with accurate data.

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