Inductive loops buried beneath the road surface or passive roadside detectors are able to provide a comprehensive view of traffic flows on the road network - but the impact of traffic conditions on individual vehicles can be difficult to understand. Detecting an identified vehicle as it passes through known locations on the network can serve as a “traffic probe”.
Probe vehicle monitoring is emerging as a critical technology that complements static sensors, CCTV cameras and ad-hoc reports, whilst also providing information on the speed and flow of a road segment. Road network monitoring is traditionally provided by static detectors such as inductive loops and CCTV cameras located on strategic roads with the highest volumes of traffic. (See Vehicle Detection) This is supplemented with ad-hoc reports from the police, emergency services and members of the public, which contributes to the development of a comprehensive view of the roads (including secondary or tertiary roads) – but the cost of monitoring a large area is often prohibitive.
Probe vehicle monitoring is where a vehicle is equipped to track journey time - for example by tracking Bluetooth signatures between successive points or by using in-vehicle navigation (GNSS). Speed and point to point journey time data is stored and communicated back to the management or control centre to:
Probe vehicles can generate more detailed information on the impact of traffic conditions – by providing the data to calculate, for example:
A probe vehicle must be capable of being reliably and uniquely identified at more than one discrete location. For example:
Roadside-based detection technologies make it possible to detect the same vehicle at different locations on the road network so it can be used as a traffic probe. The vehicles must have a specific identifier - such as a readable number plate, a Radio Frequency Identification (RFID) tag, or vehicles carrying a device (such as mobile phone, satellite navigation system, computer or portable media device) with a Bluetooth transceiver that is discoverable. Probe vehicles can also be categorised as “passive” or “active” - although sometimes this is an arbitrary distinction:
The techniques of Probe Vehicle Measurement have developed to ensure that any data collected is combined to create average road link performance and is not personally identifiable.
The most common technologies for probe vehicle monitoring can be split into roadside-based detection and mobile device reporting. Examples include:
Satellite-based in-vehicle systems are often integral to fleet management applications for freight and commercial operations. These vehicles often travel along strategic highways and can be a suitable for a fleet of probe vehicles to cover the network. (See Freight and Commercial Operations) Satellite technology can also be used for road user charging. (See Electronic Payment)
In general, the greater the number of probe vehicles, the better the quality and accuracy of the information that is derived. Small samples of probe vehicle may not deliver high enough accuracy on a specific road or for a specific road network. The behaviour of a single vehicle (such as a bus) may not reflect the general traffic flow – so it is important that the probe vehicles are a high enough proportion of the total to meet the required levels of accuracy.
A study by the Washington State Department of Transportation (WSDOT) suggested that sample sizes as low as 45 RF-tag-equipped probe vehicles per day – with average 30-minute headways - could yield useful data on the occurrence, duration, severity, and frequency of congestion. Probe vehicle sample sizes varied depending on local road networks, traffic patterns and other factors.
Increasingly, road users are being informed about road network conditions, before and during trips, through social messaging and third party navigation systems. They may receive traffic reports from other sources such as public radio stations, mobile phone alerts (Twitter, Weibo or other social messaging sites) and third party navigation applications. Traffic authorities can monitor messaging services such as Twitter (South Africa) and Weibo (China) for traffic information that may help complement static vehicle detectors.
The provision, collection and use of data from road users raises legal and institutional issues relating to privacy, data collection, ownership and sharing. (See Legal and Regulatory Issues)