Road Network Operations
& Intelligent Transport Systems
A guide for practitioners!

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Automatic Vehicle Locator / Computer Aided Despatch

Automatic Vehicle Location (AVL) is at the heart of modern fleet management, helping operators to manage fleets more effectively through technologies that can provide a direct link between vehicles, operation control centres and real-time passenger information systems. It allows for real-time tracking of vehicles, enabling improved service efficiency, asset utilisation and customer service.

The primary navigational technologies used in AVL systems include Global Positioning Systems (GPS), dead-reckoning systems, station or roadside detectors, sub-surface detector loops and wireless triangulation. In-vehicle data processing is undertaken so that the GPS receiver’s three-dimensional coordinates can be determined. The information on vehicle location is then sent to the traffic centre, the dispatch centre and bus stop as needed. (See Enabling Technologies)

Since all satellite navigation systems require the observation of at least four satellites to function, vehicle location needs complementary systems that continue to work even when a vehicle is in a tunnel, under trees, or surrounded by tall buildings. Gaps in coverage can be bridged by:

  • map matching, the basic component of popular in-vehicle navigation systems. This takes advantage of the fact that vehicle location is usually restricted to the road network. It uses a highly accurate digital map and heuristic algorithms
  • dead reckoning, using a gyroscope or related inertial guidance principles to deduce vehicle location in reference to a known starting point. However, cumulative error has to be corrected occasionally

There are other methods to determine vehicle location – such as mobile phones. These are important for emergency calls and other location-specific ITS services.


Practitioners will need to take decisions on how much to centralise the control centre. This will depend greatly on how much the dispatching function is already decentralised and on the capabilities of operating staff.


GALILEO, Europe’s Global Satellite Navigation System will provide a highly accurate guaranteed global positioning service under civilian control. The fully deployed system will consist of 30 satellites and the associated ground infrastructure. Galileo will be interoperable with GPS and GLONASS, the US and Russian military global satellite navigation systems. The high number of satellites available will allow positions to be determined to within a few centimetres, improving the availability of signals in high rise cities and providing better coverage at high latitudes.


Considerable investment is needed in data collection and software development to map the transport network and complement data generated by traffic and vehicles. ITS requires reliable databases of network links, interconnections and other features, supported by a sound location referencing system. Without an inventory of stop locations, for example, it is not possible to offer point-to-point journey planning for public transport. Similarly for road information, reliable coding of the network is needed for emergency response. Wherever possible, collection, location referencing and storage of this data in a database for use by public transport operators or an agency should be co-ordinated and compatible with data on the road network held by the road network operator.

Transport network databases need constant maintenance to keep them up-to-date. Careful checking is essential to avoid errors which can lead to features being incorrectly located.

Reference sources

Synthesis Report 73 (‘AVL Systems for Bus Transit: Update’ of the USA’s Transit Co-operative Research Program (TCRP) provides a good overview of US experience. It is available at