Satellite Navigation Systems include a routing engine - software that plans the route to the destination. In basic models the route is planned on the basis of a fixed database that describes the road network. (See Basic Info-structure and Navigation and Positioning) More sophisitcated models can take into acount current traffic conditions and provide information on:
Presentation of the information on-screen or by audio must considered in light of human factors and the underlying map interface - the "user interface".(See Systems Approach to ITS Design).
Traffic information can be coded and distributed digitally as a non-audio data stream over radio. This is how most traffic-enabled Satellite Navigation Systems receive traffic information in real-time. They can then display an icon in relation to the event, alert the driver audibly to it and also prompt the driver to re-route - or automatically amend the route accordingly.
When there is a traffic incident, congestion, or extreme weather event, a data message can be sent to the navigation device. Each event can be assigned a level of severity (impact) in terms of its geographic extent, timing and its likely duration. (See Traffic Incidents) Transmitting these details enables the satellite navigation routing engine to calculate a delay level for the location (the links and junctions in the network that are affected). The software then recalulates the travel time and route options, taking account of this additional delay. Regulations and traffic restrictions may impact on whether automatic re-routing is allowed.
In Europe and North America the main standard for transmission of traffic event data to in vehicle satellite navigation systems is known as RDS-TMC. Information on RDS-TMC can be found from the Traveller Information Services Association website including TMC coverage maps. A lot of work has been undertaken to standardise the categorisation of events, inparticular the Alert C standard: ISO 14819:2013 - parts 1 and 2 define event categorisation for the RDS-TMC digital transmission system on FM radio.
For TMC to operate there must be a location code table tied to the underlying map network. These location code tables can be developed privately or publically.
Gradually as radio bandwidth becomes scarcer, so it is likely that Digital Audio Broadcasting DAB will take over from FM Analogue radio services.
There is a more detailed standard for broadcast of traffic related data over DAB known as TPEG. Practitioners should be mindful of the TPEG standard (ISO TS 18234) for traffic and public transport incident information. This standard makes use of the additional bandwidth available via DAB and the Internet. TPEG is now being widely implemented on digital networks. More information on TPEG can be found via the Traveller Information Services Association website and more specifically in the Introductory Guideline.
Both RDS-TMC and TPEG delivery mechanisms allow for the transmission of incident, congestion and weather related messages with TPEG allowing a much richer definition of incidents.
Both TPEG and RDS-TMC can be delivered as open or encypted services dependent on whether these services are to be provided by Government or the Private Sector.
Such information is of particular benefit to tourists and those who are unfamiliar with the route/public transport in question. This may help prioritise where these systems have greatest value. Such systems must be derive information on the stops on the route and must know when these stops have been reached. Most often they are linked to other on vehicle systems, such as ticket machines or automatic vehicle location systems. (See Passenger Transport) It is critically important to ensure robust operation of such systems making sure that they can be automatically updated if the vehicle is moved between routes. As with other display types, the following must be considered: