In the area of Intelligent Transportation Systems the introduction of wireless communications is reshaping the information distribution concept, and is one of the most important enabling technologies. The distribution of real-time traffic information, scheduling and route-guidance information is helping the transportation management systems in their strive to optimize the system. The communication required to transfer all this information is rather expensive in terms of transmission power, use of the scarce resources of frequencies and also the building of an infrastructure to support the transceivers. By using information that already exists and is exchanged within the infrastructures of the GSM and UMTS networks, a lot of the resource problems are solved. The information that could be extracted from these cellular networks could be used to obtain accurate road traffic information to support real-time traffic information. In this way the cellular networks not only becomes the means to distribute information but also a source of road traffic information.

From the analysis made it is obvious that the potential of retrieving valuable road traffic information from cellular systems in a cost efficient way, i.e. by using already existing signalling data, is very high. It has however not been clear what to expect from these systems in terms of accuracy, availability and coverage. In this chapter the basics for this is laid out and discussed in detail. A practical trial has also been performed and the results show clearly the potential as well as the differences in using the GSM compared to the UMTS system. The advantages and drawbacks are discussed and backed up by real measurements from an existing road segment environment. The main advantages of using the existing signalling data, i.e., passive monitoring compared to active monitoring where the terminal sends extra data is discussed and could be summarized in three components, no user acceptance is necessary, no extra signalling is necessary and it does not drain the terminal battery.

In the future it is likely that vehicles need to communicate more frequently with each other and with some kind of traffic control centre. This traffic will also be very useful in order to estimate road traffic information using the signalling information obtained from the cellular system. However, the enhanced communication systems will also change traffic patterns in the cellular networks which will affect the potential of estimating road traffic from cellular systems. The evolvement indicates that the terminals will be in active state almost constantly, and hence the updating information will be more frequent and the information more accurate.