Performance Measurement of a Bluetooth-based Floating Car Observer

Abstract

The German Aerospace Center (DLR) developed a traffic monitoring approach, called DYNAMIC, which combines the advantages of Floating Car Data (FCD) and Floating Observer Data (FOD) principles. DYNAMIC is based on detections which are made by floating traffic observers using wireless radio-based technologies such as Bluetooth while passing other traffic objects (vehicles, cyclists, pedestrians). For the evaluation of the performance of DYNAMIC it is crucial to know how likely it is that a detectable traffic object (i.e. with Bluetooth device on board) within the detection range will be monitored. The major point to answer this question is the inquiry process which sets up the connection between Bluetooth devices and which can take up to several seconds. Given the possibly high speed of the vehicles and the relatively small detection range this poses a major problem to this detection mechanism. Within the paper an analytical model of the time it takes an observer to discover traffic objects nearby is introduced. Therefore, the device discovery process will be described as an exponential distribution, that is, the number of detections based on Bluetooth is a sequence of independent respectively seen or not seen trials each of which occurs with a certain probability. This follows from the assumption that the number of vehicles equipped with Bluetooth devices and the number of observer vehicles within the network is small, so that the chances to encounter are statistically independent events. To evaluate the analytical model, simulations as well as laboratory and field experiments with Bluetooth receivers and senders were conducted to measure detection rates as a function of inquiry times. On that base the performance is analysed using measures as the penetration rate, the encounter rate as well as the detection and redetection rate. By introducing a theoretical model, it is possible to describe the detection process as an exponential distribution and to determine its specific properties such as the encounter rate of Bluetooth-FCO and detectable traffic objects. The results of the field measurement indicate a good accordance to the simulation model results. Still the functions look different enough to require some further adaptions of the parameters. Since broader field test (e.g. with a higher amount of observing vehicles) are difficult to conduct due to economic reasons, the simulation offers a wide range of possible further research studies.