Marginal Benefit of Adding Antennas to Bluetooth Sensor Arrays in Freeway Travel Time Data Collection

Abstract

The use of Bluetooth detection systems for collecting travel time data is becoming increasingly popular because data can be collected continuously at a relatively low cost. However, representativeness of the data can be problematic, given the relatively low detection efficiency of Bluetooth monitoring systems. This paper investigates the use of multiple sensors with individual antennas to the same Bluetooth data collection device to increase the number of identifications from passing vehicles at each data collection point. Increased detection potentially increases the number of travel time measurements, increasing the potential for the measurements to better represent the travel conditions on the roadway. To test this approach, devices with up to 10 Bluetooth sensors were deployed concurrently on a freeway overpass. The marginal benefit of additional Bluetooth sensors in the array was investigated. The additional detections captured by adding up to six Bluetooth sensors in a single sensor array outnumber the detection losses associated with possible radio interference between the Bluetooth sensors or in device-to-sensor communication. On average, additional Bluetooth device media access control (MAC) addresses were detected by increasing the number of sensors from one to two (61% increase), two to three (27% additional increase), three to four (16% additional increase), four to five (11% additional increase), and five to six (8% additional increase). Also, it was observed that additional travel time measurements were collected by adding the number of sensors from one sensor at both locations to a two-sensor setup at both locations (between 110 and 156%), from a two-sensor setup to a three-sensor setup (between 44 and 60%), from a three-sensor setup to a four-sensor setup (between 30 and 47%), from a four-sensor setup to a five-sensor setup (between 10 and 20%), and from a five-sensor setup to a six-sensor setup (between 8 and 17%). This field study indicates that there are potential benefits to using multisensor Bluetooth arrays in lieu of the current practice of using a single sensor configuration to capture additional travel time data from passing vehicles.