Bluetooth is a telecommunications industry specification that defines the manner in which mobile phones, computers, personal digital assistants, car radios, and other digital devices can be easily interconnected using short-range wireless communications. One example of the use of this technology is the interconnection of a mobile phone with a wireless earpiece to permit hands-free operation. The technology received its unusual name in honor of Harald Bluetooth, king of Denmark, who united Norway and Denmark during the mid-tenth century.
The Bluetooth capability is implemented using a low cost transceiver chip, which exchanges information with other Bluetooth devices within a globally available frequency band of 2.45 GHz. Each device has a unique 48-bit address. Bluetooth transmission ranges include 10 meters (33 feet) for Class 2 radios and 100 meters (330 feet) for Class 1 radios. However, the Bluetooth traffic detection technology described here has been able to increase these ranges through appropriate adjustments to receiver sensitivity and device placement. Because of its relatively low transmission power and narrow bandwidth, Bluetooth technology offers the advantages of low power requirements that permit the use of relatively small batteries and/or solar cells for field operation. The challenge of the technology is the implementation of roadside monitoring equipment with the sensitivity and rapid acquisition time necessary to receive Bluetooth signals from high speed traffic.
Bluetooth transceivers continuously transmit their 48 bit ID (address) for the purpose of identifying a device with which to communicate. This “inquiry mode” is used to establish a link with the “responding devices”. Inquiries are made by a Bluetooth transceiver, even while it is already engaged in communication with another device. The continuous nature of this process facilitates the identification of passing vehicles or pedestrians containing Bluetooth devices, since all equipped and activated devices will be transmitting inquiries as long as they have their discovery mode enabled.
The 100 meter (330 foot) detection range can be considered the radius of a circle with the Bluetooth receiving antenna at its center. Any Bluetooth transmitter entering this circle will be detected. This is a potential source of error for recording vehicle passage times, since the vehicle could be detected at any point of the circle that impinges on the roadway. The worst case error is 330 feet corresponding to a Bluetooth receiver placed at the edge of the roadway. Thus, for two successive receivers the worst case error would be 660 feet. Since receivers are typically placed at approximately 2 mile intervals, this would correspond to a maximum error of 660 ft. per10,560 ft., or 6%. However, this is an unbiased random error since vehicles could be detected at any point within the circumference of the circle, and furthermore, it would be a rare installation for which the receiver is so close to the edge of the roadway that the detection zone is as large as 330 feet. The maximum error of 6% is considerably less than the accuracy with which travel times can be estimated using conventional detectors, for which error rates as high as 30% have been found.
2. Adaption of Bluetooth to Traffic Monitoring
In its most basic form, the Bluetooth technology calculates travel times by matching public Bluetooth wireless network IDs at successive detection stations. The time difference of the ID matches provides an accurate measure of travel time and space mean speeds based on the distance between the successive stations. (Figure 1) Accurate measurement of distance between successive Bluetooth data collection sites is accomplished using GPS equipment installed in the Bluetooth devices that record location as a header record for the collected data. An equally significant application of the Bluetooth technology is its ability to collect Origin-Destination (O-D) information which is derived by tracing a Bluetooth transceiver's path through a series of Bluetooth units with known locations. In the past, it has been necessary to acquire O-D data using either expensive license plate matching equipment or unreliable post-card surveys.
The use of Bluetooth technology offers a number of advantages over existing methods, in that this approach:
- Directly measures travel time and space mean speed. This is a leapfrog advance over existing point detection technology (inductive loops, radar detectors, image processors, etc.) commonly used by most transportation agencies. The greater accuracy of the Bluetooth units results from the fact that travel times and space mean speeds are measured directly by the equipment, while these variables must be inferred from the point detection technology using speed measurements at discrete locations.
- Measures travel times and O-D's for a variety of modes (highway vehicles, rail, and pedestrian) since the Bluetooth devices are associated with people rather than vehicles.
- Can be applied globally due to the proliferation of the Bluetooth standard protocol. Similar techniques are available that detect the passage of toll tags (such as EZPass). However, adequate samples of toll tags are only available in the vicinity of toll facilities.
- Simplifies field installation procedures because of the low-power and omni-directional antenna patterns.
- Offers a greater degree of privacy than that which can be provided with toll tag tracking, license plate surveys or cellular telephone geolocation due to the fact that there are no databases of Bluetooth addresses that can be used to associate addresses with individual owners or their vehicles.
3. Test Results
Freeway Test Results
To date, the Bluetooth equipment has been tested at many different freeway and arterial sites at many locations throughout the United States and in Brisbane Australia. The most extensive deployment was made for the purpose of validating GPS-based vehicle probe data procured by the I-95 Corridor Coalition from the Inrix Corporation in connection with its six-state Vehicle Probe Project. In all cases, the equipment performed flawlessly and provided reliable travel time measures. The data presented here represents a very small sample of the overall results.
The initial testing for the I-95 Coalition project was on a heavily traveled 15 mile section of the Capital Beltway (I-495) near Washington, DC, as shown in the map of figure 4. The twenty units were deployed in connection with this data collection activity, were relocated to different sites as the data collection proceeded. Three data sets were developed in connection with these tests:
- The Bluetooth data, which was intended to represent "ground truth" due to its anticipated accuracy and large sample sizes.
- Floating car data, which was collected because of its historical use for collection of travel time data as a representation of ground truth, and to provide a check for the Bluetooth data
- GPS data acquired from Inrix, which is being purchased on an area-wide basis by the I-95 Corridor Coalition.
The results of these tests are shown in figure 5 which provides a comparison of the Bluetooth and floating car data as well as the GPS data acquired from Inrix. The data shown in this figure is the eastbound travel time between I-270 and Connecticut Avenue (MD 185) between 2:30 PM and 7:30 PM.
The comparison between the Bluetooth with floating car data shows good agreement between the two data sources except for the single floating car data point at the beginning of the Bluetooth data set. This single data point shows higher travel times than the Bluetooth data because the car making the floating car runs had stopped to install the Bluetooth equipment on the I-495 median. The mismatch between the floating car and Bluetooth data quantifies the time required for "field installation" of the Bluetooth equipment at approximately eight minutes (the excess travel time for this particular run). The differing number of data points on these two curves demonstrates the potential of the Bluetooth technique to deliver statistically reliable data due to its large sample sizes as opposed to floating car runs for which only three data points could be acquired during the same time period. The comparisons of Figure 5 also show good agreement between the GPS and Bluetooth data. The GPS data does not suffer from the limited sample size of the floating car data, although here again, the Bluetooth sample size appears to be significantly greater. The GPS data exhibits a tendency to oscillate at times when there is a large variance in the travel times of the Bluetooth data, possibly because of inadequate sample sizes in the GPS data. Clearly, the large Bluetooth sample size facilitates the ability to analyze other data sources and to better understand traffic flow characteristics.
Arterial Test Results
A sample arterial data set comparing Bluetooth and floating car data is shown below. Here again, good agreement is shown between the two data sets, with the Bluetooth data providing a significantly higher sample size. The arterial data differs from the freeway data in a number of respects including:
- Greater variability, undoubtedly caused by the presence of traffic signals and mid-block facilities.
- Smaller sample sizes, characteristic of relatively lower volume arterials
- Increased presence of outliers due to vehicles making brief diversions along the arterial which has significant strip development.
These arterial characteristics are typical, and demonstrate the importance of using Bluetooth technology with its higher sample sizes, for these types of roadways, if reliable data is to be collected.
Conclusion
Many additional applications exist for the Bluetooth technology that can leverage its O-D capabilities while insuring a relatively high level of privacy to owners of Bluetooth equipped devices. Potential applications include the use of O-D data to:
- Support traditional planning activities associated with the development of new facilities.
- Evaluate impacts of dynamic message sign (DMS) messages on traffic diversion percentages
- Support the operation of toll facilities with variable pricing features. This data can be used to support the evaluation and levels of service provided by these facilities to ensure that "paying" customers are receiving the improved services for which they are being charged. i.e. comparing the travel times of tolled vs. facilities without tolling
- Measure pedestrian flows at major events
Bluetooth equipment offers the benefit that it is an open system, not based on proprietary communications protocols. Development of the Bluetooth equipment has taken advantage of the large consumer marketplace that supports the production of the Bluetooth chips. This marketplace has ensured the availability of inexpensive, low power products. The equipment described in this paper has been developed in a manner that will permit its adaptation to inevitable future advances in the state-of-the-art of short range communications devices. As Bluetooth technology evolves or is replaced, the chips can be replaced with the newer technologies.
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[1] Wasson, J.S., J.R. Sturdevant, D.M. Bullock, "Real-Time Travel Time Estimates Using MAC Address Matching", Institute of Transportation Engineers Journal, ITE, Vol. 78, No. 6, pp 20-23, June 2008.
[1] Information describing the I-95 Corridor Coalition vehicle probe project can be found at http://www.i95coalition.org/vehicle-probe.html
[1] "How Stuff Works - Bluetooth Basics", http://electronics.howstuffworks.com/framed.htm?parent=bluetooth.htm&url=http://www.bluetooth.com
