Abstract
We have developed a wireless sensor package to instrument roadways for Intelligent Transportation Systems. The sensor package counts passing vehicles, measures the average roadway speed, and detects ice and water on the road. Clusters of sensors can transmit this information in near real-time to wired base stations for use controlling and predicting traffic, and in clearing road hazards. The sensor package draws a maximum time-averaged current of 17mA from an internal lithium battery, allowing it to operate in the roadbed for at least 10 years without maintenance. The nodes cost well under $30 to manufacture, and can be installed without running wires under the road, facilitating wide deployment. Unlike many other types of traffic sensors, these sensors count vehicles in bumper-to-bumper traffic just as well as in widely separated traffic. The devices detect vehicles by detecting the perturbations in the Earth’s magnetic field caused by the vehicles. They measure this perturbation using an
anisotropic magnetoresistive magnetic field sensor. The radio transmitters in the sensor are frequency-agile, and the sensors use a randomized sparse TDMA protocol, which allows several transmit-only devices to share a channel. The sensor package includes a custom-designed, compact, broadband, inexpensive printed circuit microstrip antenna for the 915 MHz U.S. ISM band. We built a prototype sensor package, and installed it in a pothole in a city street. We used the sensor to monitor the traffic flow rate during free-flowing traffic and a traffic jam.
Introduction
The congestion level on our nation’s roadways is spiraling out of control. The Texas Transportation Institute estimates that the total cost of congestion to commuters in 1997 was $72 billion, or $750 per commuter. [Schrank99, V-13] The amount of congestion-induced delay experienced by the average commuter in a large city such as Los Angeles or Boston has more than doubled since 1982. [Schrank99, III-9] Better public transportation is the only sustainable long-term solution to road congestion. However, a nearer term solution is also required. The most obvious is to build new roads, but, in urban areas, this is generally not feasible, due to a lack of suitable land or insufficient funds. [DOT99] There is a major effort
underway, the National Intelligent Transportation Systems (ITS) initiative, to use information technology to make better use of our nation’s roads. One particularly compelling system envisioned by ITS workers is the Automated Traveler Information System. (ATIS) Before embarking on a trip, drivers could consult a web page to obtain accurate trip time estimates for
various departure times and modes of transportation. If they chose to drive, a dynamic route guidance system would provide them with turn-by-turn directions based on up-to-the minute information about roadway speeds and congestion levels.
At the very least, this type of system would allow drivers to make better route decisions, to be confident that they were taking the most efficient route, and to plan their activities around traffic delays. More importantly, though, this type of system would make participating in a traffic jam into a conscious, planned action, rather than the consequence of losing a perceived route-planning gamble. Drivers might postpone discretionary trips, consider local or electronic
alternatives, consider the use of public transportation, or even decide to move closer to work or to find work closer to home. One of the largest obstacles to the implementation of this type of system is the shortage of accurate, real-time traffic data. Currently available traffic sensor systems (video, sonar, radar, inductive, magnetic, capacitive, PVDF wire, pneumatic treadle) use significant electrical power, so each sensor must be connected to the power distribution network. For sensors that are installed on electrical poles (video, sonar, radar), this costs a few hundred dollars. For sensors that are installed in the road, (inductive, magnetic, PVDF wire, capacitive, pneumatic treadle) this costs several thousand dollarsi. Nevertheless, in-road sensors are still popular, because they are very accurate, provide direct information with very little ambiguity, can monitor road conditions (i.e. presence of ice), and do not require a human operator.
Here, I present the design for a wireless in-road traffic sensor system. The sensors are small, low in cost ($30), and extremely rugged. They count and measure the speed of passing vehicles using magnetic technology. They also measure information about road conditions. Each cluster of sensors transmits data to a receiver mounted on an electrical pole up to 300 meters away, which relays the data to a processing station. Each sensor node consumes so little power that it can operate from a small internal lithium battery for at least 10 years.
Ara N. Knaian
S.B., Electrical Science and Engineering (1999)
Massachusetts Institute of Technology
Submitted to the Department of Electrical Engineering and Computer Science
in Partial Fulfillment of the Requirements for the Degree of
Master of Engineering in Electrical Engineering and Computer Science
at the
Massachusetts Institute of Technology
June 2000
© 2000 Massachusetts Institute of Technology
All Rights Reserved
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