1 Synergistic Sensor Location for Cost-Effective Traffic Monitoring ManWo Ng, Ph.D. Assistant Professor Department of Modeling, Simulation and Visualization Engineering & Department of Civil and Environmental Engineering Web: April 10, 2012
2 Synergistic Sensor Location: The Idea Given that we are interested in monitoring traffic in an entire region (e.g. CBD, city, etc.), do we need to monitor all locations? Answer: Generally, yes. The answer can be no, provided that we can live with inferred traffic conditions. How do we make use of this fundamental notion of flow conservation to determine the minimum number of locations to monitor in an entire network?
3 Outline Some small examples to illustrate the methodology. Details are quite technical and, hence, omitted. Problem variations. Summary. Questions/ feedback.
4 Synergistic Sensor Location : A Simple Example S S i i i S S Vehicular flow conservation. Hence, in order to determine all 7 link flows, we ONLY need to measure 4 (v A,v R1,v R2,v R3 ). FHWA s ramp balancing/ ramp counting procedure.
5 Our Contribution: Generalization FHWA s ramp counting procedure is limited to very special network topologies. We developed a generalization of the ramp counting procedure applicable for arbitrary network topologies. Cost-savings are now within reach: What if instead of 400 locations (say) in our road network, we only need to monitor 200 locations (say), preserving the same amount of information?
6 A More Complicated Example 14 links
7 One Possible Solution? How about we monitor the traffic at the blue arcs/ links? Can we infer the uncounted red arcs/links from them?
8 How about this Solution? Hence, choosing wisely is critical!
9 Two Remarks In both suggested solutions, it was assumed that 9 links to be monitored. It can be shown that this is the minimum number of links possible. The details of the methodology are highly technical (involves abstract concepts from linear algebra), and hence, not discussed here.
10 A Last Example 20 links network How many links/ locations do we need to monitor? What is the minimum number and locations?
11 Another Solution? Hence, there are multiple optimal monitoring strategies. That is, there is flexibility for us to choose. E.g. if we must monitor location x Finally, we CANNOT count less than 14 locations/ links.
12 Large-Scale Implementation Ready These toy networks were relatively easy to address. How about for huge, real-world networks? How to determine the minimum number of monitoring locations (and where they are) is far from trivial for large-scale networks. Our research group has developed an efficient algorithm that makes it possible to address transportation networks of realistic size.
13 Other Variations of the Problem Variation 1: Given that we can monitor at most n locations (e.g. due to budgetary constraints), what locations should we choose in order to maximize the amount of info we gain? Variation 2: How do we combine this technique with existing sensor equipment on the road to maximize the monitoring coverage? Variation 3: Given that we need to observe traffic at m given/ specified locations, what is the minimum number of locations to monitor and where are they? Etc.
14 Summary The presented technique is able to determine: The minimum number of locations to be counted. Where these locations are. Consequently, it has the potential to save $$. Exact savings depend on: Network topology. Number of locations observable Number of sensors installed Budget Synergy increases with the number of monitored locations. Other traffic info, e.g. turning ratios, route flows (e.g. via blue tooth technology) can possibly be incorporated too.
15 Actual Deployment Practice-ready. (Thanks to Frank Hickman and his team for valuable feedback!) Technique is independent of sensor technology (blue tooth, camera s, loop detectors etc.). Currently looking for opportunities to have this technique implemented.
16 Thank you! Questions/ Thoughts?