Distributed surveillance on freeways emphasizing incident detection and verification

Efficient freeway management requires continuous decision-making based on conditions on the network and an understanding of the impacts of the decisions made. These conditions are usually measured with fixed-point surveillance systems, most of which are deployed in such a manner as to require communication links that are always connected and are polled at regular intervals. All of the sensor data are typically sent to a Traffic Management Center (TMC) for assessment, yet most of the time no action is taken in response to the data, leading to unnecessarily high communication costs. To reduce communication costs without a significant loss in the quality of information received at the TMC this paper lays the foundation for an event driven communication system by examining the sequence of events at the detector stations in the context of incident detection. Where, following the broadest convention on freeways, an incident is any non-recurring event that causes a temporary bottleneck and restricts flow. Although the focus is incident detection, the proposed communication system could easily support many other applications that use aggregate data, e.g., measuring average annual daily travel (AADT). The methodology is generalizable to most common freeway geometries and care is taken in the paper to specifically address the situation where an incident interacts with a recurring bottleneck. To address the normally high communications costs, a portion of the decision-making process is transferred from the TMC to the field controllers, which would make the initial evaluation of conditions and only send data that might elicit a control response or benefit comparative decisions between detector stations. In other words, rather than relying on the conventional, centrally polled communication system, these events could be used to initiate communication from the field when the potential value outweighs the cost per communication. The process could also lead to better data handling for decision-making or archiving in a conventional, polled communication system as well. We develop the methodology by deconstructing several incidents on a freeway and identify the observable events at a pair of detector stations that may be upstream, downstream or straddle the incident. This analytical process could be repeated for any other condition of interest.