Deployment of a Wireless Mesh Network for Traffic Control

Introduction

Adaptive traffic control systems are employed in cities worldwide to improve the efficiency of traffic flows, reduce average travel times and benefit the environment via a reduction in fuel consumption. One of the main and most common functions of such systems lies in adaptive control of traffic lights. This ranges from simple lengthening or shortening of green and red light durations in an intersection according to the actual presence of cars in the respective lanes, to coordination of green light phases among neighboring intersections on main thoroughfares. This adaptivity is made possible with the use of sensors (typically in the form of magnetic loop detectors embedded under the road pavement) that feed data to roadside traffic light controllers, and a communications infrastructure that connects among the intersections and a traffic management centre, as well as, in some cases (typically in large cities), a hierarchy of regional computers (RC) that perform the control decisions for respective portions of the system.

Traditionally, the communications layer of traffic control systems has been based on wired connections, either private or leased from public telecommunications operators. While for many years such leased lines (operating at 300bps) have served their purpose well, they have several shortcomings, such as a significant operating cost, inflexibility, and difficulty of installation in new sites. In certain cases, alternative solutions, operating over public infrastructure, have been deployed for specific sites where private or leased lines were not a viable option; these ranged from ADSL, regular dialup, or cellular (GPRS). However, using public network for traffic control could suffer from inconsistent delay jitters and reliability issues. For example, previous experimental studies (Chakravorty, 2002) have shown GRPS links could have very high RTTs (>1000ms), fluctuating bandwidths and occasional link outages.

In recent years, there has been considerable interest in wireless mesh networks and their deployment in metropolitan areas, from both a commercial and a research perspective (Lundgren, 2006). Trials in several major cities in the US (e.g., Philadelphia, New Orleans, Tropos networks (http://www.nici.nat.gov.tw/content/application/nici/english/)) have shown mesh networks to be a viable technology that can compete well with alternative “last-mile” connectivity solutions to the public. Correspondingly, most of the research on metropolitan-area wireless mesh networks (MAWMN) has focused on maximising the throughput that can be extracted from them, in the anticipation that their major use will be public, for purposes such as accessing the Internet or conducting voice calls (Ganguly, 2006). On the other hand, little attention has been directed to the aspects of reliability and latency, which are most important if MAWMN are to be considered for replacement of mission-critical infrastructure, such as traffic control system communications.

In this chapter, we describe a testbed (Lan, 2007) that has been built with a goal to develop protocols that enhance the reliability and reduce the latency of mesh networks, and thereby enable them to be used as the communications layer of traffic control systems. Our initial testbed covers seven traffic lights in the suburban area of Sydney. These intersections are chosen because they represent a typical suburban area with lots of traffic, foliages, pedestrians and high-rise residential buildings. In addition, the inter-node distance (ranging from 200 to-500m) is representative of 90% of the distance between traffic controllers in the Sydney CBD (Central Business District) area. The nodes have been custom-built to meet the need of research.