Adaptive Traffic Signals, Comparison and Case Studies

Interest in adaptive traffic signals in the USA is increasing and the use of adaptive signals is strongly supported by FHWA. There are several systems (notably SCOOT and SCATS) that are widely used throughout the world and several new adaptive products have recently been released. This paper compares and contrasts the systems that have been successfully installed in the USA and discusses their advantages and disadvantages. The discussion includes several case studies in which the authors investigated the options available to agencies, discusses the reasons why different systems were selected for installation, and presents the results of detailed evaluations of the effectiveness of each installed system. The paper also describes the evaluation techniques that were used to provide statistically reliable evaluation results, overcoming the shortcomings that are often found in traffic engineering surveys. These include survey design based on statistical principles and the use of new technology to increase sample size and reduce the resources needed for detailed data analysis. INTRODUCTION Adaptive traffic signal systems have been operating successfully in many countries since the early 1970’s. The two most widely deployed systems are SCATS (Sydney Coordinated Adaptive Traffic System) and SCOOT (Split Cycle and Offset Optimization Technique) being. (See, for example: Schroeder, 1989; Hadi, 2002; Fehon, 2004.) Since 1990, FHWA has recognized the benefits to be achieved by the implementation of adaptive control over the traditional time-of-day (TOD) selection of fixed cycle length timing patterns, and has sponsored several programs aimed at developing adaptive signal systems, such as RT-TRACS in the 1990’s and ACS-Lite during the 2000’s. (See, for example: Ghaman, et al., 2002; Ghaman, et al., 2004; and FHWA, 2010) In recent years, several new systems have come onto the market in the USA, and these are generating significant interest. There are still substantial barriers to the widespread introduction of adaptive control. With over 272,000 traffic signals in USA (NTOC, 2007), less than 1% are operating adaptively. In contrast, while there appear to be no published statistics, the authors estimate that possibly 50% of the signals in Australia operate adaptively, and the majority of coordination in the larger cities is adaptive. ADAPTIVE SYSTEMS IMPLEMENTED IN USA Overview Stevanovic (2010) undertook a very detailed survey of all known adaptive traffic signal system operators in North America during 2008-9 as part of an NCHRP project entitled “Adaptive Traffic Signals State of the Art, Foreign and Domestic” (Stevanovic, 2010). He also surveyed a sample of adaptive system operators in UK, Australia, New Zealand, Canada, Chile, Ireland and China. Responses were received from 45 agencies, 34 of