Evaluation of the Performance of the Sydney Coordinated Adaptive Traffic System (SCATS) on Powell Boulevard in Portland, OR

Abstract

The Sydney Coordinated Adaptive Traffic System (SCATS) is used to mitigate traffic congestion along urban arterial corridors. Although there has been research on SCATS’ performance, this report combines three different areas of research about SCATS that are not known to be represented in any research literature. These include: (a) the relationship between SCATS, traffic volumes, and Transit Signal Priority (TSP); (b) between TSP and traffic conditions; and (c) the correlation between signal timing and air quality; in particular, human exposure to the air pollutant PM2.5 at intersections. In addition, this research looked at the key factors affecting transit user exposure to traffic-related pollutants at bus shelters. All areas of study present the results of statistical tests and regressions to determine SCATS or traffic variables impacts. SCATS did show statistically significant improvements regarding traffic speeds at one minor intersection, even when traffic volumes showed a statistically significant improvement. At a major intersection, results were mixed and not conclusive. Overall, it was determined that the improvements available through SCATS vary depending on the time of day and the direction of travel. TSP was not negatively affected by SCATS. In controlling for both priority and traffic conditions, each were shown to have a distinguished and significant impact on bus travel time. Non-priority signals had a much greater impact on travel time than priority signals (11.0 and 0.6 seconds for the corridor model, respectively). In controlling for both priority and traffic conditions, each were shown to have a distinguished and significant impact on travel time. Utilizing a regression model, results in an intuitive ranking of the intersections’ delay was produced; major intersections with high traffic volumes on crossing streets are likely to not experience TSP benefits. To a high degree, this research has shown that pedestrian exposure can be considered as an outcome of traffic-signal timing decisions made by cities and counties. The statistical results have shown the high impact that signal timing and queuing have on pedestrian level exposure. Heavy vehicle volume was a significant variable as well as the presence of buses. The reduction of bus idling time through more efficient operations and transit-signal priority is likely to reduce pedestrian and transit users’ pollution exposure levels. Longer green times along the main corridor are able to significantly reduce particulate matter for transit users and pedestrians waiting at the sidewalk of the intersection, whereas time allocated to cross the street increases queuing and exposure along the main corridor. The impact of heavy-duty diesel engines is also clear. The reduction of bus idling time through more efficient operations and transit-signal priority is likely to reduce pedestrian and transit users’ pollution exposure levels. Transit agencies can also reduce pollution significantly by improving the efficiency and cleanliness of their engines. TriMet (the local transit agency) initiatives to improve fuel efficiency by installing EMP engine-cooling devices not only improve fuel efficiency, but also air quality. Finally, significant reductions in transit users’ exposure to traffic-related pollution can be made at bus stops by properly orienting the shelter and by reducing bus idling.