Saturation Flow at a Signalised Intersection Approach with a Downstream Bus Stop and Bicycle Lane
Arterial roads in urban areas are more prevalently experiencing traffic congestion during peak periods. It is commonplace that arterial roads are used by private vehicles along with transit buses and bicycles. The interaction between these modes influences roadway capacity, so it is imperative to understand the impact of transit buses and bicycles on the traffic stream. Signalised intersections are important control elements of an arterial road. Saturation flow rate (SFR) of general traffic
... general traffic lanes is an important feature of traffic operation and a vital determinant of signalised intersection capacity. This study reviewed factors that influence SFR under diverse traffic, geometric and environmental conditions. The literature acknowledges that the presence of bus stops can significantly influence traffic flow at signalised intersections. In this study, a review on the influence of bus stops on SFR, and development of bus blockage adjustment factors were considered. A knowledge gap was found when studying bus stops downstream of signalised intersections and the interaction of bicycles that pass a downstream bus stop. No study was found that provides an adequate means of analysing the impact of a far-side, offline bus stop with adjacent bicycle lane upon SFR across the stop line at a signalised intersection. The aim of this research was established to study how SFR on the adjacent general traffic lane at the study signalised intersection was impacted due to the presence of a busy far-side, offline bus stop and the presence of a bicycle lane. This study addressed this complex situation by developing a case study approach at a signalised intersection in Brisbane, Australia. The Brisbane metropolitan area has an extensive bus system and a busy arterial road network. The first step of the study was to select a busy arterial road. Old Cleveland Road was selected as the most suitable candidate. Then, the signalised intersection of Old Cleveland Road/Cavendish Road was selected for study, because it experiences high traffic and high bus flows during peak periods. The study signalised intersection has a far-side offline bus stop and bicycle lane in the inbound direction. These conditions satisfied the specific site conditions required by the study. A suitable data collection methodology was required to capture the multiple events occurred due to complex site conditions. The multiple events occurred due to vehicles passing the SATURATION FLOW AT A SIGNALISED INTERSECTION APPROACH WITH A DOWNSTREAM BUS STOP AND BICYCLE LANE vi stop line, bicycle passage and stopping buses. These multiple events were recorded on a laptop computer using a special macro program developed for this purpose. Time headway data of passenger cars crossing the stop line were obtained from the recorded data. Based on analysis of literature, steady state of traffic flow occurs during the middle interval of the green period. Therefore, middle interval headways were used for data analysis. The following four scenarios were identified for analysis, according to the multiple events described above; A: "Passenger cars only pass", B: "Passenger cars pass as a bicycle passes", C: "Passenger cars pass as a bus re-enters" and, D: "Passenger cars pass as a bus re-enters and a bicycle passes". The variation of headways between above four scenarios were analysed comparatively. A significant variation of headways between the middle interval scenarios was observed, and statistical tests indicated that scenario C (Passenger cars pass as a bus re-enters) was the most critical situation. Further statistical tests were conducted to analyse the scenario C, and from the results of boxplot analysis, it was observed that a re-entering bus increased the headway of the car coincident with bus movement. But the headway returned to steady state after the second car crossed the stop line. Because it was found that re-entering buses influence the SFR, an adjustment factor to SFR due to the re-entering buses of the far-side off line bus stops at signalised intersections was developed, thereby achieving the aim of this research. Adjusted SFR was estimated in the end using new bus blockage adjustment factor. The existing bus blockage adjustment factor given in SFR model of HCM (2016) represents the case of online bus stops. When analysing the cases of offline bus stops, the new adjustment factor developed in this study can be used in the SFR model of HCM (2016) . This study brought significant contributions into the area of research related to analysis of SFR at signalised intersections with complex site characteristics. A specific site selection approach for multiple needs was developed to select the unique study site. A novel data collection methodology was employed to record the multiple event types occurred at the study site. Also, a statistical analysis methodology was developed to comparatively analyse the complex site characteristics and select the critical characteristic that has significant impact on SFR. This methodology can be incorporated in the development of a generalised estimation method that would yield an improved SFR adjustment factor for all sites containing downstream off-line bus stops.