EFFECT OF BUS-STOP SPACING ON MOBILE EMISSIONS IN URBAN AREAS

Abstract

The operational effect of bus-stop spacing has been a critical issue. Closely spaced bus stops disrupt the traffic flow on the bus route, particularly during peak hours because buses make frequent stops to provide services to customers. The disruption of traffic flow results in energy loss, increased delay/congestion, transit fleet-size requirement and mobile emissions. This paper describes a set of analytical models developed to assess the effect of bus-stop spacing on travel time and mobile emissions in urban areas. It is hypothesized that mobile emissions in urban areas can be significantly reduced if excessive transit-related stops are minimized through optimal spacing of bus stops. As part of the study, a nationwide survey of transit agencies was conducted to determine the prevalent bus-stop spacing policy in urban areas. The survey results show that the average bus stop spacing was approximately 330 meters (m), which is much less than the optimal threshold of approximately 700 m to 800 m obtained from the models described here. The analysis shows that for a typical bus route in an urban area, the peak hourly reduction in mobile emissions from optimal spacing of bus stops is not considered substantial for hydrocarbon, Nitrogen oxides, and carbon monoxide, which actually increased. A substantial reduction was observed for carbon dioxide and fuel consumption. However, for major urban areas with several bus routes, the aggregate system-wide impact for the combined morning and afternoon peak hours is expected to be substantial also for hydrocarbon and nitrogen oxides. INTRODUCTION As the federal government’s regulations to protect the environment intensify and as transportation funding becomes increasingly competitive, transportation operational issues have been receiving a high level of attention from transportation professionals and decision-makers. The classical operational issues include congestion mitigation, travel-time reduction, air-quality improvement, reduction of operating costs, and safety improvement. Conformance to one or more of these operational issues is a requirement for receiving most transportation-related federal funding. Public transit is widely considered as being environmentally friendly because of its high loading capacity. The number of passengers carried by a typical bus in some urban areas can exceed the equivalent of 40 passenger cars during rush hours. In most urban areas of the United States buses share the same rights-of-way with other vehicles (namely passenger cars, trucks and motorcycles). Buses are known to make frequent stops, particularly during peak hours, to provide services to transit patrons. As buses stop at designated transit stops, they also impede the flow of traffic, which depending on the traffic intensity can result in congestion and excessive emission of air pollutants on the bus route. Frequent stops are also costly to the transit operators because travel times are increased, as is the fleet size requirement to sustain the policy headway. Conversely, when bus stops are distantly spaced to avert the problems associated with closely spaced stops, the transit-operators risk providing inaccessible services, which may lead to loss of patrons. Transit operators face the challenging task of increasing farebox revenue to offset operating deficits. As an effort to encourage transit patronage by providing highly accessible bus services, transit operators typically provide too many stops, particularly at high-density land use locations, which sometimes are counter-productive. Like poorly timed or coordinated traffic signals, overly close bus-stop spacing engenders frequent stops and excessive delay of traffic, resulting in high mobile emissions and vehicle operating costs. Hypothesis It is hypothesized here that a significant reduction of mobile emissions can be achieved through proper spacing of bus stops in urban areas. Nationwide Survey on Bus Stop Spacing As part of the effort to estimate the operational benefit of optimizing bus stop spacing, a nationwide survey of transit agencies in urban areas was conducted to determine the prevalent bus-stop spacing policy. The summary of the survey results is shown in Table 1. It should be noted that the results of the bus-stop spacing survey, summarized in Table 1, are probably biased because the majority of the responses received came from the western, mid-western, and southern parts of the U.S. Nationwide surveys with adequate representation of the northeastern urban areas (which are the most congested regions of the U.S.) may result in much lower thresholds of average bus-stop spacing than was presented in Table TRB 2003 Annual Meeting CD-ROM Paper revised from original submittal.