Abstract
In the estimation of saturation flow at signalized intersections using the HCM model, the influence of area-type characteristics on flow is accounted for by the area-type adjustment factor (fa). The recommendation by the manual to use fa=0.9 for CBD-intersections and 1.0 for all others tacitly assumes that the nature of non-CBD areas does not influence flow. This may not hold for many developing country cities and metropolitan areas where the roadside environments associated with non-CBD intersections may be laden with a flurry of activities that impact traffic flow. For signalized intersections in such areas, the HCM-recommended area-type adjustment factor could lead to over-estimation of saturation flow rates if the activities create substantial roadside friction to flow. This study set out to develop area-type adjustment factors for non-CBD signalized intersections located within the Kumasi Metropolis in Ghana, to accurately reflect the impact of roadside character on saturation flow rates. Using field saturation flow data collected at selected signalized intersections in the metropolis, area-type adjustment factors were developed for three distinct roadside friction environments categorised in the study as low/none, medium, and high. Average fa values developed were 0.99 for low/none, 0.98 for medium, and 0.94 for high roadside friction environments. Also, a multiple linear regression model that attempts to relate fa to pedestrian traffic interfering with flow within the environment of the intersection was developed as an alternative procedure for determining site specific area-type adjustment factor. It is recommended to use the fa values developed in this study in the HCM saturation flow model in order to estimate more accurately the saturation flow rates at non-CBD signalized intersections within the Kumasi Metropolis.
Highlights
Direct field measurement of saturation flow rates associated with vehicular discharge through signalized intersections forms part of the routine for assessing the performance of such intersections
Rahman et al [3] derived adjustment factors based on data collected from Yokohama City, Japan, and noted that saturation flow rates increased by 20% which corresponded to a professional driver adjustment factor of 1.20 if the signalised intersection was identified to have a high volume of taxi drivers
The paper has presented the procedures involved in the development of area-type adjustment factors for saturation flow estimation for use in non-CBD areas within the Kumasi Metropolis
Summary
Direct field measurement of saturation flow rates associated with vehicular discharge through signalized intersections forms part of the routine for assessing the performance of such intersections. The alternative procedure to field measurements recommended by the Highway Capacity Manual (HCM) [1] is to adjust the ideal flow of 1900pcphgpl by a number of factors to reflect all the parameters that cause the flow to deviate from ideal. The manual provides empirical expressions for evaluating the adjustment factors based on parameters associated with the traffic stream, geometrics, and external interference at the intersection. Shao et al [2] studied the influence of traffic composition, lane widths, and approach grades on signalised intersections in China and developed factors that were inconsistent with corresponding factors in the HCM [1]. Rahman et al [3] derived adjustment factors based on data collected from Yokohama City, Japan, and noted that saturation flow rates increased by 20% which corresponded to a professional driver adjustment factor of 1.20 if the signalised intersection was identified to have a high volume of taxi drivers
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