Speed-Flow-Density Relationships for Traffic on Two-Lane Two-Way Rural Highways (The Case of Ethiopia)
No Thumbnail Available
Date
2023-07
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
Understanding the relationship between speed, flow, and density is essential for policymakers, designers, traffic engineers, and other professionals in determining operational capacities, reducing traffic congestion, improving traffic safety, developing effective traffic management strategies, designing highways, and improving overall traffic stream performance.
The Highway Capacity Manual, which were developed based on traffic and geometric conditions of the United States of America, were used in Ethiopia to study the Speed-Flow-Density relationship for Two-lane Two-Way Highways, which was necessary for capacity analysis and assessment of the overall operational performance. Moreover, no established travel time-flow and travel time-density relationships were studied, which is needed for transportation planning and traffic demand management purposes. To that end, it is crucial to examine those relationships by calibrating data obtained from those highways in order to obtain a capacity value that better reflects the actual traffic conditions and make conclusions regarding travel time relationships.
On three selected two-lane, two-way rural highways, traffic data was collected from both sides of the flow during the peak hour in the morning using video recording. Using an electric pole nearby the road stretch as a reference, travel time data and vehicle classes were recorded. Data processing was done using Adobe Premiere CS6. The average speed was calculated by dividing the measured distance between the reference points to the vehicle travel times averaged for every 1-minute and 15-minute time intervals. The average flow was computed by counting the vehicles passing, converting heavy trucks into equivalent passenger car equivalents, and using the steady state equation density was determined easily.
This study employed a regression approach for assessing different classical speed-density models, including Greenshields, Greenberg, Underwood, Drake, May and Schofer (North Western University Model), Drew, Multi-Regime Matrix, and Composite Models. At both 1-minute and 15-minute time intervals, the analysis revealed that the multi-regime matrix model better explained the data than the other models. With this model, the steady state equation was used to establish the corresponding speed-flow and flow-density relationships.
Finally, based on the developed speed-flow relationship, the capacity for both time interval analyses were less than the Highway Capacity Manual's recommended maximum capacity value, and the value of capacity obtained using the 15-minute time interval was less than the corresponding 1-minute time interval analysis. Furthermore, the travel time relationships matched the curves obtained with previous studies for similar highway types.
Description
Keywords
Speed-Flow-Density Relationships; Capacity; Two-Lane Two-Way Highways; Travel time relationships