Rationale for signal free urban arterial operation with grade separated u-loops in Dhaka city

Abstract

Oversaturated condition, prevalent in urban arterials of Dhaka and similar other emerging mega cities of the World, has rendered the commonly used signalized traffic control strategies at arterial corridor intersections inadequate. In space constrained conditions of Dhaka and few other similar cities, grade separated U-Loop pair have been implemented at a few locations to alleviate congestion. However, a comprehensive design rationale for grade separated U-loops in urban arterials of developed cities, especially, in heterogeneous traffic condition of developing cities is currently unavailable. This study aims to develop rationale for designing U-loop layouts on urban corridors and analyze the potential impact of grade separated U-loop location and configuration on the operational performance of an intersection under varying traffic conditions. A number of set scenarios depending on traffic volume and turning proportion data collected from secondary sources as well as varying geometric configurations of the U-Loops have been developed for the purpose of the study. Signal optimization timing plan for conventional traffic signals are done with Synchro macro-simulation model for use as reference scenario of conventional traffic signals. Micro simulations models with VISSIM have been developed and calibrated for conventional traffic signal, junction with unconventional U-loops and U-loop system for group of junctions to simulate these configurations under varying traffic conditions. The simulation results show that introduction of grade separated U-loop can significantly increase the junction throughput (about 200% increase of throughput for most normal flow scenario) along with significant decrease in junction delay (over 90% decrease in delay for most normal range flow scenario) when compared with the conventional signalized intersections. It is observed that for both balanced and unbalanced volume conditions U-Loop system’s performance deteriorates with the increase of approach volumes, right and cross traffic proportion. Again, the study finds that a 7.3m wide 2 lane-based U-loop system performs much better when U-loops are placed at 150m distance rather than at longer distances from junction. It is encouraging that U-loop holds its potential even for 2-4 intersections group served by a single U-loop pair. The range of results in Tables, graphs and bar charts produced in this research will work as a rational framework for planners and traffic engineers alike for selection of a suitable U-loop plan using different combinations of traffic volume, turning proportion, layout plan and widening scope in certain situation.