COMPARISONS OF DELAYS AND CO₂ EMISSIONS BETWEEN FIXED-TIME AND WEBSTER'S OPTIMAL SIGNAL TIMING AT INTERSECTION

Abstract

This study aims at comparing the average delay and carbon emissions between the existing conditions, in which traffic flows are controlled by a fixed-time signal controller, and the scenarios where signal timings are optimized using Webster’s method. This study opts to optimize signal timing at an isolated signalized intersection by minimizing the total vehicular delay that occurs as a result of queuing. The layout of Saphan Khwai Intersection, an at-grade four-legged intersection located in Bangkok, Thailand, was adopted as a case study for comparing the two types of signal controls. Sixteen scenarios with varying traffic flows, ranging from 100 pc/h/ln to 1,600 pc/h/ln, were coded in Microsoft Excel using macros to compare the resultant delays and carbon emissions between five different fixed-time signal controls, spanning a cycle length of 50 seconds to 250 seconds, and the optimized signal timing using Webster's method. The results show that Webster’s method produces lower delays and carbon emissions than adopting the fixed cycle length at low to moderate traffic flows. However, the effectiveness of Webster’s method starts to be diminished as the average demand flow is approaching the saturation flow rate under the same conditions. This study intends to investigate how much delays and carbon emissions would be reduced if a real-time signal controller using Webster’s method is adopted to replace the current fixed-time controller at the intersection.