Jacobs Journal of Civil Engineering

Traffic Capacity of Interchange Circular Loops

*Hashem R
Department Of Civil Engineering, Jordan University Of Science And Technology, Jordan

*Corresponding Author:
Hashem R
Department Of Civil Engineering, Jordan University Of Science And Technology, Jordan
Email:Almasaeid@yahoo.com

Published on: 2019-03-05

Abstract

The major objective of this research was to determine the capacity of circular loops at suburban interchanges. To achieve this objective, ten interchange loops with different radii were selected. Data on traffic operation variables at loop entrance, loop exit, and middle part of the loop (proper) were obtained using video camera. Also, data on loop geometric were obtained through field survey. Results of analysis indicated that loop proper capacity under Jordanian conditions is significantly lower than those presented in the Highway Capacity Manual. Two loop proper capacity models were developed using loop radius or free-flow speed as determinate variable. The analysis also revealed that loop exit capacity is influenced by the traffic flow on the outer arterial lane. Using regression analysis, an exponential model was developed to estimate loop exit capacity. Furthermore, a linear model was recommended to estimate loop entrance capacity, using loop radius as determinate variable. Finally, results of the study showed that loop entrance and exit capacity values in Jordan are comparable with those presented in the literature, and the exit capacity is the critical value that determines the capacity of the loop ramp.

Keywords

Interchange; loop capacity; circular loop; exit; entrance; proper; regression

Introduction

Today with the increase in traffic congestion levels, many interchanges were constructed to eases traffic movements. Cloverleaf and trumpet interchanges are widely developed in rural and suburban areas. These highway facilities are normally made of ramps and loops. In fact, a loop is provided to handle left-turn movements and its capacity is a critical issue in selecting a specific interchange type. For example, American Association of State Highway Officials (AASHTO 2011) revealed that the maximum capacity of a loop, regardless of its width, varied from 800 to 1200 vph [1]. In Jordan, Highway Capacity Manual (HCM) is widely used in estimating capacities of highway facilities without any modifications to account for possible differences. These differences are anticipated and attributed to differences in driver’s behavior, design factors, and traffic mix. Estimation of loop capacity and determination of influencing geometric or operational factors would be very crucial for traffic and highway engineers.The objective of this study was to determine loop capacity under Jordanian local condition and identify design factors that influence the estimated capacity. The scope of this study included circular loops in urban and suburban interchanges. In fact, in Jordan most of loops are constructed as circular loops. Furthermore, loops in rural areas are rarely operated at/near capacity