Traffic Stream Relationships of Two-Lane Highways: A Case of Akure-Ondo Road in Southwest Nigeria

Abstract

In the design and planning process of highway infrastructure, speed-flow-density relationships are useful tools for predicting the roadway capacity, determining adequate level-of-service of traffic flow and travel time for a given roadway. The speed-flow-density relationships currently used for transportation studies in Nigeria is derived from the Highway Capacity Manual, which does not reflect the true traffic situation on twolane roads in Nigeria. Developing cost effective tools for describing these relationships in the context of a developing country like Nigeria is imperative. The aim of this study was to develop models to describe the relationship between traffic flow, speed and density on Akure-Ondo two-lane highway in southwest Nigeria. Moving observer technique was employed to collect traffic stream data over a stretch of 5km on the study segment during periods of uniform flow on weekdays. To describe the traffic stream relationships, two approaches namely: related and nonrelated traffic stream models were developed. The nonrelated traffic stream models gave inaccurate relationships while the related traffic stream modelling approach performed well at describing speed-flow, flow-density and flow-speed relationships with R 2 values 0.62, 0.75, and 0.80 respectively. The relationships developed from related traffic stream models estimated maximum flow on the study segment as 330 veh/h at an optimum density of 4.44 veh/km. The speed at maximum flow was estimated as 74.5km/h, while the free flow speed was estimated as 149.027km/h.