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

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 two-lane 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 R2 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.