VOL. 11, NO. 20, OCTOBER 2016 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences © 2006-2016 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com 11877 SPATIAL EVALUATION OF SPEED-FLOW-GEOMETRY RELATIONSHIP ON TWO-LANE RURAL HIGHWAYS Muttaka Na’iya Ibrahim 1,2 , Othman Che Puan 1 and Mushairry Mustaffar 1 1 Faculty of Civil Engineering Universiti Teknologi Malaysia, Skudai, Johor, Malaysia 2 Department of Civil Engineering, Bayero University, Kano, Nigeria E-mail: mnibrahim132@gmail.com ABSTRACT The mean travel speed of drivers on uninterrupted flow facilities such as two-lane rural highways is deemed as good performance indicator for the subject road class; as the variable relates well with user perception. However, the operating conditions on two-lane roads relating to travel speed is different from those on other types of facilities, as fast moving vehicles in either direction are usually impeded by slower moving ones in the same travel direction and also facing oncoming traffic in the opposing lane. Thus, impeded vehicles may be compelled to travel at lower speeds than desired; particularly, in the absence of sufficient sight distance and permissible gap in the opposing traffic stream, being the appropriate lane used for passing maneuvers. This implies that the operating speed on two-lane highways substantially depends on the level of traffic flow and perhaps, its composition as well as the roadway geometric features. This paper examines the effects of traffic level, composition and road geometric features on the operating speed on two-lane highways based on a spatial approach. An empirical model relating mean travel speed to traffic flow parameters and highway geometric features was derived for prediction of mean travel speed on two-lane rural highways based on easily observable variables. Keywords: speed-flow-geometry relationship, two-lane highways, operating condition, spatial approach. INTRODUCTION Two-lane rural highways constitute a significant proportion of roads in many countries. This type of facility is well known for high vehicular interactions in both traffic directions as only two opposing lanes are used for vehicular movements. The effect of the interactions among the vehicles, traffic level and composition as well as the road geometric features made the operating conditions on this class of road relating to mean travel speed different from that on other roads classes. On two- lane roads, fast moving vehicles are usually impeded by slower moving ones in the same travel direction and equally facing oncoming vehicles in the opposing lane. Consequently, impeded vehicles may be forced to travel at lower speeds than their desired. The decline in the travel speed tends to be more with increase in traffic volume; particularly, for streams with high proportion of heavy vehicles. Because, presence of heavy vehicles within a traffic stream reduces speed and in turn cause a reduction in capacity due to their size and lower desired speed [1]. More so, restricted passing opportunities along the segment result in higher effect of the slow moving vehicles [2] as passing maneuver is performed using the opposing traffic lane subject to availability of sufficient sight distance and acceptable gap in the opposing stream. This implies that the mean travel speed on two-lane highways considerably depends on the amount of traffic flow and perhaps, its composition as well as the characteristics of the highway geometry. The mean or average travel speed (ATS) of traffic stream is used and deemed as good performance indicator for two-lane highways, as the variable relates well with user perception. Further, ATS is one of the service measures used in evaluating the operational performance of two-lane highways upon which the operating condition can be described and an appropriate level-of-service (LOS) be assigned. Despite the significance of ATS in the LOS analysis of two-lane highways and its variability with changes in traffic flow level and roadway geometric features over extended segment, previous studies regarding the effect of traffic flow and road geometric features [3-9] were conducted based on spot observations. This approach may not accurately reflect the effects of variations in traffic level and geometric features along the segment. Hence, the impacts of traffic level, composition and variations of roadway geometric features over extended segment were not given the desired attention. This paper examines the impacts of variations in traffic level, composition and roadway geometric variables on mean travel speed on two-lane highways based on spatial observation. METHODOLOGY In the course of this investigation, the required data for the work were fully gathered from field observation. Data relating to traffic flow and roadway geometric parameters on various segments of two-lane highways with varying characteristics were sampled. A total number of twenty directional segments drawn from Johor and Pahang States, Malaysia, were utilized for the data sampling. Table 1 presents the list of the study sites and number of segments used at each of the locations.