Journal - The Institution of Engineers, Malaysia (Vol. 73, No. 1, March 2012) 20 AN INVESTIGATION OF THE 85 TH PERCENTILE OPERATING SPEED MODELS ON HORIZONTAL AND VERTICAL ALIGNMENTS FOR TWO-LANE RURAL HIGHWAYS: A CASE STUDY (Date received: 28.12.2009/Date accepted: 1.2.2012) Syed Khairi Syed Abbas 1 , Muhammad Akram Adnan 1 , Intan Rohani Endut 1,2 1 Faculty of Civil Engineering, 2 Malaysia Institute of Transport, Universiti Teknologi MARA, 40450 Shah Alam, Selangor E-mail: 1 s_k_5421@yahoo.com ABSTRACT The number of accidents in Malaysia increases relatively from year to year, although there are many programmes organised by the authority to reduce them. There are several factors that lead to serious accident problems which are human behaviour, vehicle’s condition, weather condition, road surface and road alignments (vertical and horizontal curves). This research paper presents an empirical research and presents design consistency models to estimate the 85 th percentile operating speed models for the horizontal and vertical alignments at two-lane rural highways. The speed data were carried out by using a laser gun metre detector to obtain the spot speed data in the selected location along the Lenggong to Sauk two-lane rural highway in Perak. Multiple linear regression analysis was conducted to develop the 85 th percentile speed models by combining the operating speed data and geometric elements data from different selected horizontal and vertical alignments. Finally, the horizontal and vertical alignment models have successfully developed in the paper. The indings are hoped to provide a starting point towards the national design standard for the highway designer and planner to improving the consistency geometric design elements of the two-lane rural highways. Keywords: Design Consistency, Horizontal Alignment, Operating Speed Model, Vertical Alignment, Rural Highways 1.0 INTRODUCTION A good quality road infrastructure means providing an appropriate level of mobility and land use access for drivers and pedestrians while maintaining a high degree of safety. Maintaining a high degree of safety for the movement of people and goods on the road should be emphasised in any road design [1, 2, 3, 4]. Geometric design can be referred to the selection of roadway elements that includes the horizontal alignment, vertical alignment, cross section and roadside of the highways or streets [5]. Many researches have applied several elements of the geometric design such as horizontal alignments and vertical alignments in other countries [4, 6, 7, 8, 9, 10, 11]. However, to date there has been no in-depth investigation reported on the model of the 85th percentile operating speed for horizontal and vertical alignments on the two-lane rural highways in Malaysia [5, 12]. This study is actually very important and crucial as there is a need to explore and develop a new model to enhance the existing guidelines and standards focussed on the horizontal and vertical alignments, especially for two-lane rural highways where high trafic speed is endured on the rural highways [1]. So far, the evaluation of the horizontal and vertical alignments’ models are based on the relationship of operating speeds and other geometric elements [4, 6, 7, 8, 9, 10, 11]. Therefore, this study proposes models for evaluating the 85th percentile operating speed for horizontal and vertical alignments on the two-lane rural highways by developing operating speed models based on the local empirical data. 2.0 RESEARCH OBJECTIVES The purpose of the study is to develop 85 th percentile operating speed models that can be integrated in giving effects of highway geometry in horizontal and vertical alignments. The model can be used to estimate the operating speed of passenger vehicles. Subsequently, the parameters that are highly correlated between driver speed and geometric design elements can be justiied based on R5 design standard and can assist the practitioners towards best practice in highway geometric design on two lane rural highways condition in the country. 3.0 LITERATURE REVIEW 3.1 Terminology The deinition of speed can take many forms and is one of the most important parameters in the geometric design of highway. The term ‘speed’ is a general term typically used to describe the actual speed of a group of vehicles over a certain section of roadway [9]. Speed is a fundamental factor in transportation engineering; it is often denoted by different terms while applied in different situations such as a design criterion, a measure of the level of service and as an operational control parameter [13]. A summary of the types of speed terminologies are presented in Table 1.