Toward an operating speed profile model for rural two-lane roads in Egypt

The geometric design, especially the horizontal and vertical alignments, of rural two-lane highway facilities is considered one of the most important factors affecting the quality of traffic service and safety. A consistent highway geometric design is defined to be one that conforms to the driver's expectations. In order to calculate the main measures of design consistency, an accurate operating speed profile model for road alignment is needed. Studies have shown that operating speed models are country dependent due to varying demographics, driver attitudes, habits, etc. This paper develops a speed profile model for two-lane rural roads in Egypt. This includes the development of operating speed models for horizontal curves and tangents, as well as the study of the characteristics and relationships between acceleration and deceleration rates before and after horizontal curves. The study uses a desert rural two-lane, two-way road sections that connect the city of Sohag with the city of Hurghada, in Upper Egypt. All geometric characteristics were obtained from highway authority. Speed data regarding individual drivers traveling on selected two-lane rural road sections were sampled using an on-board GPS system. The study confirms the finding in previous research that curvature is the most important factor in determining the speed on horizontal curves. Moreover, tangent length is the most important factor in determining operating speeds at tangents. The acceleration and deceleration characteristics were derived to gain an understanding of the behavior of individual vehicles traveling through curves of varying radii and lengths as well as preceding tangent length. Several operating speed models were developed for tangents and curves as well as for acceleration and deceleration rates. Incredibly effective, these models can be used for design consistency evaluations.