Evaluation of speed distribution at horizontal curve on two-lane rural arterial highway under mesopic and photopic scenario

Abstract

Drivers travel at speed they assumed to be comfortable to achieve prevailing conditions and road geometry. The relationship between road geometrical design and speed of travelling vehicle is very prominent especially when travelling at the horizontal curve of a roadway. Crash events on horizontal curve are higher by the range of 1.5 and 4 times compared to the tangent sections. This is because speed varies according to the perception of constrain imparted to the drivers by the road geometrical design. The purpose of this study is to investigate the relationship between design speeds, operating speeds, and speed limits while drivers negotiate the horizontal curve. Furthermore, this study also investigates the behaviour of driver manoeuvre on the horizontal curve under mesopic and photopic visions. The speed data measurement was based on spot speed data at specific points and locations using the laser gun detector. The study was carried out by analysing the speed characteristics during daytime at three different segments along the horizontal curve (i.e. transition at entering of curve, middle of curve, and transition at leaving of curve). Apart from that, the difference in the mean speed on the contrast sensitivity under mesopic and photopic conditions was determined via t-test. It discovered that vehicles travelling on transition entering the curve tend to travel at higher speed than on the middle of curve and on transition leaving the curve. Based on the preliminary finding, further testing on middle of curve at night time was carried out to find significant effect under mesopic and photopic visual conditions. The research also found that the existing speed limit of the selected road stretches was lower than the 85th percentile speed. The 85th percentile speed is a commonly used measure to decide the speed limit on a road.