TR3-1 Building Tomorrow’s Society Bâtir la Société de Demain Fredericton, Canada June 13 – June 16, 2018/ Juin 13 – Juin 16, 2018 EFFECT OF RAINY DARKNESS INTENSITY AT TWO-LANE HIGHWAY ON TRAVEL TIME DIFFERENTIALS Makinde, Opeyemi Oluyemisi 1,3 and Ben-Edigbe, Johnnie 2 1 University of KwaZulu-Natal, South Africa 2 University of KwaZulu-Natal, South Africa 3 216073398@stu.ukzn.ac.za Abstract: In this paper travel time differentials prompted by rainy darkness on Two-Lane Highway were investigated. The investigation raises the issue of night travel on roads without lights. Even though drivers react to rainfall and darkness simultaneously, studies have not been carried out insufficiently. Hence the paper wants to provoke debate on this issue. Based on the hypothesis that rainy darkness irrespective of intensity will cause travel time increase, with and without rainy darkness studies were carried out at three selected sites in Nigeria during the rainy season (June-August). Traffic and rainfall data were collected continuously for eight weeks. Rainfall intensity was classified according to the conventional rate of precipitation: Light rain — when the precipitation rate is < 2.5 mm (0.098 in) per hour. Moderate rain — when the precipitation rate is between 2.5 mm (0.098 in) - 7.6 mm (0.30 in) or 10 mm (0.39 in) per hour. Collated data on which the study was based were analyzed. Travel times for with and without rainy darkness were estimated and compared. Results show that there is speed reduction of 4.1% due to light rain effect, 8.6% and 14.8% due to moderate and heavy rain effect respectively. Further, travel time increase relative to speed decrease during rainy darkness, thus suggesting that motorists reduce vehicle speed when it is dark and rainy irrespective of intensity. The paper concluded that rainy darkness has a significant effect on travel time. 1 Introduction The highway is an important element of transportation because it connects all other forms of transportation. Traffic flows on the highway are expected to be safe, efficient, give good driving comfort and with minimum delay. Vehicle performance on highway depends on the road geometry, traffic volume, traffic composition and weather condition like fog, snow, wind, darkness and rainfall. These weather conditions can be mild and could be harsh depending on the intensity of rain and fog, wind speed and their duration. Harsh weather condition can bring about total traffic flow disruption on the highway. The weather impacts on traffic flows affect delays, safety, travel demand, and road accident (FHWA, 2008 and Cools et al. 2010). Weather impact on traffic flow may increase accident, delay, speed reduction and traffic flow contraction (Alhassan and Ben-Edigbe, 2014). Rainfall disrupts traffic flow by reducing driver’s visibility, thereby leading to speed reduction, and increase in headways (Ben-Edigbe, 2010). Other studies by Chung et al, (2005) and Mukhlas et al (2016) showed that increase in rainfall intensity causes speed and capacity decrease and increase in travel time. Under darkness condition where there is no road lightning, there is poor or no illumination, drivers are dependent on the performance of their vehicle headlights. This condition affects the driver’s visibility because of deterioration in colour vision which could lead to a reduction in sight distance. If rainfall is thrown in the mix, then driving conditions deteriorate even further. Rainy darkness depicts rainfall and darkness conditions. When driving under rainy darkness, the driver will be subjected to the combined effect of rainfall and darkness. This might