Effect of Geometric Design Consistency on Road Safety: A Case Study of Nairobi Southern Bypass (UCA-2) Road

Abstract

The National Transport Safety Authority reported that between January and November 2014, about 15 people were involved in fatal accidents along the Nairobi Southern Bypass. This high number of deaths has made this bypass to be declared a high risk road in Nairobi. The safety performance of a road can be significantly improved by identifying and correcting any design inconsistencies present. The main objective of the study was to determine the effect of geometric design consistency on road safety focusing on the Nairobi Southern By-pass (UCA-2) road. To achieve this, two specific objectives were investigated. The first specific objective was to determine the frequency, type and severity of accidents on Nairobi Southern bypass (UCA-2) road in Nairobi County. Primary data was collected from filled questionnaires while secondary data was collected from the accident data recorded at Karen, Langata and Industrial Area Police Stations. From the analysis of the collected data, the results showed that total of 87 accidents had occurred between June 2016 and April 2019.It was also established that driver carelessness was the main cause of accidents that occurred along this road. To achieve the second objective, which was to determine the design consistency measures and evaluate their applicability on road safety along Nairobi Southern bypass (UCA-2) road in Nairobi County, the analysis of the spot speed data, “as-built” drawings and traffic volume data acquired from the Kenya National Highways Authority and the China Road and Bridge Corporation was done. Using the indicated criteria and models developed by other researchers, all the measures of geometric design of the Nairobi Southern Bypass, that is, operating speed, vehicle stability, alignment indices and driver workload were found to be consistent. The study findings indicated a strong positive correlation between speed reduction and accident occurrence. Two-lane highways designed solely on the concept of design speed have proved adequate in several cases. From the results of this study, it is recommended that the operating speed of vehicles (which is more realistic) be used as much as possible for the design of these roads. The results obtained from the study showed that the geometric design was adequate for criteria of a good design and therefore required warning signs to be placed before the transitions especially for the black spots as the criteria of a good design. This therefore indicated that the geometric design of the Nairobi Southern Bypass was sufficient and that the traffic accidents that occurred were mainly cause by reasons other than the geometric design.