Galadima et al., 2017 330 Nigerian Journal of Environmental Sciences and Technology (NIJEST) www.nijest.com ISSN (Print): 2616-051X | ISSN (electronic): 2616-0501 Vol 1, No. 2 July 2017, pp 330- 336 Improving Roadway Operations and Safety for Large Truck Vehicles by Optimizing some Critical Geometric Design Parameters Galadima S.P. 1 , Ogirigbo O.R. 1 and Ehiorobo J.O. 1,* 1 Department of Civil Engineering, University of Benin, Benin City, Nigeria Corresponding Author: *jacehi@uniben.edu ABSTRACT Large truck vehicles are constrained by their physical and operational attributes such as length, width, height and axle loading, which affect their performance on highway infrastructure. In recent times, car-truck and truck only crashes have increased due to the complex interactions of such vehicles on geometrically deficient roadway systems. By adjusting geometric design parameters in accordance with stipulated requirements across deficient roadway sections it is possible to estimate using crash rate index the degree of improvements observed. A total of 189.4 kilometers of three roadway lengths (Benin-Ore, Benin-Agbor and Benin-Sapele) were investigated for geometric design deficiencies across crash sections. Adjustments were made using AAHSTO design specifications based on speed limit, terrain type and roadway functional class for three critical geometric design parameters namely: degree of horizontal curvature, vertical grade and roadway lane width. Comparison of parameter estimates before and after adjustments showed corresponding cumulative percentage improvements of 6.5%, 13% and 4.7% for Benin-Ore, Benin-Agbor and Benin-Sapele roadways respectively. This implies that adjustments to critical design parameters at deficient roadway sections can help in mitigating large truck crash rates and allow for better accommodation and operation of such truck vehicles on plied roadways. Keywords: highway infrastructure, highway improvement, geometric design parameters, Nigerian highways, truck crash rates. 1.0. Introduction The necessity of highway geometric design is to cater for the safe operation of vehicles of known configuration. When plied road sections are geometrically deficient, there is an increased likelihood that the safety of road users will be jeopardized. The AASHTO highway design manual (AASHTO, 2011) recognizes four (4) different classes of vehicles for which roadways may be designed for. These include: passenger cars, buses, trucks, and recreational vehicles. Large trucks otherwise referred to as long combination vehicles (LCVs) represent the most complex design vehicles due to their overall physical dimensions and operational attributes (Elefteriadou et al., 1997). Sections of existing roadway systems with small curb return radii at intersections and interchanges, steep vertical grades as well as narrow or reduced lane widths pose haulage difficulty and high crash susceptibility for large truck vehicles especially among traffic mix on significantly plied routes (Galadima et al., 2017). Harkey et al. (1996) documented several operational characteristics of long combination vehicles, including their off-tracking tendency, stability, speed, acceleration, braking and stopping distance, believed to have impact on transportation safety as well as close relationship to geometric design. The study was geared towards understanding how such large truck vehicles operate in order to better accommodate them through geometric design or regulate them through more stringent laws and better enforcement. The operational characteristics of these vehicles have an impact on transportation safety and relationship with geometric design features. The authors suggested the need for additional study in the following areas: