Horizontal curve radii versus turning abilities of vehicle combinations for timber transport

State Forests make various road investments in cooperation with local self-governments. Jointly modernized roads facilitate local transport for both parties. The prerequisite is proper selection of technologies and project parameters for the modernized roads. The paper aimed to analyze the minimum horizontal curve radii for various elements of roads (on exits, road junctions and vehicle maneuvering areas) depending on the vehicle combinations for timber transport. The scope of works comprised: - analysis of technical conditions for public roads; - analysis of kinematic relationships for vehicle combinations for timber transport; - determining the types of vehicle combinations for timber transport; - empirical determination of the turning abilities of timber haulage vehicle combinations and fire engines. Potential vehicle combinations used for timber transport were determined on the basis of data collected from the customers. Tests of turning abilities of vehicle combinations used for long and short timber transport and for forest fire fighting used by the State Fire Service units were conducted on a maneuvering area, at the curves with radius R = 11.0 m; 13 and 15 m and steering angle α = 90º. A combination of a truck with a trailer or steered cart revealed the best turning parameters. It was demonstrated that the rounding radius R = 11.0 m without widening of a 3.5 m wide road may be used only when entering very wide public roads with a not so heavy traffic. A circular curve rounding a with a minimum radius R = 13.0 and in specific cases with R = 15.0 m.