Reviews of design guidelines for rural roads in Germany, France, and Switzerland reveal that highway designers adhere to controls on maximum and minimum lengths of tangents between successive curves. Minimum tangent lengths are prescribed to promote operating speed consistency, and maximum lengths are suggested to combat driver fatigue. Current U.S. practice does not set maximum or minimum lengths of tangents; instead current AASHTO policy favours long tangent sections for passing purposes on two-lane, rural roads. This paper presents a recommended strategy for U.S. highway designers to consider tangent lengths explicitly in rural highway design. The proposed approach uses recommended operating speed differences between successive horizontal geometric elements (curves and tangents) and acceleration or deceleration profiles derived from car-following tests to establish limits. Recommendations are also provided for transition lengths (tangent length) between successive curved roadway sections for (a) tangents that should be regarded as "non-independent" design elements; that is, the sequence "curve-to-curve" is the most important element of the design process and (b) tangents that should be regarded as "independent" design elements; that is, the sequence "tangent-to-curve" is the most important element of the design process.
Abstract