NCHRP Report 439, "Superelevation Distribution Methods and Transition Designs," evaluates and recommends revisions to the horizontal curve guidance presented in the 1994 AASHTO publication, A Policy on Geometric Design of Highways and Streets (Green Book). The two principal design elements evaluated were the use of superelevation and the transition from a tangent to a curve, though all elements of a curve were considered in the analysis. The transition recommendations were incorporated into the 2001 edition of the Green Book while the superelevation recommendations were being considered for the subsequent edition.

Chapter III of the 1994 AASHTO Green Book contains information on superelevation design procedures for the full spectrum of highway conditions, including rural highways and high-speed urban streets, low-speed urban streets, turning roadways, and intersection curves. These design procedures include distribution of both the superelevation rate (e) and the side friction factor (f) as well as the design of superelevation transitions. These procedures are based on limited empirical data from the 1930s and 1940s.

Five methods are discussed in the 1994 Green Book for distributing the superelevation rate and side friction factor, but there is limited understanding of the operational characteristics of these different methods. The use of multiple methods by different agencies produced inconsistent designs. Different agencies had also adopted different methods of transitioning from a tangent, crowned cross section to the superelevated cross section used on a horizontal curve.

Following a thorough literature review and survey of the domestic and international practice, the researchers collected data at 55 curves in 8 states to quantify the relationship between side friction demand, speed, curve radius, and superelevation rate. Simulation was then used to evaluate the effect of alternative transition designs on vehicle lane position and control. Recommendations for the Green Book were then developed to make the design of curves easier and more consistent throughout the United States.

NCHRP Report 439 summarizes the results of NCHRP Projects 15-16 and 15-16A, conducted by the University of Nebraska at Lincoln and the Texas Transportation Institute at Texas A&M University, respectively. Two contracts were needed because the principal investigator changed employment.