Potential Changes to 2004 Green Book's Adjustment Factors for Entrance and Exit Terminals

AASHTO's Policy on Geometric Design of Highways and Streets (Green Book) notes that acceleration and deceleration lengths should be adjusted on graded ramps. Factors are available in the 2004 Green Book to adjust acceleration lane and deceleration lane lengths for grades of 3% to 6%. The source of the adjustment factors was provided in the 1954 Policies on Geometric Highway Design, in which they first appeared as being based on applying “principles of mechanics to rates of speed change for level grades.” Reviews of this and other documents did not reveal a procedure for determining adjustment factors. A potential source for an adjustment factor for entrance terminals is the calculation of the distance needed to accelerate from one speed to another on different grades by means of vehicle performance equations available in the literature. Potential acceleration length adjustment factors were developed and presented in this paper. The 2004 Green Book provides equations to calculate stopping sight distances for different grades. This methodology was applied to the equations used to calculate deceleration lengths so as to determine deceleration lengths for different grades. The ratio of the deceleration length on a grade to the deceleration length on a level surface formed the basis for adjustment factors for deceleration. Actual performance of vehicles on grades and on a level surface should be measured and compared with the suggested adjustment factors to determine the accuracy of those factors.