Recommendations for Acceleration Lane Length for Metered On-Ramps

Abstract

AASHTO’s A Policy on Geometric Design of Highways and Streets (Green Book) is currently used by most state departments of transportation in the United States in determining the design length of acceleration lanes of metered on-ramps; however, the recommended acceleration lengths have not been updated for several decades. This study aimed to develop a method for determining acceleration lengths at metered on-ramps. Vehicle location versus time information was collected via parallel cameras at seven metered on-ramps in California; then, a piecewise constant acceleration model was proposed to calculate the spot speeds of individual samples at predetermined locations. The percentile distance-versus-speed profiles at each ramp were built, and regression models were generated to predict the required acceleration length at a given merge speed. The 85th percentile data were recommended as the minimum acceleration length to accommodate most drivers in accelerating to a safe merging speed. The new recommendation was compared with the existing guidance in the Green Book. On the basis of 1,658 individual samples, it was found that the recommended acceleration lengths were shorter than the Green Book guidelines by 10% to 35%. Also, results showed that acceleration lengths for tractor trailer trucks were approximately 60% greater than the Green Book guidelines.