Determination of Freeway Acceleration Lane Length for Smooth and Safe Truck Merging
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2019-09-01
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Edition:Final Report
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Abstract:Freeway entrance ramp has intensive vehicle merging activities that lead to delay and potentially high risk of collision between merging and freeway vehicles, especially for the ramps with a high percentage of heavy truck volume. Sufficient length of acceleration lane is critical for the traffic operations at the ramp merging sections. Currently, most DOTs employ the guidelines provided in the American Association of State Highway and Transportation Officials (AASHTO) Green Book, “A Policy on Geometric Design of Highways and Streets” (AASHTO 2011). However, AASHTO’s acceleration lane length design guideline was developed based on passenger cars and hasn’t been updated for years since 1965. In addition, AASHTO other design guidelines and previous studies mainly considered the speed differential between the on-ramp and freeway vehicles on determining freeway acceleration lane length. Only a few of them considered traffic volume, and even less gave special considerations on the heavy trucks that need more time to accelerate and find larger gaps to merge. This research developed an analytical model for estimating the required freeway acceleration lane lengths for both heavy trucks and passenger cars. The model considers both the acceleration and gap searching needs of the merging vehicles. As a result, it can take account of the impacts of the freeway traffic volume into the estimation of the required acceleration lane length. A case study was conducted in Houston, TX to demonstrate the model application. Finally, this research provides a chart and a lookup table for estimating required acceleration lane lengths for both heavy trucks and passenger cars under different traffic volume conditions for different combinations of the freeway and entrance ramp design speeds.
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