Acceleration Characteristics at Metered On-Ramps

Abstract

Vehicle acceleration capability is an important parameter for traffic operation analysis, traffic simulation, and transportation facilities design. Most of the existing studies on acceleration characteristics, however, do not fully represent the realistic traffic conditions or are based on outdated data; these deficiencies may result in errors in modeling vehicle acceleration capability. In this paper, actual acceleration characteristics at metered on-ramps were investigated according to field data collection and analysis. With the assumption that vehicles make uniformly accelerated motion in each short space or time interval, the entire acceleration length was partitioned into several short segments. Then, video cameras, magnetic counters, or both were used to document the spot speed at predetermined locations; accordingly, distance–acceleration profiles of various ramp configurations were determined. Results showed that the acceleration rate at metered on-ramps was not constant; vehicles usually accelerated at a higher acceleration rate when the speed was lower and vice versa, and the maximum acceleration rate usually occurred at about 50 to 100 ft downstream from the stop bar. Also, a ramp with taper merging configuration usually produced higher acceleration rates than did a parallel merging ramp. This study also pointed out that acceleration rates documented in the AASHTO Green Book, widely used by most state departments of transportation in the United States as the acceleration length design guidance for metered on-ramps, were much lower than the values measured at existing metered on-ramps.