Vehicle Speed and Acceleration Profile Study for Metered On-Ramps in California

Ramp metering has been proven as an effective freeway-management strategy; however, one of the critical challenges in design and operation of ramp metering is the lack of up-to-date design guidance for acceleration lanes. This paper aims to investigate the actual acceleration characteristics for metered on-ramps with various geometric features and also figure out speed profiles to guide the design of acceleration-lane length. A piecewise-constant acceleration model, which assumes vehicles making uniformly accelerated motion within each short space or time interval, was employed to model vehicle acceleration characteristics. The approach of using distance versus speed profiles for acceleration-length design was also introduced. Results show that acceleration rate at metered on-ramps is not constant; vehicles usually accelerate at a higher acceleration rate when speed is lower and vice versa. It was observed that taper ramps usually produce higher acceleration rates than ramps with an auxiliary lane, and existing acceleration length would also affect drivers’ acceleration behavior. Finally, this study pointed out that the acceleration performance data documented in the AASHTO Green Book, which are now widely employed by several state DOTs in the United States as acceleration length design guidance for metered on-ramps, is approximately 1.7 times greater than the value measured at typical taper-type metered on-ramp in California.