Guangchuan Yang

Feasibility of Using a Constant Acceleration Rate for Freeway Entrance Ramp Acceleration Lane Length Design

AbstractWhen estimating the required acceleration length of vehicles accelerating from a stopped position, a constant acceleration rate is sometimes assumed for the sake of simplicity. Nevertheless...

Recommendations for Acceleration Lane Length for Metered On-Ramps

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.

Impacts of traffic flow arrival pattern on the necessary queue storage space at metered on-ramps

ABSTRACT This paper compares queue lengths at the two different on-ramp configurations: arterial-to-freeway ramp and freeway-to-freeway connector. Mesoscopic queue length simulation models are developed based on the input-output method. The input-output method aims to model the traffic flow arrival profile; then, the mesoscopic simulation model generates random traffic volume to simulate the stochasticity of traffic flow. The simulation models are validated using field-collected data. Simulation results indicated that the traffic flow arrival pattern has significant impacts on ramp queue length; vehicle platoons released from the upstream signalized intersection tend to exacerbate ramp queue length. At metered ramps, the required queue storage length is found to be 5.7% of demand when demand is less than 500 vehicles per hour per lane (vphpl), or 3.9% when demand is greater than 500 vehicles per hour per lane (vphpl). In comparison, at metered connectors, the percent numbers are 4.7 and 2.1%, respectively.

Modeling the Impacts of Traffic Flow Arrival Profiles on Ramp Metering Queues

An adequate queue storage length is critical for a metered on-ramp to prevent ramp queue spillback to the upstream signalized intersection. Previous research on queue length estimation or queue storage length design at metered ramps has not taken into account the potential impact of various on-ramp traffic flow arrival profiles on ramp queue lengths. This paper depicts the traffic flow arrival profiles and queue generation processes at three different metered ramp categories. Based on a large number of microscopic simulation runs, it is found that, under a given demand-to-capacity scenario, the queue at a metered ramp with two on-ramp feeding movements is more likely to be cleared in a cycle than at a metered ramp with three on-ramp feeding movements. Also, the platoon dispersion effect significantly reduces the ramp queue length, and hence the queue storage needs at a metered ramp. In addition, this paper reveals that ramp queue length tends to increase linearly with upstream signal cycle length. The design of queue storage length for a metered on-ramp hence needs to fully consider the various ramp configurations and upstream signal timing settings.

Queue length estimation for a metered on-ramp using mesoscopic simulation

Abstract This paper proposed an analytical method for queue length modeling at a metered on-ramp, and developed a mesoscopic simulation model for queue length estimation under various demand-to-capacity scenarios. Queue length data were collected at four representative ramp metering locations for model validation; results showed that the queue length modeling method could properly capture the realistic queue profile, and the estimated queue lengths were close to the field observations. It was found that for under-saturated scenarios, queue length showed an exponential increasing trend with demand-to-capacity ratio; while for over-saturated scenarios, the queue length tended to increase linearly with demand-to-capacity ratio. Simulation results indicated that for under-saturated conditions, the required queue storage length was approximately 5.7 percent of on-ramp demand when demand was less than 500 vphpl, or 3.9 percent when demand was between 500 and 900 vphpl.

Acceleration Characteristics at Metered On-Ramps

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.