Exploratory Analysis of Ramp Metering on Efficiency, and Safety of Freeways Using Microsimulation

Abstract

The microscopic Verkehr In Städten – SIMulations Model (VISSIM) stochastic simulator program was used to explore the effectiveness of ramp metering on efficiency, Level of Service, and safety of freeways. Three different geometric configurations of ramp-freeway junctions were evaluated using different traffic volume conditions of the ramp and the freeway. Different signal timing scenarios were designed for the different traffic volume and geometric configuration scenarios. Calibration process was conducted for the collected traffic data that were obtained from cameras and detectors. Two-hundred-eighty models were built and run to explore the effectiveness of the performance and safety of the ramp meters on freeways. Average speed and average travel time of the vehicles passing a 3,000-ft long freeway segment were used as measures of effectiveness of the freeway efficiency evaluation. Average density in the ramp influence area was used to obtain the freeway level of service as a measure of effectiveness of the freeway capacity evaluation. Frequency, types, and severity of vehicle conflicts, which occurred on the 3,000-ft freeway segment, were used as measures of effectiveness of the freeway safety evaluation. The Surrogate Safety Assessment Model (SSAM) program, which was developed by the Federal Highway Administration (FHWA), was used to find the frequency and types of vehicle conflicts, while the severity of vehicle conflicts was separated by a designed method that was retrieved from the previous literature studies. Minitab statistical software was used for some tests such as normality test to determine the appropriate number of samples, and F-tests. A sensitivity analysis was also conducted for better understanding the effectiveness of two assumption changes on the results that were obtained from running the models. The assumptions were car following headway in the ramp influence area and traffic composition on the freeway. The findings of the study provided different results related to the different geometric configurations, signal timing designs, and traffic volumes. Ramp metering at the Type I geometric configuration provided positive effects on the efficiency and safety of the freeway when using the two designed signal timing scenarios when the freeway traffic volume was equal to or greater than 1,250 vehicle per hour per lane (vphpl) and the ramp traffic volume was equal to or greater than 800 vphpl. Ramp metering provided negative effects on the efficiency and safety of the freeway when using it for the Type II geometric configuration. In the geometric configuration of Type III, ramp metering using the signal timing of 2 seconds green and 4 seconds red provided the best efficiency and safety increases when the freeway traffic volume was equal to or greater than 1,250 vphpl and the ramp traffic volume was equal to or greater than 800 vphpl. Conclusively, ramp metering increases efficiency and improves safety of freeways only at specific situations regarding geometric configuration of the ramp-freeway junction type, traffic volume of the freeway and the ramp, and the designed traffic signal of the ramp meters.