Effect of Geometry and Control on the Probability of Breakdown and Capacity at Freeway Merges

Abstract

This paper investigates the effect of geometric and operational features on freeway merge capacity by analyzing prebreakdown flow rates and using nonparametric and parametric (survival) analysis and semiparametric analysis (Cox’s proportional hazard model) techniques. The probability of breakdown and capacity were quantified as a function of the number of lanes, ramp flow rate, presence of lane drop, and presence of ramp metering. The number of lanes showed a negative relationship with the per-lane average prebreakdown flow rate. Sites with ramp meters also showed higher prebreakdown flow rates than nonmetered sites by 150 to 300 veh/h/ln. In survival analysis, the number of lanes and presence of ramp meters were found to have a statistically significant effect on the breakdown probability. Capacities, defined as the 15th percentile breakdown probability flow rate, were 2,043 and 1,952 veh/h/ln at three-lane and four-lane unmetered sites, respectively. Capacities were 2,316 veh/h/ln at three-lane and 2,011 veh/h/ln at four-lane metered sites. Higher ramp flows resulted in higher breakdown probability and lower capacities at two of the four sites that were investigated, suggesting that further research is needed to evaluate this relationship.