Identifying the Effects of Geometric and Operational Parameters on Merging Speed and Breakdown Probability at Expressway Merge Segments

Abstract

Merging segments frequently experience traffic breakdowns as traffic flow shifts from free-flow conditions to congestion. This transition is primarily attributed to the arrival of vehicles from on-ramps onto expressways, which disrupts the flow on the mainline and leads to deceleration waves. The severity and duration of these disruptions escalate when merging speeds drop below the free-flow speeds observed on the mainline. This study investigated the impact of geometric and operational factors on the merging speed of vehicles entering the mainline from on-ramps, thereby affecting the probability of traffic flow breakdown. A mixed linear regression methodology was used to analyze collected data on vehicle speeds at 11 merge segments in Tehran, Iran. Results revealed that the curvature change rate of ramps, ramps’ width at expressway junctions, and the number of expressway lanes upstream of the merge area negatively impacted the merging speed, thereby increasing the breakdown probability. For instance, expressways with three lanes upstream of the merge area resulted in an average decrease in the merging speed of 10.32 km/h compared to the two-lane cases. Conversely, the acceleration lane’s length positively affected on-ramp vehicle speed, reducing the breakdown probability. Operational factors were also found to play a significant role, particularly the spacing between merging and leading vehicles on the mainline at the time of insertion, which was positively associated with the merging speed. Meanwhile, both ramp flow rate and free-flow speed were identified as exerting opposing influences on the speed of vehicles merging from the on-ramp. The findings of this study offer valuable insights for traffic engineers, facilitating the design of ramp layouts and merge areas. These insights enable the implementation of strategies aimed at minimizing the traffic flow breakdown probability and ensuring smooth traffic flow on expressways. Alleviating congestion in merge areas is a significant challenge for transportation engineers and policymakers. Traditional strategies such as ramp metering and variable speed limits often address symptoms but overlook the probabilistic nature of traffic breakdowns, leading to overly conservative or aggressive controls. This study examined the impact of geometric and operational factors on the merging speed and the breakdown probability at merge segments in urban expressways, offering practical insights for optimizing design and traffic management. The findings revealed that higher on-ramp curvature change rates reduce merging speeds and increase breakdown risk. Therefore, designing on-ramps with lower curvature change rates—by increasing the radius or length—is preferable. In addition, longer acceleration lanes improved merging speeds and reduced breakdown likelihood. From a management perspective, this study’s identification of the relationship between the ramp flow rate and the breakdown probability offers valuable guidance for optimizing dynamic ramp metering by determining the ideal flow rate. Moreover, the findings on the relationship between the free-flow speed and the breakdown probability can guide engineers in optimizing variable speed limits to reduce the likelihood of deceleration waves in merging areas, improving overall traffic flow efficiency.