Roundabout Capacity Survey Method Using Vehicle Trajectory Data and Gap Acceptance Cycles

Abstract

The objective of this paper is to describe a new roundabout capacity survey method using vehicle trajectory data. The survey uses gap acceptance cycles based on a traffic signal analogy approach used for gap acceptance capacity and performance model development. Real-world vehicle trajectory data were collected at a single-lane roundabout with moderate traffic conditions in Raleigh, North Carolina (NC), USA. The survey method defines reliable gap acceptance cycles for measuring follow-up headway, critical headway, and capacity cycle by cycle by observing saturated unblocked periods occurring when all entry vehicles are queued. The results compared well against values obtained from some established methods. Average blocked time, unblocked time, and gap acceptance cycle time estimates were assessed using M3D, M3B, M3T, M1, and M2 arrival headway distribution models with observed follow-up and critical headway values. Gap acceptance capacity estimates were assessed for these models as well as the Highway Capacity Manual (HCM) default model and the general Siegloch M1 model used in the HCM. The results confirmed the validity of the survey method, supporting the novel signal analogy concept with real-world data. The assessments implied low levels of bunching in circulating flows. The survey method can be used for determining gap acceptance factors for different vehicle classes in entry lanes. Future research is recommended on bunching models for headway distributions, and further assessments of the new survey method by applying it to multi-lane roundabouts as well as unsignalized (stop and give-way / yield controlled) intersections and permitted turns, unsignalized bypass lanes and turn on red at signalized intersections.