Modeling Queue Lengths at Roundabouts at Freeway Ramp Terminals

In the US, roundabouts have been widely constructed as an alternative replacement of signalized intersections at freeway ramp terminals as a safety treatment. However in practice, queue spillback from the off ramp to freeway mainline is a typical operational and safety issue particularly pertaining to roundabout interchanges. Knowing the possible maximum queue length under various conditions is essential for appropriate design of storage length of the off ramp and arterial approaches as well as establishment of signal metering warrant to avoid the queue spill back. The Highway Capacity Manual (HCM) does not provide such queue length model specifically developed for roundabouts at ramp terminals. The existing roundabout queue length model is of deterministic nature. With these two concerns, it is highly needed to establish a queue length estimation model for roundabout interchanges based on stochastic data. This research, through a strictly calibrated microscopic simulation platform, analyzes and models the queue lengths at roundabout interchanges. Both single-lane and double-lane roundabout interchanges were modeled in VISSIM simulation platform. Capacity at each roundabout entrance were calibrated and validated via calibrating the critical and follow-up headways. The calibration was completed separately for passenger car and heavy vehicles as both vehicle types have different critical and follow-up headways. Based on the calibrated simulation model, queue length data was collected from multiple random simulation experiment runs. Analysis was performed to establish models that estimate off ramp and arterial queue lengths under various traffic demand conditions, along with consideration of effects of (1) interchange ramp spacing and (2) off-ramp heavy vehicle percentage. Results reveal that at both single-lane and double-la ne roundabout interchanges, the off ramp queue length is significantly impacted by the summation of entrance and circulating demands at the off ramp roundabout entrance. Ramp spacing does not impact off ramp queue length at roundabout interchanges. Ramp heavy vehicle percentage impacts off ramp queue length for both single-lane and double-lane cases. Arterial queue length is only impacted by the summation of entrance and circulating demand. Based on the analysis results, numerical models were established to predict off ramp and arterial queue lengths under various traffic demand and heavy vehicle percentage conditions for both single-lane and double-lane roundabouts at ramp terminals. A set of easy-to-use look-up tables were eventually developed to facilitate estimating off ramp and arterial queue lengths.

• Supplemental Notes:
  • This paper was sponsored by TRB committee ANB75 Standing Committee on Roundabouts.
• Corporate Authors:

  Transportation Research Board

  500 Fifth Street, NW
  Washington, DC  United States  20001
• Authors:
  • Li, Zhixia
  • Chitturi, Madhav V
  • Bill, Andrea R
  • Noyce, David A
• Conference:
  • Transportation Research Board 96th Annual Meeting
  • Location: Washington DC, United States
  • Date: 2017-1-8 to 2017-1-12
• Date: 2017

Language

• English

Media Info

• Media Type: Digital/other
• Features: Figures; References; Tables;
• Pagination: 21p
• Monograph Title: TRB 96th Annual Meeting Compendium of Papers

Subject/Index Terms

• TRT Terms: Estimating; Interchange ramps; Mathematical models; Off ramps; Queuing; Roundabouts; Traffic congestion; Vehicle mix
• Subject Areas: Highways; Safety and Human Factors;

Filing Info

• Accession Number: 01628204
• Record Type: Publication
• Report/Paper Numbers: 17-06665
• Files: TRIS, TRB, ATRI
• Created Date: Mar 7 2017 10:25AM