Summary Analysis of Roundabouts

Author: Bill and Visa Siino

Date: September 17, 2015

We had the fortunate opportunity to engage a national leader in roundabout engineering to perform an updated simulation using the latest standards and procedures. Here are the results for your enjoyment.

 Analysis of Traffic Light Group's Report

• According to Kittelson Associates Inc (KAI), a national leader in roundabout engineering, the traffic light group’s report is not fully consistent with the current state of modern roundabout planning and design.  For instance, with regard to:
  • Lane Imbalance –
    • Older roundabout guidance in the U.S. has included the generalized notion that roundabouts operate poorly under unbalanced traffic flows. This notion has not been corroborated in any of the recent research on the performance of roundabouts in the United States, including NCHRP Report 572 (2007) or the more recent FHWA TOPR 34, Accelerating Roundabouts in the United States (publication pending). In particular, updated capacity models can be used to verify the performance of any combination of flows, rendering it unnecessary to resort to such generalizations.
  • Capacity –
    • Synchro 8 and other software packages implement a version of the HCM 2010 capacity models, which are now considered out of date and overly pessimistic in terms of capacity. These have been updated based on research for FHWA, with publication pending in 2015. The Transportation Research Board (TRB) Committee on Highway Capacity and Quality of Service has adopted the updated capacity models, which will be published in a major update of the HCM 2010, expected to be released around the end of 2015. FHWA staff recommends the use of the updated models, which have been presented at the TRB International Roundabout Conference in 2014 in Seattle, Washington.
  • Vehicle Queues-
    • A queue analysis considering access control mitigations is not provided in the traffic light group's report at any project intersection. Management of vehicle queues is a critical performance measure when evaluating intersection improvements and the overall performance of the transportation system. The roundabout at Via de la Valle will provide superior flexibility to access nearby streets and driveways, such as La Fremontia, compared to a signal alternative. The signal design will likely limit access in and out of nearby streets and driveways or will require additional lanes be constructed beyond those identified in the current signal analysis.

KAI Study 

• KAI performed a second independent study, using updated capacity models and parameters derived from driving characteristics of roundabout users in California. Using data based on driving styles of Californians, the roundabouts on Paseo Delicias will provide less delay compared to traffic lights, showing results between the original SD County engineering report, but better than the Urban Systems Report. The California parameters used in the analysis yield similar results as those typically found with the current national research being conducted by KAI for the Federal Highway Administration (FHWA).
• For the existing project condition, single lane roundabouts operate with approximately 10 seconds of delay with LOS A/B.
• For the 2030+ project condition, single lane roundabouts operate with 21 seconds or less of delay at Via de la Valle and El Monte Video. Congestion at El Camino del Norte could be mitigated by adding a simple westbound right turn lane for morning congestion and a second eastbound lane for 200’ in each direction for afternoon congestion. Each of these improvements at El Camino del Norte can be added on an as-needed basis, if and when 2030 forecast traffic demand is realized. The year these improvements may be needed can be estimated by applying compound growth rates for peak hour movements between the Existing and 2030 design year volumes.

KAI Credentials

• KAI is a national leader in the research, planning, and design of roundabouts with over 800 across the US and Canada. KAI helped research and develop the roundabout design guidelines and roundabout capacity models that all Departments of Transportation and engineering firms use for roundabout planning and design in the United States. Kittelson has an unparalleled resume of roundabout research and guidance documents developed at the national, state, and local levels. The following is a representative sample of roundabout research projects that are ongoing or have been completed by KAI the past 15 years.
  • FHWA Roundabouts: An Informational Guide (1^st Edition)
  • FHWA Roundabouts: An Informational Guide - 2nd Edition (NCHRP Report 672)
  • Caltrans Roundabout Geometric Design Guidance, June 2007
  • FHWA Roundabout Brochure – Roundabouts: A Safer Choice
  • NCHRP Project 3-65: Applying Roundabouts in United States (NCHRP Report 572)
  • NCHRP Project 3-78 – Crossing Solutions at Roundabouts and Channelized Turn Lanes for Pedestrians with Visual Disabilities (NCHRP Report 674)
  • NCHRP Project  3-100 – Evaluating Performance of Corridor with Roundabouts
  • FHWA TOPR 34 - Accelerating Roundabout Implementation in the United States
  • 2010 Highway Capacity Manual (Roundabout Chapter)
  • Contributions to the 2009 MUTCD (Roundabout Content)
  • FHWA Mini Roundabout Technical Summary
  • FHWA Roundabout Technical Summary