Ourston Roundabout Engineering | Analysis, Planning & Reviews

By: Roundabouts  09-12-2011
Keywords: Traffic Control

Analysis

The exceptionally strong, analytical background of ORE, years of detailed roundabout knowledge, and effective communication are the ingredients we provide when asked to analyze roundabouts.  Ourston can develop a collaborative and comprehensive approach to conduct operational field evaluations, develop data sets, perform statistical analysis and reporting.

In analysis of roundabouts we often compare safety performance of before and after conditions.  Arcady 7 software is the world’s only analysis tool that can accurately predict capacities and crash frequency for normal and mini roundabouts. ORE is affiliated with Transport Research Laboratory, the researchers and programmers of Arcady 7. TRL has a partnership agreement to assist ORE on research assignments.

Planning and Site Selection

ORE Inc. pioneered Intersection Control Studies (ICS) involving roundabouts.  Our study method, now widely used, compares the recommended modifications with a roundabout to determine the preferred means of traffic control at the intersection.  We can provide these studies to include:

  • Developing a roundabout concept to facilitate the comparison.  Concepts would be fully functional in terms of geometry (i.e. achievement of entry deflection, accommodation of design vehicle) such that there would be very little change in its overall footprint should it proceed to detailed design.
  • Determining current and future, 20 years hence, traffic operations.  The existing conditions would be modelled using Synchro or HCS, and the roundabout concept will be modelled using ARCADY 7, RODEL or VISSIM.
  • Determining current and future injury collision performance.  Future injury collisions under Stop control can be predicted using trend analysis or Safety Performance Functions if available.  Current and future injury collision performance for a roundabout can be predicted using an aggregate of several methods including NCHRP 3-65 research. 
  • Comparing capital costs.  This requires knowledge of the local construction industry for the unit prices.
  • Comparing life cycle costs.  This would include capital costs plus any operating and maintenance costs.  This would also include the present value of reduced injury collision frequency with a roundabout over a 20-year life cycle.
  • Discussion of qualitative criteria such as vehicle fuel consumption and emissions, speed control, truck accommodation, road and utility maintenance, construction and staging, compatibility with adjacent land uses and site accesses, public acceptance, and aesthetics.
  • Recommending whether or not to proceed with a roundabout at the subject intersection based on comparison of capacity, cost and safety analysis results along with qualitative assessments.

The above review tasks are normally used to generate the following deliverables:

  • Intersection Level of Service and analysis letter report
  • General intersection geometric modifications in CAD format
  • Brief discussion of the pros, cons, and any potential pitfalls and a comparison table for stakeholders
  • A presentation for the staff regarding our analysis, findings and recommendations

Out of these studies we have developed a streamlined initial screening checklist that provides a relatively quick assessment of the feasibility of a roundabout at a particular location.  The checklist can provide enough information to assist in deciding whether or not to proceed to a more detailed study to investigate the feasibility of a roundabout.

Field Audits and In-service Reviews

What we have learned from practicing and studying empirically based roundabout design is that roundabout operating problems stem from the following: 

  • A poor understanding of the way site context influenced design
  • Incorrect lane configurations resulting in unbalanced lane use and incorrect geometry
  • Designers don’t always recognize the operating effects of their geometric design choices
  • Overall composition of markings, geometry and signs is often overlooked in the pursuit of details

Fortunately in many cases, the remediation of roundabout operating problems caused by poor design composition can be inexpensive but requires renewed effort to integrate the geometry with the project context.  With regard to in-service reviews, Ourston Roundabout Engineering under the direction of Mark Lenters has undertaken more roundabout rehabilitation projects than any other roundabout firm operating in the U.S.  Mark's experience record shows that highly complex sites that were both congested and crash-prone can be remedied:

  • Brigham Road Interchange, St. George, Utah (25,000 ADT with six legs)
  • Edmonton, Canada (Four roundabout intersections with entering ADT ranging from 25,000 to 60,000)

Our history in design auditing and inservice reviews for conventional intersections has been coupled with our knowledge of how roundabouts should operate.  We are able to relate what you see in the field to principles of roundabout operation that are evidence based and as a result we can competently identify countermeasures and holistic improvements to geometry, markings and signs.

Keywords: Traffic Control

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