Ourston Roundabout Engineering | Direct Design

By: Roundabouts  09-12-2011
Keywords: Planning And Design

Ourston prides itself in having the best geometricians in the business.  Why is geometric design so important? Effective geometry can:

  • Reduce vehicle delay
  • Increase capacity of the roundabout
  • Promote traffic ergonomics (positive guidance)
  • Promote pedestrian safety
  • Reduce intersection speeds

Roundabout design is an iterative process that involves tradeoffs between safety and capacity.  Achieving a balanced design where capacity, safety and minimal cost are optimized is what makes roundabout design complex.  Modern roundabout design practice requires a heuristic principle-based approach that recognizes the holistic nature of the way roundabouts should be designed to achieve safe and efficient operations.  Understanding the trade-offs of setting priorities among competing objectives of cost, safety and capacity in design is necessary to achieve an optimal balanced design.

Guides don't specify how to address trade-offs.  Ourston has the expereince to balance designs to address site context and constraints. Based on our proven expertise, the essential demands of a good design for any roundabout that has multi-lane entries are:

  • An effective lane configuration that rationalizes the number of lanes approaching and exiting with the traffic flow proportions can result in the development of spiraling circulating lanes to avoid exit path conflicts.
  • Locating and sizing the circle to fit approach alignments while introducing adequate entry speed reduction and natural path development for multilane entries.
  • Composition of the geometry, markings and signs to elicit an error free response from users.  This aspect of design is established by our heuristic knowledge more than by just following guides.  We know this first-hand because we wrote the first roundabout design manual for the U.S. in 1995 and released again in 2001.

Detailed Design Assignment Scope

We can provide components of the full contract design and documentation, or we can undertake the full production of plans and specifications. Our background in intersection planning and design and highway design projects for over 25 years underpins our availability to perform these services with a high value-added methodology.

Operational Analysis

  • Develop a lane configuration and volume diagram, evaluating the need for exclusive turn lanes and spirals.
  • Operational performance analysis of a roundabout based on delay and queuing criteria using Arcady 7 for the AM/PM/OP peak hours.
  • Review alternative roundabout geometric solutions meeting pre-established operational performance criteria.
  • Documentation of roundabout operational analysis.

Design and Plan Preparation

  • Terrain model from survey data
  • Determine applicable design criteria
  • Drainage design
  • Prepare plan sheets
  • Typical sections 
  • Construction details
  • Plan details
  • Jointing plans/paving grades
  • Erosion control
  • Storm sewer
  • Landscaping plan
  • Permanent signing
  • Lighting plan
  • Pavement marking
  • Traffic control and construction staging
  • Alignment detail
  • Miscellaneous quantities
  • Plan & profile
  • Earthwork summary
  • Cross sections
  • Detour route plan
  • Overhead sign supports
  • Right of way plat
  • Lighting plans

Keywords: Planning And Design

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