des Carrefours Giratoires Roundabout Design Principles NCLUG Roundabout Luncheon Tuesday, March 4, 2014 Prepared by:
des Carrefours Agenda Giratoires Roundabout design process: Planning and Policy Lane Configurations Roundabout Staging Design Checks Tool Box Pedestrians and VRU s
des Carrefours Giratoires Planning and Policy
des Carrefours Planning Giratoires and Policy Where to Use Roundabouts? 1. Safest at-grade intersection 2. High capacity 3. Appropriate for low volume intersections 4. Easily modified 5. Flexible geometry 6. Accommodate trucks 7. Good for sites with poor visibility 8. Simple to navigate 9. Safe and simple for pedestrians 10. Low maintenance 11. Aesthetically pleasing
des Carrefours Planning Giratoires and Policy Roundabout design philosophy Design is context sensitive and must be driven by principles We must resist the temptation to create templates Rarely are retrofits addressed Standard drawings and rules do not address unusual conditions
des Carrefours Planning Giratoires and Policy Core Principles Circle Size: Must accommodate the design vehicle Speed reduction Circle location and approach alignments Sight distances Vulnerable Road Users Positive Guidance Signage, markings and curbs
des Carrefours Planning Giratoires and Policy They re not perfect - disadvantages Space is usually required at the intersection Typically more expensive up front costs Driver s line of sign to yield line is vital Challenges associated with VRUs, especially at multi-lane roundabouts Disrupts traffic platoons Retrofits can be complicated for construction staging Designs can be complex
des Carrefours Staging Giratoires Considerations Consider building single lane initially: Operationally simpler Lower cost initially (but increases ultimate cost) Better safety performance If initial volumes do not justify multi-lane, drivers may ignore markings
des Carrefours Staging Giratoires Considerations 2 options for staging Build outward (final central island and splitters) Smaller ICD - Better safety performance Simpler design Lower initial cost Must disturb outside properties more than once May have to relocate drainage, pedestrian and cycling features Build inward (final inscribed circle) Larger ICD more capacity Higher initial cost Eliminates future impacts on adjoining property
des Carrefours Giratoires Initial Design Steps
des Carrefours Initial Giratoires Design Steps 1. Traffic volume diagram 2. Test the entry/exit lane capacity assumptions 3. Draw a conceptual lane configuration diagram 4. Computer analysis 5. Adjust design 6. Develop CAD scheme 7. Develop design checks 1. Deflection (Fastest Path) 2. Natural paths 3. Design vehicle turning 4. Pedestrian facilities 8. Rerun computer analysis
des Carrefours Next Giratoires Design Steps 1. Confirm traffic volume diagram 2. Re-test the entry/exit lane capacity assumptions 3. Confirm lane configuration diagram 4. Re-run Computer analysis 5. Adjust design 6. Further develop CAD scheme 7. Modify design checks 1. Deflection (Fastest Path) 2. Natural paths 3. Design vehicle turning 4. Pedestrian facilities 8. Finalize computer analysis
des Carrefours Design Giratoires Checks 1. Composition 1. Lane configuration 2. Truck turning 3. Stopping sight distance 4. Entry and exit paths (overlap for multi-lane) 2. Details 1. Lane widths and transitions 2. Grading 3. Sight triangles 4. Lighting, markings, signage and pedestrian facilities
des Carrefours Giratoires Roundabout Toolbox
des Carrefours Toolbox Giratoires It s important to build to toolbox! Good roundabout design can achieve the following: Reduced delay Maximum intersection capacity Optimal driver safety Enhanced pedestrian safety Slower speeds REMEMBER.roundabout design is an iterative process which must balance all of these factors
des Carrefours Toolbox Giratoires Geometric Parameters V half width E entry width L flare length R entry radius Φ entry angle D inscribed circle diameter
des Carrefours Toolbox Giratoires Parameter Single Lane Dual Lane Entry Width 18-22 ft 24-28 ft Flare Length 15-300 ft (for capacity) Half Width Lane width Entry Radius 55-90 ft 65-90 ft Entry Angle 16-30⁰ 16-30⁰ Inscribed Circle Diameter 13-150 ft 150-180 ft Circulatory Road Width 1-1.2 times the width of widest entry Exit Road Width 200-1000 ft
des Carrefours Toolbox Giratoires Entry Path Curvature The entry path radius must be checked for all turning movements. It must not exceed 230 feet at Compact Roundabouts in urban areas (where the speed limit and the design speed within 330 feet of the yield line on any approach do not exceed 40mph). At all other roundabout types, the entry path radius must not exceed 100m. (TD 16/07)
des Carrefours Toolbox Giratoires Entry Angle Larger Roundabout For a large roundabout where the arms are well separated, the angle measured is in effect that between the projected path of an entering vehicle and the path of a circulating vehicle. To determine the entry angle: construct the curve EF as the center of the midpoint between the nearside curb and the median line (or the edge of any splitter island or central reserve); construct BC as the tangent to EF at the yield bar; construct the curve AD as the center of the midpoint of (the used section of) the circulatory; the entry angle, ϕ, is the acute angle between BC and the tangent to AD)
des Carrefours Toolbox Giratoires Entry Angle Compact Roundabout The angle between the projected entry and exit paths is measured and then halved to find ϕ: construct line BC; construct the curve JK in the next exit as the middle of points midway between the nearside curb and the median line; construct the line GH as the equivalent of line BC i.e. the tangent to the curve JK at the point where JK intersects the border of the inscribed circle; the lines BC and GH intersect at L. The entry angle, ϕ, is half of angle HLB. ϕ = [angle HLB]/2 Note: if angle GLB exceeds 180 degrees, ϕ is defined as zero.
des Carrefours Toolbox Giratoires Path Overlap http://www.youtube.com/watch?v=4zv1naa8bms 40-50 feet Where do I go??
des Carrefours Toolbox Giratoires Right turn bypasses Can improve capacity, but can reduce pedestrian/cyclist safety Can be used to avoid additional entry lanes Can assist in accommodate right turning for larger vehicles
des Carrefours Toolbox Giratoires Sight Distances
des Carrefours Toolbox Giratoires Signage and Markings Typical signage sequence: Roundabout ahead sign Map-type roundabout sign Yield ahead sign Lane use control sign (multi-lane) Keep right sign Pedestrian signs Yield signs One-way/chevron sign Exit signs
des Carrefours Toolbox Giratoires Markings Debate around: Left turn arrows Fishhook arrows vs standard Arrows/lane lines in the circulatory
des Carrefours Toolbox Giratoires Vulnerable Road Users at Roundabouts Basis for opposition to roundabouts Consultation with ADA, CNIB, other groups Research Hot topic in roundabout design Canadian roundabout design guide still in planning stages
des Carrefours Toolbox Giratoires Vulnerable Road Users at Roundabouts Visually and mobility impaired Pedestrians Cyclists Equestrians Motorcyclists Children Elderly
des Carrefours Toolbox Giratoires Vulnerable Road Users at Roundabouts The most vulnerable road users at roundabouts in the UK are cyclists 10% of all accidents involving cyclists occur at roundabouts 11% of these tend to be serious or fatal accidents 14 times more likely to have an accident than a car
des Carrefours Toolbox Giratoires Visually Impaired Pedestrians Well defined walkway edges Tactile markings and truncated domes Colour contrast Pedestrian lighting Mobility Impaired Pedestrians Gentle slopes Ensure landscaping does not encroach Eliminate ledges
des Carrefours Giratoires Thank You