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DEFINITIONS
TRAFFIC CIRCLE: Any intersection designed
around a central island
ROUNDABOUT: A traffic circle with certain
prescribed characteristics
ROUNADBOUT BENEFITS
Community enhancement Traffic Calming Safety Reduced delay at lower volumes Improved left turn treatment Complex intersections
CURRENT ISSUES
Ped/bicycle treatment High capacity applications Residential street applications Visibility requirements Design details Cost of implementation
ROUNDABOUT CHARACTERISTICSYield on entryNo internal controlEntry deflection for speed
control
DeflectionDeflection
Min Radius = 100 mMin Radius = 100 m
OTHER REQUIREMENTS
Easily identified in the road system.
Layout must be clearly visible and must encourage drivers to enter slowly.
Adequate lighting is necessary.
A SIMPLE YIELD CONTROLLED JUNCTION
Because a roundabout is a series of yield-controlled junctions, this provides a good place to start.
FIRST SIDRA RUN
Set up the basic parameters and intersection configuration
Enter traffic volumes Enter flow scale limit at 250% Run SIDRA Observe performance measures
VOLUME GROWTH FACTOR 100% TO 250 % Capacity reached at 242%
726 vph EB 484 vph NB NB Delay = 42 sec/veh
Tabl e S. 10 - MOVEMENT CAPACI TY AND PERFORMANCE SUMMARY - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Mov Mov Arv Total Lane Deg. Aver. Eff . 95% Perf . No. Typ Fl ow Cap. Uti l Satn Del ay Stop Back of I ndex (veh (veh Rate Queue / h) / h) (%) x (sec) (veh) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - West: West Approach 12 T 726 1800 100 0. 403 0. 0 0. 00 0. 0 15. 07 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - South: South Approach 33 R 484 484 100 1. 000* 41. 7 2. 69 26. 1 32. 68 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
CAPACITY: 484 V/C RATIO : .40 DELAY: 41.7 s/v
STOP RATE: 2.7 95% BACK OF
QUEUE: 26.1 veh
NB Delay: 19.2 sec/veh
MOVEMENT CAPACITY AND PERFORMANCE SUMMARY --------------------------------------------- Mov’t Arv Total Deg. Aver. Eff. 95% Flow Cap. Satn Delay Stop B.O.Q---------------------------------------------- EBT 675 1800 0.375 0.0 0.00 0.0 --------------------------------------------- NBR 450 530 0.849* 19.2 1.97 15.5
NEXT ...
Let’s make up a roundabout composed of four of these junctions
Entering Volume 450 vph Circulating Volume 675 vph
HOW? ...
Each LT is included in the circulating volume at 2 junctions
Each Thru movt is included at 1 junction
RT’s do not appear anywhere
THIRD SIDRA RUN
Set up the roundabout configuration
Enter new volumes Run SIDRA Compare the performance with a
single YIELD junction
ROUNDABOUT GAP ACCEPTANCE PARAMETERS ------------------------- Turn Critical Follow Gap Up (s) Headway ------------------------- Left 4.43 2.49 Thru 4.43 2.49 Right 4.43 2.49 -------------------------
STOP & YIELD CONTROLGAP ACCEPTANCE PARAMETERS
Crit. Fol.up Min. % Exit Flow Gap Hdway Deps OpposingLeft turns : 7.0 3.40 1.0 50Through : 6.5 3.30 1.0 50Right turns: 5.5 2.60 1.0 50Opposed turns from priority road: 5.5 2.60 1.0 0
FOURTH SIDRA RUN
Return to YIELD example Change gap acceptance
characteristics Run SIDRA Observe performance measures
FIFTH SIDRA RUN
Change island diameter to 4 meters and rerun SIDRA
Change island diameter to 100 meters and rerun SIDRA
Compare performance measures
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