ROUNDABOUT CHARACTERISTICS TTE 6815
DEFINITIONS TRAFFIC CIRCLE: u Any intersection designed around a central island ROUNDABOUT: u A traffic circle with certain prescribed characteristics
ROUNADBOUT BENEFITS n Community enhancement n Traffic Calming n Safety n Reduced delay at lower volumes n Improved left turn treatment n Complex intersections
CURRENT ISSUES n Ped/bicycle treatment n High capacity applications n Residential street applications n Visibility requirements n Design details n Cost of implementation
ROUNDABOUT CHARACTERISTICS n Yield on entry
ROUNDABOUT CHARACTERISTICS n Yield on entry
ROUNDABOUT CHARACTERISTICS n Entry deflection for speed control
ROUNDABOUT CHARACTERISTICS n Yield on entry n No internal control n Entry deflection for speed control Deflection Min Radius = 100 m Min Radius = 100 m
ROUNDABOUT CHARACTERISTICS n No parking on circulating roadway
ROUNDABOUT CHARACTERISTICS n No parking on circulating roadway
ROUNDABOUT CHARACTERISTICS n No pedestrians on center island
ROUNDABOUT CHARACTERISTICS n Counter clockwise circulation
ROUNDABOUT CHARACTERISTICS n Counter clockwise circulation
ROUNDABOUT CHARACTERISTICS n Counter clockwise circulation
ROUNDABOUT CHARACTERISTICS n Counter clockwise circulation
ROUNDABOUT CHARACTERISTICS n Must accommodate design vehicle
ROUNDABOUT CHARACTERISTICS n Splitter islands
ROUNDABOUT CHARACTERISTICS n Splitter islands
ROUNDABOUT CHARACTERISTICS n Splitter islands
ROUNDABOUT CHARACTERISTICS n Pedestrians cross one car length back from the entry point
ROUNDABOUT CHARACTERISTICS n Pedestrians cross one car length back from the entry point
ROUNDABOUT CHARACTERISTICS n Pedestrians cross one car length back from the entry point
ROUNDABOUT CHARACTERISTICS n Deflection by physical features
ROUNDABOUT CHARACTERISTICS n Deflection by physical features
ROUNDABOUT CHARACTERISTICS n Deflection by physical features
OTHER REQUIREMENTS n Easily identified in the road system. n Layout must be clearly visible and must encourage drivers to enter slowly. n Adequate lighting is necessary.
A SIMPLE YIELD CONTROLLED JUNCTION n Because a roundabout is a series of yield-controlled junctions, this provides a good place to start.
n Press F7 to see these graphics of the intersection layout.
TRAFFIC VOLUMES n 60 % EB n 40% NB n Start with … u 300 EB u 200 NB
FIRST SIDRA RUN n Set up the basic parameters and intersection configuration n Enter traffic volumes n Enter flow scale limit at 250% n Run SIDRA n Observe performance measures
VOLUME GROWTH FACTOR n 100% TO 250 % n Capacity reached at 242% u 726 vph EB u 484 vph NB u NB Delay = 42 sec/veh
VOLUME GROWTH FACTOR v/c Ratio
VOLUME GROWTH FACTOR v/c Ratio Delay
VOLUME GROWTH FACTOR Capacity
VOLUME GROWTH FACTOR Capacity Spare Capacity
n CAPACITY: 484 n V/C RATIO :.40 n DELAY: 41.7 s/v n STOP RATE: 2.7 n 95% BACK OF QUEUE: 26.1 veh
TRAFFIC VOLUMES n 60 % EB n 40% NB n Now Try… u 675 EB u 450 NB n About 85% v/c
SECOND SIDRA RUN n Enter new volumes n Disable flow scale n Run SIDRA n Observe performance measures
n 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 NBR *
NEXT... n Let’s make up a roundabout composed of four of these junctions n Entering Volume 450 vph n Circulating Volume 675 vph
HOW?... n Each LT is included in the circulating volume at 2 junctions n Each Thru movt is included at 1 junction n RT’s do not appear anywhere
SO... n 450 vph on each approach n 60% Left (270 vph) n 30% Thru (135 vph) n 10% Right (45 vph)
CIRCULATING VOLUME 270 x 2 = x 1 =135 === Total675
THIRD SIDRA RUN n Set up the roundabout configuration n Enter new volumes n Run SIDRA n Compare the performance with a single YIELD junction
COMPARISON n YIELD V/C =.85 Del = 19.2 s/v n ROUNDABOUT V/C =.64 Del = 9.3 s/v
ROUNDABOUT GAP ACCEPTANCE PARAMETERS Turn Critical Follow Gap Up (s) Headway Left Thru Right
STOP & YIELD CONTROL GAP ACCEPTANCE PARAMETERS Crit. Fol.up Min. % Exit Flow Gap Hdway Deps Opposing Left turns : Through : Right turns: Opposed turns from priority road:
FOURTH SIDRA RUN n Return to YIELD example n Change gap acceptance characteristics n Run SIDRA n Observe performance measures
YIELD control with roundabout gap acceptance parameters n V/C Ratio =.646 n Delay = 7.6 s/v
FIFTH SIDRA RUN n Change island diameter to 4 meters and rerun SIDRA n Change island diameter to 100 meters and rerun SIDRA n Compare performance measures
EFFECT OF CENTRAL ISLAND DIAMETER 4 METERS n C. G. = 4.72 n F.O.H = 2.65 n V/C =.696 n Del = METERS 4.14 sec 2.32 sec sec/veh