ROUNDABOUT CHARACTERISTICS

TTE 6815TTE 6815

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 entry

ROUNDABOUT CHARACTERISTICSYield on entry

Yield on entry

ROUNDABOUT CHARACTERISTICSEntry deflection for speed

control

ROUNDABOUT CHARACTERISTICSYield on entryNo internal controlEntry deflection for speed

control

DeflectionDeflection

Min Radius = 100 mMin Radius = 100 m

ROUNDABOUT CHARACTERISTICSNo parking on circulating

roadway

ROUNDABOUT CHARACTERISTICSNo parking on circulating

roadway

ROUNDABOUT CHARACTERISTICSNo pedestrians on center

island

ROUNDABOUT CHARACTERISTICSCounter clockwise

circulation

ROUNDABOUT CHARACTERISTICSCounter clockwise

circulation

ROUNDABOUT CHARACTERISTICSCounter clockwise

circulation

ROUNDABOUT CHARACTERISTICSCounter clockwise

circulation

ROUNDABOUT CHARACTERISTICSMust accommodate design

vehicle

ROUNDABOUT CHARACTERISTICSSplitter islands

ROUNDABOUT CHARACTERISTICSSplitter islands

ROUNDABOUT CHARACTERISTICSSplitter islands

ROUNDABOUT CHARACTERISTICSPedestrians cross one car

length back from the entry point

ROUNDABOUT CHARACTERISTICSPedestrians cross one car

length back from the entry point

ROUNDABOUT CHARACTERISTICSPedestrians cross one car

length back from the entry point

ROUNDABOUT CHARACTERISTICSDeflection by physical

features

ROUNDABOUT CHARACTERISTICSDeflection by physical

features

ROUNDABOUT CHARACTERISTICSDeflection by physical

features

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.

Press F7 to see these graphics of the intersection layout.

TRAFFIC VOLUMES

60 % EB 40% NB

Start with … 300 EB 200 NB

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

VOLUME GROWTH FACTOR

v/c Ratio

VOLUME GROWTH FACTOR

v/c Ratio

Delay

VOLUME GROWTH FACTOR

Capacity

VOLUME GROWTH FACTOR

Capacity

Spare Capacity

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

TRAFFIC VOLUMES

60 % EB 40% NB

Now Try… 675 EB 450 NB

About 85% v/c

SECOND SIDRA RUN

Enter new volumes Disable flow scale Run SIDRA Observe performance measures

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

SO ...

450 vph on each approach60% Left (270 vph)30% Thru (135 vph)10% Right (45 vph)

CIRCULATING VOLUME

270 x 2 = 540

135 x 1 = 135

===

Total 675

THIRD SIDRA RUN

Set up the roundabout configuration

Enter new volumes Run SIDRA Compare the performance with a

single YIELD junction

COMPARISON

YIELD

V/C = .85

Del = 19.2 s/v

ROUNDABOUTV/C = .64

Del = 9.3 s/v

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

YIELD control with roundabout gap acceptance parameters

V/C Ratio = .646 Delay = 7.6 s/v

FIFTH SIDRA RUN

Change island diameter to 4 meters and rerun SIDRA

Change island diameter to 100 meters and rerun SIDRA

Compare performance measures

EFFECT OF CENTRAL ISLAND DIAMETER

4 METERS C. G. = 4.72 F.O.H = 2.65 V/C = .696 Del = 11.6

100 METERS

4.14 sec

2.32 sec

.575

7.4 sec/veh