ROUNDABOUT
CHARACTERISTICS
TTE 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
CHARACTERISTICS
Yield
on entry
ROUNDABOUT
CHARACTERISTICS
Yield
on entry
Yield
on entry
ROUNDABOUT
CHARACTERISTICS
Entry
deflection for speed
control
ROUNDABOUT
Deflection
CHARACTERISTICS
Yield
on entry
No internal control
Min
Radius = 100
Entry
deflection
formspeed
control
ROUNDABOUT
CHARACTERISTICS
No
parking on circulating
roadway
ROUNDABOUT
CHARACTERISTICS
No
parking on circulating
roadway
ROUNDABOUT
CHARACTERISTICS
No
pedestrians on center
island
ROUNDABOUT
CHARACTERISTICS
Counter
clockwise
circulation
ROUNDABOUT
CHARACTERISTICS
Counter
clockwise
circulation
ROUNDABOUT
CHARACTERISTICS
Counter
clockwise
circulation
ROUNDABOUT
CHARACTERISTICS
Counter
clockwise
circulation
ROUNDABOUT
CHARACTERISTICS
Must
accommodate design
vehicle
ROUNDABOUT
CHARACTERISTICS
Splitter islands
ROUNDABOUT
CHARACTERISTICS
Splitter islands
ROUNDABOUT
CHARACTERISTICS
Splitter islands
ROUNDABOUT
CHARACTERISTICS
Pedestrians
cross one car
length back from the
entry point
ROUNDABOUT
CHARACTERISTICS
Pedestrians
cross one car
length back from the
entry point
ROUNDABOUT
CHARACTERISTICS
Pedestrians
cross one car
length back from the
entry point
ROUNDABOUT
CHARACTERISTICS
Deflection
features
by physical
ROUNDABOUT
CHARACTERISTICS
Deflection
features
by physical
ROUNDABOUT
CHARACTERISTICS
Deflection
features
by physical
OTHER REQUIREMENTS
Easily
identified in the road
system.
Layout must be clearly
visible and must encourage
drivers to enter slowly.
Adequate lighting is
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
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
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
 NB
Delay:
19.2 sec/veh
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 approach
60% 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
 ROUNDABOUT
V/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 CONTROL
GAP ACCEPTANCE PARAMETERS
Crit. Fol.up Min. % Exit Flow
Gap Hdway Deps Opposing
Left turns : 7.0 3.40
1.0
50
Through : 6.5 3.30
1.0
50
Right turns: 5.5 2.60
1.0
50
Opposed 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
 V/C
 Del
=
=
= 2.65
.696
11.6
100 METERS
4.14
sec
2.32
sec
.575
7.4
sec/veh