This document summarizes key points from a traffic engineering course lecture on roundabouts. It discusses conflict points, geometric characteristics of roundabouts like entry width and exit width. It covers calculating volumes using passenger car equivalents and peak hour factors. An example is provided on calculating entry, exit, and circulating volumes. Finally, it discusses methods for determining roundabout capacity like the HCM 2000 and Kimber methods, and using software like Synchro to analyze roundabouts.

1. TRAFFIC ENGINEERING COURSE
(PWE 8322)
ROUNDABOUTS
Instructor: Usama Elrawy Shahdah, PhDLecture # 05

2. Conflict Points
2

3. Conflict points
3

4. Geometric Characteristics
Entry width
Inscribed
circle
diameter
Exit width
Splitter island width
Circulatory roadway width

5. Geometric Characteristics
5

6. 6

7. 7

8. Number of Lanes
8
vc
va

9. Roundabout Volumes
Turning
movements
Roundabout
volumes

10. Steps
Vehicle Type
Passenger Car
Equivalent (pce)
Car 1.0
Single-unit truck or bus 1.5
Truck with trailer 2.0
Bicycle or motorcycle 0.5
 Step 1: Convert trucks and other vehicle types to
passenger car equivalents (pce)
 Step 2: PHF volume adjustment
 Step 3: Entry volume
 Step 4: Exit volume
 Step 5: Circulating volume

11. Entry Volume
 Entry volume = sum of entering turning movements

12. Exit Volume
 Exit volume = sum of turning movements as shown

13. Circulating Volume
 Circulating volume = sum of turning movements as
shown

14. Example: Volume Conversion
Turning
movements
Roundabout
volumes
110
350
140
50
240
100
North/south:
4% SU trucks/buses, 2%
combo trucks
East/west:
2% SU trucks/ buses
?
PHF = 0.94

15. Example Step 1: PCE Calculation
 SB TH: 530 veh (4% SU/bus, 2% combo)
% cars (0.94)  1.0 pce/veh = 0.94
% SU/bus (0.04)  1.5 pce/veh = 0.06
% combo (0.02)  2.0 pce/veh = 0.04
fhv = 1.04
 530 veh
551 pce
Vehicle Type
Passenger Car
Equivalent (pce)
Car 1.0
Single-unit truck or bus 1.5
Truck with trailer 2.0
Bicycle or motorcycle 0.5

16. Example Step 1: Completed PCE
Calculation
Raw Counts PCEs
110
350
140
50
240
100 111
354
141
51
242
101

17. Example Step 2: PHF Factor
PCEs (hourly)
PCEs (peak 15
minutes)
118
377
150
54
258
107111
354
141
51
242
101
141 / 0.94 = 150

18. Example Step 3: Calculate Entry
Volume
 Entry volume = sum of entering turning movements
118
377
150
54
258
107 645
?
?
419
?
642
797

19. Example Step 4: Calculate Exit Volume
 Exit volume = sum of turning movements as shown
645
?
?
419
?
642
797
118
377
150
54
258
107

20. Example Step 5: Calculate Circulating
Volume
 Circulating volume = sum of turning movements as
shown
645
628
826
419
453
642
797
118
377
150
54
258
107

21. Example: Solution
Turning
movements
Roundabout
volumes
110
350
140
50
240
100
North/south:
4% SU trucks/buses, 2%
combo trucks
East/west:
2% SU trucks/ buses
645
628
826
419
453
642
797
PHF = 0.94

22. 22
Capacity Determination

23. HCM 2000 Method
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444...111 222...666
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444...666 333...111
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cc
tv
tv
ca
e
e
vc
vc
va

24. HCM method
24

25. CONTROL DELAY & LOS
25

26. Kimber Method
26
 Qe (capacity of entry flow) is linearly dependent
upon circulating flow crossing the entry (Qc).

27. Kimber Method
27
 Factors affecting the capacity of a roundabout:
 Inscribed circle diameter D (m)
 width of flare of each entry. e (m),
 Approach width v (m)
 effective length over which flare is developed l' (m)
 entry angle (φ)
 entry radius (r) (m)

28. Kimber Method
28

29. Kimber Method
29
 Range of geometric variables in data set used by
Kimber to develop capacity expression

30. Kimber Method (simplified method)
30
 Single-lane roundabouts
 This constraint implies that the capacity of the
circulating roadway of the roundabout is 1800 pcph.

31. Kimber Method (simplified method)
31
 Two lane roundabouts

32. Example
32
 A single-lane roundabout having the following
geometric characteristics:
 e = 4 m
 v = 4 m
 l’= 40 m
 r = 20 m
 Φ = 30 degrees
 D = 40 m
 circulating volume = 285 pcph
 the entry leg demand = 315 pcph.
 Compute the average delay
 Use an analysis period of 15 minutes.

33. Solution
33
 Use Simplified Kimber method
 Delay

34. Trafficware Synchro
34
 Demonstrated in Class

35. Home Reading
35
 Synchro Studio 9 User Guide
 NCHRPREPORT 672 “Design Roundabouts: An
Informational Guide”, Second Edition

36. 36
Thanks for your time