1. Analyses of Unsignalized Intersections
Lectures 12 to 13
(Chapters 23)
Analyses of Unsignalized Intersections

2. Types of Unsignalized Intersections
Uncontrolled
Stop Sign Controlled
TWSC
AWSC
Yield Sign Controlled
Roundabout

3. TWSC Intersections
Priority movements

4. Gap Acceptance Critical gap, tc Follow-up time, tf Veh1 Veh2 tf tmv
Minimum gap in the major stream that a minor street driver can accept
Cannot be measured directly in the field
Drivers behave differently
Follow-up time, tf
Minimum headway between two consecutive vehicles in the minor stream
Can be measured directly in the field
Veh1
Veh2 tf
tmv

5. tc and tf Movement tcb, sec tfb, sec Two-lane Major Street
Four-lane Major Street
Major LT
Minor RT
Minor TH
Minor LT
4.1
6.2
6.5
7.1
6.9
7.5
2.2
3.3
4.0
3.5

6. Adjustments on tc and tf
Values
tcHV
1.0, Two-lane major street
2.0, Four-lane major street
tcG
0.1, Movements 9 and 12
0.2, Movements 7, 8, 10 and 11
1.0, Otherwise
tcT
1.0, With two stage process
0.0, With single stage process
t3LT
0.7, Minor-street LT at T-intersection
0.0, Otherwise
tfHV
0.9, Two-lane major street
1.0, Four-lane major street

7. Capacity Conflicting volume, vcx Potential capacity, cpx
Figure 23.3 (page 670)
Potential capacity, cpx
Movement capacity, cmx
For minor street LT at T intersection

8. Average Control Delay, sec/veh
Delay and LOS
LOS
Average Control Delay, sec/veh A B C D E F
0-10 (0-10)
>10-15 (10-20)
>15-25 (20-35)
>25-35 (35-55)
>35-50 (55-80)
>50 (>80)
* (xx ) for signalized intersections

9. Exercise
Using HCS to solve the sample problem given. After the model is set up, try to answer the following questions.
Can you duplicate the results given in the example?
How many minor movements are involved in the example?
Which minor movements have the same potential capacity and movement capacity, i.e., which minor movements are not impeded by higher priority minor movements?
What is the basic critical gap and final critical gap for the minor street LT movement?
What is the conflicting flow rate for minor street LT movement? Verify the result manually.
What is the probability of major street LT movement not blocking the minor street LT movement (i.e., probability of queue free for major street LT movement)?
what is the impedance factor for minor street LT movement? How is this factor used for the calculation?
What are the movement capacities for the minor street LT and RT movements?
What is the capacity for the minor street approach (shared LT and RT lane)
What is the control delay for the minor street approach?

10. Complexities * Upstream Signal Effect Two-stage crossing
Flared approach

11. Roundabout Roundabout is a special type of traffic circle
Major differences between roundabout and traffic circle
Speed
Traffic control
Priority of circulating traffic
Design vehicle
Pedestrian access
Parking
Direction of circulation
Types of roundabout
Mini
Urban: compact, single-lane, double-lane
Rural: single-lane, double-lane

12. Roundabout Design Elements
Inscribed circle diameter
Exit width
Entry width
Splitter island width
Circulatory roadway width

13. Roundabout Operations - HCM
Roundabouts introduced in 1997 HCM
Method unchanged in HCM 2000
Uses analytical method
Only estimates capacity; no guidance on delay or level of service
Measured in PCE

14. FHWA Method Consideration of vehicle types (cars, trucks, motorcycles)
Empirical regression model
Delay and LOS vc va

15. FHWA Method vs. HCM
FHWA
HCM

16. Steps to Calculate Roundabout Volumes
Turning movements
Roundabout volumes

17. Steps
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

18. Entry Volume
Entry volume = sum of entering turning movements

19. Exit Volume
Exit volume = sum of turning movements as shown

20. Circulating Volume
Circulating volume = sum of turning movements as shown

21. Example: Volume Conversion
Turning movements
Roundabout volumes
PHF = 0.94
East/west: 2% SU trucks/ buses
530
120 70 ? 50
240
100
110
350
140 80
410 90
North/south: 4% SU trucks/buses, 2% combo trucks

22. 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

23. Example Step 1: Completed PCE Calculation
Raw Counts
PCEs
530
120 70
551
125 73 50
240
100 51
242
101
110
350
140
111
354
141 80
410 90 83
426 94

24. Example Step 2: PHF Factor
PCEs (peak 15 minutes)
PCEs (hourly)
586
133 78
111
354
141 51
242
101 83
426 94
551
125 73 54
258
107
118
377
150 88
454
100
141 / 0.94 = 150

25. Example Step 3: Calculate Entry Volume
Entry volume = sum of entering turning movements
797
586
133 78 ? ? ? 54
258
107 ?
419
118
377
150
645 ? ? ? 88
454
100 ?
642

26. Example Step 4: Calculate Exit Volume
Exit volume = sum of turning movements as shown
797
586
133 78
626 ?
424 54
258
107 ?
419
118
377
150
645 ?
610 ? 88
454
100
843
642

27. Example Step 5: Calculate Circulating Volume
Circulating volume = sum of turning movements as shown
797
586
133 78
626
453
424 54
258
107
826
419
118
377
150
645
660
610
628 88
454
100
843
642

28. Example: Solution Turning movements Roundabout volumes 797 PHF = 0.94
East/west: 2% SU trucks/ buses
530
120 70
626
453
424 50
240
100
826
419
110
350
140
645
660
610
628 80
410 90
843
North/south: 4% SU trucks/buses, 2% combo trucks
642

29. Example: Capacity Calculation
Roundabout volumes
Roundabout category?
797
626
453
424 ?
826
419
645
660
610
628
843
642

30. Example: Capacity Calculation
424
826
Check capacity of each entry
645

31. Example: Capacity check
200
400
600
800
1000
1200
1400
1600
2000
2400
Circulatory Flow (veh/h)
Maximum Entry Flow (veh/h)
Entering and circulating
flow = 1800 veh/h
826
607
762
Urban & Rural Single- Lane Roundabouts
Urban Compact
Roundabouts

32. Example: Capacity Check
Entering volume = 645 pce/h
Capacity of entry:
Single-lane: 762 pce/h
Urban compact: 607 pce/h
Volume-to-capacity ratio:
Single-lane: 645 / 762 = 0.85 AT
Urban compact: 645 / 607 = 1.06 OVER
What to do when v/c >1.0?
Repeat calculation for other approaches
What to do
when v/c >1.0?

33. Solution: Capacity Calculation
Urban Compact
Single-Lane
0.90
0.83
OVER CAPACITY
OK (TODAY)
0.57
0.49
1.06
0.85
0.85
0.74

34. Delay Control delay Geometric delay Total delay = Control + Geometric
Includes initial deceleration delay, queue move-up time, stopped delay, and final acceleration delay
Geometric delay
Delay experienced by a single vehicle with no conflicting flows
Caused by geometric features
Total delay = Control + Geometric
Typical measure used: control delay

35. Control Delay Delay equation where: d = control delay, s/veh
v = entry flow, veh/h
c = capacity, veh/h
T = time period = 0.25 h