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Lectures 12 to 13 (Chapters 23) Analyses of Unsignalized IntersectionsPowerPoint Presentation

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

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Types of Unsignalized Intersections

- Uncontrolled
- Stop Sign Controlled
- TWSC
- AWSC

- Yield Sign Controlled
- Roundabout

TWSC Intersections

- Priority movements

Gap Acceptance

- Critical gap, tc
- 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

tc and tf

Adjustments on tc and tf

Capacity

- Conflicting volume, vcx
- Figure 23.3 (page 670)

- Potential capacity, cpx
- Movement capacity, cmx

For minor street LT at T intersection

Delay and LOS

* (xx ) for signalized intersections

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?

Complexities

- * Upstream Signal Effect
- Two-stage crossing
- Flared approach

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

Roundabout Design Elements

Inscribed circle diameter

Exit width

Entry width

Splitter island width

Circulatory roadway width

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

vc

va

FHWA Method- Consideration of vehicle types (cars, trucks, motorcycles)
- Empirical regression model
- Delay and LOS

http://www.tfhrc.gov/safety/00068.pdf

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

Entry Volume

- Entry volume = sum of entering turning movements

Exit Volume

- Exit volume = sum of turning movements as shown

Circulating Volume

- Circulating volume = sum of turning movements as shown

Example: Volume Conversion

Turning movements

Roundabout volumes

PHF = 0.94

East/west:2% SU trucks/ buses

70

530

120

?

50

240

100

110

350

140

80

410

90

North/south:4% SU trucks/buses, 2% combo trucks

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

Example Step 1: Completed PCE Calculation

Raw Counts

PCEs

70

530

120

73

551

125

50

240

100

51

242

101

110

350

140

111

354

141

80

410

90

83

426

94

551

125

51

242

101

111

354

141

83

426

94

Example Step 2: PHF FactorPCEs (peak 15 minutes)

PCEs (hourly)

78

586

133

54

258

107

118

377

150

88

454

100

141 / 0.94 = 150

Example Step 3: Calculate Entry Volume

- Entry volume = sum of entering turning movements

797

78

586

133

?

?

?

?

54

258

107

419

118

377

150

645

?

?

?

?

88

454

100

642

Example Step 4: Calculate Exit Volume

- Exit volume = sum of turning movements as shown

797

78

586

133

626

?

424

?

54

258

107

419

118

377

150

645

?

610

?

843

88

454

100

642

Example Step 5: Calculate Circulating Volume

- Circulating volume = sum of turning movements as shown

797

78

586

133

626

453

424

826

54

258

107

419

118

377

150

645

660

610

628

843

88

454

100

642

Example: Solution

Turning movements

Roundabout volumes

797

PHF = 0.94

East/west:2% SU trucks/ buses

70

530

120

626

453

424

826

50

240

100

419

110

350

140

645

660

610

628

843

80

410

90

North/south:4% SU trucks/buses, 2% combo trucks

642

Example: Capacity Calculation

Roundabout volumes

Roundabout category?

797

626

453

?

424

826

419

645

660

610

628

843

642

1200

1000

Entering and circulating

flow = 1800 veh/h

762

800

Maximum Entry Flow (veh/h)

600

607

400

Urban & Rural Single-Lane Roundabouts

Urban Compact

Roundabouts

200

0

826

0

400

800

1200

1600

2000

2400

Circulatory Flow (veh/h)

Example: Capacity checkExample: 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?

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

Delay

- Control delay
- 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

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