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Improving left turn safety at signalizedImproving left turn safety at signalized

intersectionsintersections

Calvin J. Mollett

Regional Municipality of York

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Traffic flows

Operational Characteristics Signal Design

Geometry

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Clear Zone

Conflict Zone

SDa

SDr

Left Turn Conflict

Left Turn Conflict

Permissive Left Turn (LT) Vehicle

Stationary LT vehicle in opposing LT lane

Available LT sight distance is insufficient

Either one of Scenarios (a), (b) or (c) below

(a) (b) (c)

OR OR

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EELL

Exposure toL

eft Turn ConflictsExposure toL

eft Turn Conflicts

C = Cycle length (sec)

ru = Probability of there being an opposing LT vehicle

PSD = Probability that LT sight distance is insufficient

PZ = Probability of vehicle/s in conflict zones

EL = 3600 (Lu * ru * PZ * PSD)

C

Lu= Number of vehicles that can perform a permissive LT

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500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900

q'T - Opposing Through Volume (vph)

3

4

5

6

7

8

9

Lu

g = 0 and g' = 0

q'L= 250 vph

LLuu -- Number of permissive left turn vehiclesNumber of permissive left turn vehicles

250 vph

qT

g and g =Length of

leading protected left

turn phases

EL = 3600 (Lu

* ru * PZ * PSD)

C

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500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900

q'T - Opposing Through Volume (vph)

3

4

5

6

7

8

9

Lu

g = 12 sec and g' = 12 sec

g = 0 and g' = 0

q'L= 250 vph

LLuu -- Number of permissive left turn vehiclesNumber of permissive left turn vehicles

250 vph

qT

g and g =Length of

leading protected left

turn phases

55%

EL = 3600 (Lu

* ru * PZ * PSD)

C

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rruu --P

robability of an opposingL

T vehicleP

robability of an opposingL

T vehicle

100

200

300

400

500

600

700

800

900

1

000

1

100

1

200

1

300

1

400

1

500

1

600

1

700

1

800

1

900

2

000

2

100

2

200

2

300

2

400

2

500

2

600

2

700

2

800

2

900

3

000

Approach Through Volume - qT

0.0

0.2

0.4

0.6

0.8

1.0

1.2

ru

-Probabilityofrestrictedsightd

istance

250

150

50 Opposing Left Turn Volume - q'LT

250 vph

800 vph

qT

qLT

EL = 3600 (Lu * ru * PZ * PSD)

C

g and g = 0 sec

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PPZZ --

Probability of vehicle/s in conflict zones

Probability of vehicle/s in conflict zones

100 300 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500 2700q'T

- Opposs ing Through Volume

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

PZ

v = 90 km/h

v = 80 km/h

v = 70 km/h

qT

EL = 3600 (Lu * ru * PZ * PSD)

C

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40 50 60 70 80 90 100

Percentile (%)

70

80

90

100

110

120

130

140

150

160

SD

a-AvailableSightDista

nce(m)

Methodology

by McCoy et. al

3.5 m

PPSDSD --

Probability of insufficient

LT sight distance

Probability of insufficient

LT sight distance

For 80 km/h the required LT Sight Distance = 133 m

56 %

-2 m

3 m

3 m3.75 m

35 mPSD = 1 0.56 = 0.44

EL = 3600 (Lu * ru * PZ * PSD)

C

5.0 m

3.5 m

3.75 m

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200 vph

800 vph

800 vph

50 vph

100 vph

600 vph

1500 vph

50 vph

Lu = 6.7 Lu = 3.3

ru = 0.130 ru = 0.64

PZ = 0.128 PZ = 0.121

EL = 2.49 EL = 3.41

Which scenario has

the highestexposure for NB left

turn conflicts ?

Scenario A Scenario B

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Improvement StrategiesImprovement Strategies

Improve LT

sight distance

Reduce

Speed

Reduce PSD Reduce PZ Reduce Lu Reduce ru

Protected

LT Phasing

Intersection

Geometry

Detector

Loop

Position

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ImproveL

eft Turn Sight DistanceImproveL

eft Turn Sight Distance

4050 60 70 80 90 100

Percentile (%)

60

80

100

120

140

160

180

200

SD

a-AvailableSightD

istance(m)

Offset = -2mWLL= 0

56%

PSD = 0.44

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Offset

WLL

ImproveL

eft Turn Sight DistanceImproveL

eft Turn Sight DistanceNo road widening

Reduce through lane widths

Introduce shadow lane

Offset remains the same

4050 60 70 80 90 100

Percentile (%)

60

80

100

120

140

160

180

200

SD

a-AvailableSightD

istance(m)

Offset = -2mWLL= 0

56%

PSD = 0.44

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Offset

WLL

ImproveL

eft Turn Sight DistanceImproveL

eft Turn Sight DistanceNo road widening

Reduce through lane widths

Introduce shadow lane

Offset remains the same

4050 60 70 80 90 100

Percentile (%)

60

80

100

120

140

160

180

200

SD

a-AvailableSightD

istance(m)

Offset = -2mWLL= 0.3 m

Offset = -2mWLL= 0

56% 66%

PSD = 0.44 PSD = 0.34

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ImproveL

eft Turn Sight DistanceImproveL

eft Turn Sight DistanceWith road widening

Offset

WLL

40 50 60 70 80 90 100

Percentile (%)

0

100

200

300

400

500

SD

a-Availab

leSightDistance(m)

Offset = -2 mWLL = 0

Offset = -1.25 mWLL = 0.75 m

56% 96%

Decrease offset

Introduce shadow lane

PSD = 0.44

PSD = 0.04

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Protected/

Permissive

Left Turn

Phasing

Protected/

Permissive

Left Turn

Phasing

500 700 900 1100 1300 1500 1700 1900 2100 2300 2500 2700 2900

qT - Approach volume (vph)

0

2

4

6

8

10

12

14

16

EL

No Loops

250 vph

800 vph

qT

150 vph

Length of protected left turn phases = 12 sec

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Protected/

Permissive

Left Turn

Phasing

Protected/

Permissive

Left Turn

Phasing

500 700 900 1100 1300 1500 1700 1900 2100 2300 2500 2700 2900

qT - Approach volume (vph)

0

2

4

6

8

10

12

14

16

EL

No Loops

3rd Vehicle position

250 vph

800 vph

qT

150 vph - 63 %

Length of protected left turn phases = 12 sec

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Protected/

Permissive

Left Turn

Phasing

Protected/

Permissive

Left Turn

Phasing

500 700 900 1100 1300 1500 1700 1900 2100 2300 2500 2700 2900

qT - Approach volume (vph)

0

2

4

6

8

10

12

14

16

EL

No Loops

3rd Vehicle position

2nd Vehicle position

1st Vehicle position

250 vph

800 vph

qT

150 vph - 63 %

- 17 %

Length of protected left turn phases = 12 sec

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Conclusions and RecommendationsConclusions and Recommendations Current design standards for signalized intersections

often result in insufficient LT sight distance (PSD > 0) Safety also depends on:

qL - Opposing left turn volume

qT Approach through volume

Increase LT sight distance Decrease LT lane offset

Introduce a shadow lane

IntroduceP

rotected OnlyL

T phase Introduce Protected/Permissive LT phasing

Consider detector loop positionsqL

qT

qT

qL

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Intersection Design GuidelinesIntersection Width = 35 m

Speed (km/h)

3.5 m Lane 3.35 m Lane

WLL

Offset WLL

Offset

40 0 -2 0 -2

50 0 -2 0 -2

60 0.2 -1.8 0.25 -1.75

70 0.4 -1.6 0.45 -1.55

80 0.5 -1.5 0.55 -1.45

90 0.6 -1.4 0.60 -1.40

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Thank You !