8-1 low cost safety improvements the tools – traffic signals – session #8

8-1

LOW COST SAFETY IMPROVEMENTS

The Tools –

Traffic Signals – Session #8

8-2

Traffic Signals

Learning Outcomes:

1. Identify countermeasures for operation and design deficiencies of traffic signals

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Traffic Signals

• New Tools:

NCHRP 440 – Accident Mitigation Guide for Congested Rural Two-Lane Highways

NCHRP 500 – Volume 5: A Guide for Addressing Unsignalized Intersection Collisions

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• New & Proven Technologies

Traffic Signals

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Traffic Signals

How can Traffic Signals Reduce crashes?

Which Signal Configurations, Equipment, and Operations Reduce Crashes?

What is the Safety Effect of each?

Discussion

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Safety Benefits of Traffic Signals

Install new Traffic Signal

Upgrade Traffic Signal

(Fatalities)

(Injuries)

CRF=7%

38%22%

Install new Traffic Signal – All Crashes (CTRE 00-61, 2001) CRF=27%

Proven

Tried

3 Approaches

4 ApproachesTried

Increase Crashes 2%

8-7

Safety Benefits of Traffic Signals

Change type of Traffic Control (3 and 4 approach Unsignalized to Signal Control)

Tried

ALL Right-angle Rear-end Intersection type AMF AMF AMF

Three-leg 0.93 0.80 1.75

Four-leg 1.02 0.40 1.74

CRF = 60%

Table 13.3: AMFs for Urban-Intersection Signalization (Injury-related crashes ONLY) (Persaud et al., 2002)

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Is this traffic signal as safe as it could be?

Discussion

What are some Traffic Signal Features/Equipment that affect Safety?

Let’s list them

8-9

Traffic Signals Countermeasures

1. Update yellow Clearance timing

2. Add All-Red Clearance phase

3. Improve visibility (12” sections, suppl. heads, etc)

4. Add Back Plates

5. Change Permissive Lefts to Protected Only

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Traffic Signals Countermeasures

6. Add Advance Warning signs with active flashers

7. Add Supplemental Signal Heads

8. Use Overhead Red “T” Heads

9. Change Late Night Yellow/Red Flash to Full Time Signal

10. Coordination of Signals

11. Controller/Actuation Upgrades

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Frequency of Red Light Running:

*TTI, Bonneson, 2003

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Engineering Countermeasures to Red-Light Running:


Tried

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Update Clearance Intervals

All-Red TimeYellow Time

ITE Traffic Engineering Handbook

*NCHRP 500, Objective 17.2 A2 – Optimize Clearance Intervals

Proven

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Red TimeYellow Time

ITE Traffic Engineering Handbook

CP = t + V/2a + V/20*64.4*g + (W+L)/V

For 85th percentile approach speed of 45 mph, curb radius (Stop bar to curb line of intersecting street of 50’, And Intersection width of 36 feet

= t + V/2a + V/20*64.4*g = 1.0 + 45*88/60/2*10 + 45*88/60/20*64.4*0% = 1.0 +66/20 = 1.0 + 3.3 = 4.3 seconds

Yellow Time

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-reduces RLR frequency by 50-70%- reduces RLR crashes by 25%

-reduces RLR frequency by 50-70%CRF = 4 to 31% Total CrashesCRF = 1 to 30% Right Angle Crashes

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* From ITE Traffic Signal Handbook

Proven

*NCHRP 500, Objective 17.2 A2 – Optimize Clearance Intervals

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Add All-Red Clearance Interval

ITE Traffic Engineering Handbook – All-Red Clearance Interval

T = ( W + L) / V

Example: 85th Speed=45mph, W=36, curb radius=50’, L=20’Red Time = (W+L)/V = (50 + 36 + 20)/45*88/60 = 106/66 = 1.61seconds

CRF=25% for Add 1.0 Sec All-Red *Bhesania, 1991

Tried

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Improve Visibility (12” Indications, Suppl heads)

Mast Arm Signals* Iowa

8” to 12” Indications, *Winston-Salem, NC

* Bonneson

CRF = 32% Related Crashes

CRF = 24% All Crashes

33-47% Right Angle Crashes

Tried

8-20

Improve Visibility (Mast Arms)

Tried

Treatment Finding

Replace pedestals with mast arms (166)

49% estimated reduction in all crashes.44% estimated reduction in fatal/injury 51% estimated reduction in property damage only (PDO) collisions.74% estimated reduction in right angle collisions.41% estimated reduction in rear end 12% estimated reduction in left-turn

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Improve Visibility – Signal Head per Lane)

Tried

CRF = 10 - 22%, Total Crashes, *ICBC, Winston-Salem, NC

CRF = 47%, Right Angle Crashes

1 head for 1 Lt lane

4 heads for 4 lanes

1 head for 1 Rt lane

8-22

Improve Visibility (Add a Signal Head)

Tried

Treatment Finding

Add a signal head(135)

15% estimated increase in all collisions.47% estimated reduction in right angle collisions.

Add a primary signal head (168)

10% to 25% estimated reduction in fatal/injury 30% to 35% estimated reduction in property-damage-only collisions.15% to 45% estimated reduction in right-angle 0% to 45% estimated reduction in rear-end

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Improve Visibility (Supplemental Signal Head)

Supplemental Signal Head

CRF = 15%, Total Crashes, *ICBC, Winston-Salem, NC

Tried

CRF = 47-48%, Right Angle Crashes

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Add Back Plates

No Back Plates Back Plates

Tried

CRF= 32% Right angle crashes,

CRF= 2% to 24% All Crashes

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Add Back Plates

-50% reduction in RLR

* Bonneson

Tried

CRF= 2% - 24%

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Retroreflectorize Back Plates

Canadian retro- reflectorized backplate

CRF=32% reduction in RLR Crashes

Tried

CRF = 12% increase in all crashes

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Retroreflectorize Back Plates

Canadian retro- reflectorized backplate

32% Reduction in RLR Related Crashes

Tried

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Traffic Signals Countermeasures– Exercise XI: Poughkeepsie NY

“T” Intersection 3 Thru Lanes

Westbound with ADT of 34,000; Side Street ADT is 1,400

NY 44 Westbound

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What low cost safety countermeaures would you consider?

What is the safety effect (CRF) for each?

Discussion

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Relevant Countermeasures:

• Change 8” to 12” Indications

• Add Backplates• Add All-Red Phase• Revise Change

Interval• Remove Sight

Obstruction of Parked Vehicles on Southeast Quadrant

NY 44 Westbound

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Left Turn Lanes + Left Turn Phases

Add a Left Turn Phase to Existing Signal

Tried

*NCHRP 500, Objective 17.2 A1 – Employ Multiphase Signal Operation

CRF = 23% to 48% Total Crashes

CRF = 63% to 70% Left Turn Crashes

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Add a Protected/Permissive Left Turn Phase to Existing Signal

Tried




8-33


Add a Left Turn Phase + Left Turn Lane to Existing Signal

Tried

*NCHRP 500, Objective 17.2 A1 – Employ Left Turn Signal Phase

CRF = 35% Total Crashes

CRF = 58% (Iowa), Left Turn Crashes

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Signalize and Add a Left Turn Lane without Left Turn Phase

Tried


CRF = 15%

Total Crashes



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Signalize and Add a Left Turn Lane + Left Turn Phase

Tried58% Reduction in

Crashes, *Iowa

*NCHRP 500, Objective 17.2 A1 – Provide Left Turn Signal Phase

CRF=25% to 36% Total Crashes

CRF=43% to 45%, Left Turn Crashes

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Change Permissive Left to Protected Left Only

CRF = 97% to 98% Left Turning Crashes

* Winston-Salem, NC

Proven

*NCHRP 500, Objective 17.2 A1 – Provide Protected Left Turn Signal Phase

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Add Signal Ahead Advance Warning Sign

Tried

CRF= 35-40%

Winston-Salem, NC

McGee

MN DOT

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Add Signal Ahead Advance Warning Sign

Tried

Treatment Finding

Post SIGNAL AHEAD warning signs—urban(98)

16%-35% estimated decrease in all collisions.

Post SIGNAL AHEAD warning signs—rural(98)


Post SIGNAL AHEAD signs(135)

44% estimated decrease in right-angle collisions.

Advance-warning flasher

(172)

44% decrease in all fatal/injury 53% decrease in pd crashes73% decrease in all fat/inj-angle crashes.82% increase in all rear-end fatal-injury

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Add Advance Warning Sign with Active Flashers tied to Signal Operation

CRF=35% to 67% Reduction in RLR Related Crashes, *Bonneson

Tried

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Add Advance Warning Signs & Flashers

Tried

Treatment Finding

Post SIGNAL AHEAD warning signs—urban(98)


Post SIGNAL AHEAD warning signs—rural(98)


Post SIGNAL AHEAD signs(135)

44% estimated decrease in right-angle collisions.

Advance-warning flasher

(172)

44% decrease in all fatal/injury collisions.53% decrease in property-damage-only73% decrease in all fatal/injury-angle 67% decrease in all fatal/injury left-turn 82% estimated increase in all rear-end

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Add Supplemental Signal Head(s)

Supplemental Far Left Signal Head

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Add Supplemental Signal Head(s)

Crest Vertical Curve + Right Hand CurveSight Distance is Limited By Noise Walls

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Overhead Red “T” Signal Heads

Tried

CRF = 12% total crashesCRF = 33% Reduction in Rt angle crashes, *Winston-Salem, NC

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Flashing Operation

Issue Benefit Detriment

Yellow on mainline/Red on side street - if volume ratio is three or more

Yellow on mainline/Red on side street – if volume ratio is less than three or if adequate sight distance is not available

8-45

Flashing Operation

Change late night flash of Yellow on mainline/Red on side street to normal operating traffic signal mode

Tried

CRF = 29% Total Crashes

CRF = 80% Right Angle Crashes

*Winston-Salem, NC

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Signal Coordination

Proven

*NCHRP 500, Objective 17.2 A4 – Employ Signal Coordination

CRF = 15% to 17% total crashes – 5 studies

CRF = 25% to 38% Right Angle Crashes

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Traffic Signals

Available thru ITE

ITE website: www.ite.org

Making Intersections Safer: A Toolbox of Engineering Countermeasures to Reduce Red Light Running

*NCHRP 500, Objective 17.2 E-3 – Implement Automated Enforcement of Red-Light Running

Proven

8-48

Traffic Signals

Review Question: What is the Recommended Practice re Calculation of Change Intervals?

CP = t + V/2a + V/20*64.4*g + (W+L)/VITE Traffic Engineering Handbook

-reduces RLR frequency by 50-70%CRF = 4 to 31% Total CrashesCRF = 1 to 30% Right Angle Crashes

8-49

Traffic Signals

Learning Outcomes:

1. Identify countermeasures for operation and design deficiencies of traffic signals

8-50

Questions?

Traffic Signals

8-1 low cost safety improvements the tools – traffic signals – session #8

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