hsm applications to two-lane rural highways predicting crash frequency and applying cmf’s for...

HSM Applications to Two-Lane Rural Highways

Predicting Crash Frequency and

Applying CMF’s for Two-Lane Rural

Highway Intersections

- Session #6

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Predicting Crash Frequency for Two-Lane Rural Highway Intersections

Outcomes:

► Describe the SPF Base Models for prediction of Intersection Crash Frequency

► Calculate Predicted Crash Frequency for Rural Two-lane Highway Intersections

► Describe CMF’s for Rural 2 Lane Intersections

► Apply CMF’s to Predicted Crash Frequency

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Why Intersection Safety?

►A small part of overall highway system, but -► In 2008 – 7,772 fatalities related to intersections ► (21% of Total Highway Fatalities)►Each year more than 3.17 million intersection

crashes occur (over 55% of all reported crashes)

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2008 US Total Crash Characteristics

Crash Type Total CrashesFatal/Injury

Crashes

Number % Number %

Non Intersection 2,638,000 45% 722,680 43%

Stop/No control Intersection

984,000 17% 321,520 19%

Signalized Intersection

1,182,000 20% 380,511 23%

Unclassified 1,005,000 17% 240,306 14%

Total 5,801,228 100% 1,637,476 100%

Source: USDOT Traffic Safety Facts 2008 Early Edition, A Compilation of motor vehicle crash data from FARS and GES, Table 29, Page 52

37% 42%

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Physical vs Functional

Area of an

Intersection

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Functional Area of an Intersection

►Decision Distance

►Maneuver Distance

►Queue-Storage Distance 6-6

Three Steps:

1. Predict Crash Frequency - Safety Performance Functions (SPF) Equations

2. Apply Appropriate Crash Modification Factors (CMFs)

- Adjust predicted safety performance from base conditions to existing/proposed conditions

- Are greater or less than 1: < 1.0 -- lower crash frequency > 1.0 -- increased crash frequency

3. Calibration, Cr or Ci - Accounts for local conditions/data

Process for Prediction of Crash Frequency and Application of Crash Modification Factors

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Models to Predict Crash Frequency for Rural Two-Lane Highway Intersections

►Three-Approach Stop Control (Stop of Stem of Tee)

► Four-Approach Stop Control (2-way Stop)► Four-Approach Signal Control

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SPF Models for RURAL Two-Lane Intersections with Stop Control on

Minor-Road

AADTmaj = Avg Annual Daily Volume on Major Road (veh/day)

AADTmin = Avg Annual Daily Volume on Minor Road (veh/day)

Nspf-3ST=exp[-9.86 + 0.79 ln(AADTmaj) + 0.49 ln(AADTmin)]

Three-Leg Stop Controlled Intersection (3ST):

Four-Leg 2-Way Stop Controlled Intersection (4ST):Nspf-4ST=exp[-8.56 + 0.60 ln(AADTmaj) + 0.61 ln(AADTmin)]

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SPF Models for RURAL Signalized Intersections

Four-Leg Signalized Intersection (4SG):

Nspf-4SG = exp[-5.13 + 0.60 ln(AADTmaj) + 0.20 ln(AADTmin)]

Nspf-4SG = estimate of intersection-related predicted average crash frequency for base conditions;

AADTmaj = Avg Annual Daily Volume on Major Road (veh/day)

AADTmin = Avg Annual Daily Volume on Minor Road (veh/day)

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Base Conditions for Rural Two-Lane Intersections:

►Intersection Skew Angle: 0odegrees ► Presence of Left-Turn Lanes: none► Presence of Right-Turn Lanes: none► Lighting: none

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SPF Model for RURAL Stop Controlled Intersection– Example:

►For a 1-Way STOP with an AADT of 5000 across the top of the “T” on the main Road and 500 AADT on the minor road of the “T”,

► What is the predicted # of Crashes?

Discussion

1-Way STOP on Minor Approach to a “T” Intersection (3-leg):

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Three-Leg Stop Controlled Intersection (3ST):

For AADTmaj = 5,000 and AADTmin = 500:

= 0.917 crashes per year or 4.59 crashes in a 5 year period

Nspf-3ST = exp[-9.86 + 0.79 ln(AADTmaj) + 0.49 ln(AADTmin)]

Nspf-3ST = exp[-9.86 + 0.79 ln(5,000) + 0.49 ln(500)]

= exp[-9.86 + 6.729 + 3.045]

= exp[-0.086]

SPF Model for RURAL Stop Controlled Intersection– Example:

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Safety Performance Function (SPF)

Highway Safety Manual Approach:

Average Crash Rate“one rate”

6-14

“Is this a Higher Crash Frequency Site?”

Highway Safety Manual Approach:

“Difference”

“Predicted Crash

Frequency”

“Substantive Crash Frequency”

0.5 crashes/yr

6 crashes/yr

6-15

1.2 crashes/yr

SPF Base Model for RURAL Signalized Intersection - Exercise

►For a 4-Approach signalized intersection with AADT = 9,000 on the major road and AADT = 4,500 on the minor road,

► What is the predicted # of Crashes?

Discussion

4-Approach Signalized Intersection:

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For AADTmaj = 9,000 and AADTmin = 4,500:

Nspf-4SG = exp[-5.13 + 0.60 ln(AADTmaj) + 0.20 ln(AADTmin)]

Nspf-4SG = exp[-5.13 + 0.60 ln(9,000) + 0.20 ln(4,500)]

SPF Base Model for RURAL Signalized Intersection – Example:

4-Approach Signalized Intersection:

For range of AADTmaj from zero to 25,200 and AADTmin from zero to 12,500

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= 7.5 crashes per year

Severity Index for all highways and streets

Severity index (SI) is the ratio of crashes involving an injury or fatality to total crashes

► ..however, Chapter 10 of the HSM provides “better” injury and fatal crash distribution by type of rural intersection control in Tables 10-5 and 10-6

* From NCHRP 486

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Crash Severity for Rural 2-Lane Intersections

Table 10-5

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Default Distribution of Crash Types for Rural 2-

Lane Intersections

Table 10-6: Default Distribution for Collision Types and Manner of Collisions

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Example: Two-way stop controlled 4-approach intersection with 9,000 AADT on Major and 4,500 AADT on minor; Fatal and Injury crashes are 5 of 9 total crashes

a. Compute the actual Severity Index (SI)

SI4st = Fatal + Injury Crashes = 5/9 = 0.55 Total Crashes

Applying Severity Index to Rural Two-Lane Highway Intersections

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b. Compute the Predicted Severity Index (SI)

SI4st = Fatal + Injury Crashes = 43.1/100= 0.43 Total Crashes

Example: Two-way stop controlled 4-approach intersection with 9,000 AADT on Major and 4,500 AADT on minor; Fatal and Injury crashes are 5 of 9 total crashes

a. Actual Severity Index (SI) = ?

b. Predicted Severity Index (SI) = ?


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- Is the Actual Severity Index higher or lower than the Predicted Severity Index?

?

0.55

0.43

Higher

Three Steps:1. Predict Crash Frequency - Safety Performance

Functions (SPF) Equations - Predict Crash Frequency for base conditions

2. Apply Appropriate Crash Modification Factors (CMFs)

- Adjust predicted safety performance from base conditions to existing/proposed conditions

- Are greater or less than 1: < 1.0 -- lower crash frequency > 1.0 -- increased crash frequency

3. Calibration, Cr or Ci - Accounts for local conditions/data

Process for Prediction of Crash Frequency and Application of Crash Modification Factors

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HSM Crash Modification Factors for Rural Two-Lane Highway Intersections

► Configuration - Number of Legs► Intersection Designs - Roundabouts

► Angle of Intersection (Skew) ► Left Turn Lanes► Right Turn Lanes► Lighting

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Potential Conflict Points

Comparison of 4-leg/3-leg Intersections

Cross intersection has 32 conflict points, Offset T has 22 points

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Number of Intersection Legs

►Crash Frequency for intersections with only 3 approaches is lower►Crash Frequency for intersections with 4 approaches are greater than for those intersections with only 3 approaches►Collision rates for intersections with more than 4 approaches are 2 to 8 times greater than for 4 approach Intersections

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CMF for Rural Intersection Skew Angle

•Some studies (McCoy, for example) show adverse effect of skew•Skews increase exposure time to crashes; increase difficulty of driver view at stopped approach

Intersection Angle = 350

@ 90 degrees

SKEW = Intersection Skew Angle (degrees) as the absolute value of the difference between 90 degrees and the actual intersection angle

Skew = 900 – 350 = 550

Skew Angle

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CMF for Intersection Skew Angle (CMF1i)

CMF1i = exp (0.0040 SKEW)

For 3- legged Stop Controlled Intersections:

For 4- legged Stop Controlled Intersections:

CMF1i = exp (0.0054 SKEW)

SKEW = Intersection Skew Angle (degrees) as the absolute value of the difference between 90 degrees and the actual intersection angle

*NCHRP 500, Strategy 17.1 B16 – Realign Intersection Approaches

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Intersection Skew from 90 degree side road for 4-leg ApproachesSkew= 0 15 30 45 CMF = 1.00 1.08 1.18 1.28

*Max skew of 15 degrees - Older Driver Handbook and ITE

Max skew of 30 degrees – 2004 Green Book



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For each of the four (4) intersections, calculate the safety effect of skew angle

#2 –45 deg#1 –90 deg

#3 –80 deg

#4 –75 deg

Skew = 0

CMF1i = 1.000Skew = 45

CMF1i = e0.0054(45)

=1.275

Skew = 10CMF1i = e0.0040(10)

=1.041

Skew = 15

CMF1i = e0.0040(15)

=1.062

Example:


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Solutions to Skewed Intersections


Old Alignment

New Alignment

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Locate Intersection at Mid-Point of Curve

Solutions to Skewed Intersections

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Left Turn Lanes in the Rural Highway Environment

Left turn lanes remove stopped traffic from through lanes

• mitigate rear-end conflict• enable selection of safe gap

*NCHRP 500, Strategy 17.1 B1 – Provide Left-Turn Lanes

“Capacity” is generally not the issue

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warrants for turn lanes in the rural environment see NCHRP 457

CMF for Left Turn Lanes (CMF2i)

NCHRP 500, Strategy 17.1 B1 – Provide Left-Turn Lanes

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____

Rural Left Turn By-Pass Lanes

*NCHRP 500, Strategy 17.1 B4 – Provide By-Pass Lanes

► Less cost than conventional left turn lane► At low volume intersections, may be just as effective►Minnesota study unable to conclude bypass lanes just as safe as left turn lanes

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►Right turn lanes remove slowing traffic from through lanes which are not stop controlled

*NCHRP 500, Strategy 17.1 B6 – Provide Right-Turn Lanes

“Capacity” is generally not the issue

CMF for Right Turn Lanes (CMF3i)

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NCHRP 500, Strategy 17.1 B6 – Provide Right Turn Lanes

CMF for Right Turn Lanes (CMF3i)

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____

CMF for Lighting of Rural 2-Lane Intersections (CMF4i)

CMF4i = 1-0.38pni

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NCHRP 500, Strategy 17.1 E2-Improve Visibility of Intersection by Providing Lighting (P)

CMF4i = 1-0.38pni

For 4 approach Two-Way Stop Controlled rural intersection:

= 1-0.38(0.244)

= 0.907

CMF for Lighting of Rural 2-Lane Intersections (CMF4i) – Example:

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Additional CMF’s from Part D and Research

Beyond the SPF’s and CMF’s detailed in Part C Chapter 10:

► CMF’s for Roundabouts from Chapter 14► CMF for 4-Way Stop► CMF for STOP AHEAD Pavement marking► CMF for STOP Beacons► CMF for driveways within 250 feet from TTI Research

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Roundabouts are Alternatives to conventional intersections

►Number of conflicts is reduced►Severe conflicts (angle) are eliminated►Speed differentials are reduced or eliminated

*NCHRP 500, Strategy 17.2 B5 – Construct Special Solutions – Roundabout Design

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CMF’s for Conversion of 2-Way Stop Intersection to Roundabout

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*NCHRP 500, Strategy 17.1 F3 – Provide Roundabouts

CMF (single lane) = 0.29CMF (multi-lane) = 0.56

Before After

Converting Stop-Control to Roundabout

Roundabouts in the rural environment

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Before Crash Info – 2 yrs: - 12 crashes with 4 F/Inj

Summit County Ohio

After Crash Info – 2 yrs: - 4 crashes with 0 F/Inj

Single Lane Rural Roundabout:►Approach speed limits 45 mph,►60 foot right of way

Roundabouts in the rural environment

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CMF’s for Conversion of 2-Way Stop to All-Way Stop Control

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CMF’s for STOP AHEAD Supplementary Pavement Marking

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CMF’s for Beacons

Four approach, STOP control, Two lane roads

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Table 14-42

Driveway near Rural Intersections

►Access points within 250 feet upstream and downstream of an intersection are undesirable ► Unsignalized - 20% more crashes for 3 driveways within 250 feet► Signalized - 13% more crashes for 3 driveways within 250 feet ► Consolidate multiple access points► Relocate access to the adjacent side road if possible

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CMF for Access Control for Rural Intersections

Where:dn = Number of driveways on both the major

and minor road approaches within 250 feet of the intersection

CMFnd = e0.056 * (dn-3)

Unsignalized Intersections:

Signalized Intersections:

CMFnd = e0.046 * (dn- 3)

*From TTI Roadway Safety Design Synthesis, 2005)

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CMF for Access Control for Rural Intersections: Example Calculation

CMFnd = e0.056 (dn - 3)

Unsignalized Intersections:For 4 driveways on US route and 3 driveways on County Route

= e0.056 (7 - 3)

= e0.056 (4)

= 1.25

*From TTI Roadway Safety Design Synthesis, 2005)

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Additional Low Cost Safety Measures beyond the published 2010 HSM

►Beyond the Highway Safety Manual are many proven low cost safety measures

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htpp://www.cmfclearinghouse.org

2009 MUTCD Figure 2A-4 Intersection Typical Signing

Warning

Guide

Regulatory Right-of-Way

►Applying the two guiding principles of:

- Clarify and Simplify

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Warning

Guide

Regulatory Right-of-Way

►Applying the two guiding principles of: Clarify and Simplify

Applying Simplify and Clarify

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Low Cost Intersection Safety Measures – Signing Countermeasures

1.Warning

All-Way Stop of 2 rural State Highways

CMF = 0.60 RuralCMF = 0.70 Urban

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2. Enhanced Warning


“Double-Up” CMF = 0.69


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3. Enhanced Warning


Warning Beacons CMF = 0.75


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4. Advance Guide Signs



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5. Regulatory Right-of-Way

Stop Sign on outside of large right turn radius is too far out of center attention window of driver All-Way Stop

of 2 rural State Highways


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Add Stop Sign on Island to Enhance Visibility CRF = 11%+ Right Hand Supplementary Stop Sign



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“Double Up” Stop Signs CMF = 0.89

CRF = 11% total crashesCRF = 55% Rt Angle Crashes


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7. STOP Beacon

Add Stop Beacon CMF = 0.42 angle crashes



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“Call Attention” to the presence of the Intersection

*NCHRP 500, Strategy 17.1 E3 – Install Splitter Islands on Minor Road Approaches

Install Splitter Islands on the Minor Road Approach to an Intersection 9. Splitter Island

CRF = 45% 3-ApproachCRF = 40% 4-Approach


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Concept 1 – Narrow travel lanes by striping on Main highway

Low Cost Intersection Safety Measures – Rumble Treatment

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► after 2 years, total crash reduction = 32%► Injury/Fatal crash reduction = 34%

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Concept 2 – Add splitter Island on side road approaches


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Low Cost Intersection Safety Measures – Add Splitter Island with Stop on Centerline

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Outcomes:

► Described the SPF Base Models for prediction of Intersection Crash Frequency

► Calculated Predicted Crash Frequency for Rural Two-lane Highway Intersections

► Described CMF’s for Rural 2 Lane Intersections

► Applied CMF’s to Predicted Crash Frequency

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Questions and Discussion


6-75

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