HSM Applications to Two-Lane Rural Highways Predicting Crash Frequency and Applying CMF’s for Two-Lane Rural Highway Intersections - Session #6 6-1

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 6-2

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) 6-3

2008 US Total Crash Characteristics Crash TypeTotal Crashes Fatal/Injury Crashes Number% % Non Intersection2,638,000 45% 722,680 43% Stop/No control Intersection 984,000 17% 321,520 19% Signalized Intersection 1,182,000 20% 380,511 23% Unclassified1,005,000 17% 240,306 14% Total 5,801, % 1,637, % 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% 6-4

Physical vs Functional Area of an Intersection 6-5

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:  < lower crash frequency  > increased crash frequency 3.Calibration, C r or C i - Accounts for local conditions/data Process for Prediction of Crash Frequency and Application of Crash Modification Factors 6-7

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 6-8

SPF Models for RURAL Two-Lane Intersections with Stop Control on Minor-Road AADT maj = Avg Annual Daily Volume on Major Road (veh/day) AADT min = Avg Annual Daily Volume on Minor Road (veh/day) N spf-3ST =exp[ ln(AADT maj ) ln(AADT min )] Three-Leg Stop Controlled Intersection (3ST): Four-Leg 2-Way Stop Controlled Intersection (4ST): N spf-4ST =exp[ ln(AADT maj ) ln(AADT min )] 6-9

SPF Models for RURAL Signalized Intersections Four-Leg Signalized Intersection (4SG): N spf-4SG = exp[ ln(AADT maj ) ln(AADT min )] N spf-4SG = estimate of intersection-related predicted average crash frequency for base conditions; AADT maj = Avg Annual Daily Volume on Major Road (veh/day) AADT min = Avg Annual Daily Volume on Minor Road (veh/day) 6-10

Base Conditions for Rural Two-Lane Intersections: ►Intersection Skew Angle: 0 o degrees ► Presence of Left-Turn Lanes: none ► Presence of Right-Turn Lanes: none ► Lighting: none 6-11

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): 6-12

Three-Leg Stop Controlled Intersection (3ST): For AADT maj = 5,000 and AADT min = 500: = crashes per year or 4.59 crashes in a 5 year period N spf-3ST = exp[ ln(AADT maj ) ln(AADT min )] N spf-3ST = exp[ ln(5,000) ln(500)] = exp[ ] = exp[-0.086] SPF Model for RURAL Stop Controlled Intersection– Example: 6-13

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 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: 6-16

For AADT maj = 9,000 and AADT min = 4,500: N spf-4SG = exp[ ln(AADT maj ) ln(AADT min )] N spf-4SG = exp[ ln(9,000) ln(4,500)] SPF Base Model for RURAL Signalized Intersection – Example: 4-Approach Signalized Intersection: For range of AADT maj from zero to 25,200 and AADT min from zero to 12, = 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

Crash Severity for Rural 2-Lane Intersections Table

Default Distribution of Crash Types for Rural 2- Lane Intersections Table 10-6: Default Distribution for Collision Types and Manner of Collisions 6-20

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) SI 4st = Fatal + Injury Crashes = 5/9 = 0.55 Total Crashes Applying Severity Index to Rural Two-Lane Highway Intersections 3-21

3-22 Applying Severity Index to Rural Two-Lane Highway Intersections b. Compute the Predicted Severity Index (SI) SI 4st = 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) = ? Applying Severity Index to Rural Two-Lane Highway Intersections Is the Actual Severity Index higher or lower than the Predicted Severity Index? ? 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:  < lower crash frequency  > increased crash frequency 3.Calibration, C r or C i - Accounts for local conditions/data Process for Prediction of Crash Frequency and Application of Crash Modification Factors 6-24

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 6-25

Potential Conflict Points Comparison of 4-leg/3-leg Intersections Cross intersection has 32 conflict points, Offset T has 22 points 6-26

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 6-27

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 = degrees SKEW = Intersection Skew Angle (degrees) as the absolute value of the difference between 90 degrees and the actual intersection angle Skew = 90 0 – 35 0 = 55 0 Skew Angle 6-28

CMF for Intersection Skew Angle (CMF 1i ) CMF 1i = exp ( SKEW) For 3- legged Stop Controlled Intersections: For 4- legged Stop Controlled Intersections: CMF 1i = exp ( 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 6-29

Intersection Skew from 90 degree side road for 4- leg Approaches Skew= CMF = *Max skew of 15 degrees - Older Driver Handbook and ITE Max skew of 30 degrees – 2004 Green Book *NCHRP 500, Strategy 17.1 B16 – Realign Intersection Approaches CMF for Intersection Skew Angle (CMF 1i ) 6-30

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 CMF 1i = Skew = 45 CMF 1i = e (45) =1.275 Skew = 10 CMF 1i = e (10) =1.041 Skew = 15 CMF 1i = e (15) =1.062 Example: CMF for Intersection Skew Angle (CMF 1i ) 6-31

Solutions to Skewed Intersections *NCHRP 500, Strategy 17.1 B16 – Realign Intersection Approaches Old Alignment New Alignment 6-32

6-33

6-34

*NCHRP 500, Strategy 17.1 B16 – Realign Intersection Approaches  Locate Intersection at Mid-Point of Curve Solutions to Skewed Intersections 6-35

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 6-36 warrants for turn lanes in the rural environment see NCHRP 457

CMF for Left Turn Lanes (CMF 2i ) NCHRP 500, Strategy 17.1 B1 – Provide Left-Turn Lanes 6-37 ____

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 6-38

►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 (CMF 3i ) 6-39

NCHRP 500, Strategy 17.1 B6 – Provide Right Turn Lanes CMF for Right Turn Lanes (CMF 3i ) 6-40 ____

CMF for Lighting of Rural 2-Lane Intersections (CMF 4i ) CMF 4i = p ni 6-41

NCHRP 500, Strategy 17.1 E2-Improve Visibility of Intersection by Providing Lighting (P) CMF 4i = p ni For 4 approach Two-Way Stop Controlled rural intersection: = (0.244) = CMF for Lighting of Rural 2-Lane Intersections (CMF 4i ) – Example: 6-42

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 6-43

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 6-44

CMF’s for Conversion of 2-Way Stop Intersection to Roundabout 6-45

*NCHRP 500, Strategy 17.1 F3 – Provide Roundabouts CMF (single lane) = 0.29 CMF (multi-lane) = 0.56 Before After Converting Stop-Control to Roundabout Roundabouts in the rural environment 6-46

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 6-47

CMF’s for Conversion of 2-Way Stop to All- Way Stop Control 6-48

CMF’s for STOP AHEAD Supplementary Pavement Marking 6-49

CMF’s for Beacons Four approach, STOP control, Two lane roads 6-50 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 6-51

CMF for Access Control for Rural Intersections Where: d n = Number of driveways on both the major and minor road approaches within 250 feet of the intersection CMF nd = e * (dn-3) Unsignalized Intersections: Signalized Intersections: CMF nd = e * (dn- 3) *From TTI Roadway Safety Design Synthesis, 2005) 6-52

CMF for Access Control for Rural Intersections: Example Calculation CMF nd = e (dn - 3) Unsignalized Intersections: For 4 driveways on US route and 3 driveways on County Route = e (7 - 3) = e (4) = 1.25 *From TTI Roadway Safety Design Synthesis, 2005) 6-53

Additional Low Cost Safety Measures beyond the published 2010 HSM ►Beyond the Highway Safety Manual are many proven low cost safety measures 6-54 htpp://

2009 MUTCD Figure 2A-4 Intersection Typical Signing Warning Guide Regulatory Right-of-Way ►Applying the two guiding principles of: - Clarify and Simplify 6-55

Warning Guide Regulatory Right-of-Way ►Applying the two guiding principles of: Clarify and Simplify Applying Simplify and Clarify 6-56

Low Cost Intersection Safety Measures – Signing Countermeasures 1.Warning  All-Way Stop of 2 rural State Highways CMF = 0.60 Rural CMF = 0.70 Urban 6-57

2. Enhanced Warning  All-Way Stop of 2 rural State Highways “Double-Up” CMF = 0.69 Low Cost Intersection Safety Measures – Signing Countermeasures 6-58

3. Enhanced Warning  All-Way Stop of 2 rural State Highways Warning Beacons CMF = 0.75 Low Cost Intersection Safety Measures – Signing Countermeasures 6-59

4. Advance Guide Signs  All-Way Stop of 2 rural State Highways Low Cost Intersection Safety Measures – Signing Countermeasures 6-60

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 Low Cost Intersection Safety Measures – Signing Countermeasures 6-61

5. Regulatory Right-of-Way  Add Stop Sign on Island to Enhance Visibility CRF = 11% + Right Hand Supplemen tary Stop Sign  All-Way Stop of 2 rural State Highways Low Cost Intersection Safety Measures – Signing Countermeasures 6-62

6. Regulatory Right-of-Way  “Double Up” Stop Signs CMF = 0.89 CRF = 11% total crashes CRF = 55% Rt Angle Crashes Low Cost Intersection Safety Measures – Signing Countermeasures 6-63

7. STOP Beacon  Add Stop Beacon CMF = 0.42 angle crashes  All-Way Stop of 2 rural State Highways Low Cost Intersection Safety Measures – Signing Countermeasures 6-64

 “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-Approach CRF = 40% 4-Approach Low Cost Intersection Safety Measures – Signing Countermeasures 6-65

Concept 1 – Narrow travel lanes by striping on Main highway Low Cost Intersection Safety Measures – Rumble Treatment 6-66

Concept 1 – Narrow travel lanes by striping on Main highway Low Cost Intersection Safety Measures – Rumble Treatment 6-67

Concept 1 – Narrow travel lanes by striping on Main highway Low Cost Intersection Safety Measures – Rumble Treatment 6-68

► after 2 years, total crash reduction = 32% ► Injury/Fatal crash reduction = 34% 6-69

Concept 2 – Add splitter Island on side road approaches Low Cost Intersection Safety Measures – Rumble Treatment 6-70

Concept 2 – Add splitter Island on side road approaches Low Cost Intersection Safety Measures – Add Splitter Island with Stop on Centerline 6-71

Concept 2 – Add splitter Island on side road approaches Low Cost Intersection Safety Measures – Add Splitter Island with Stop on Centerline 6-72

Concept 2 – Add splitter Island on side road approaches Low Cost Intersection Safety Measures – Add Splitter Island with Stop on Centerline 6-73

Predicting Crash Frequency for Two-Lane Rural Highway Intersections 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 6-74

Questions and Discussion Predicting Crash Frequency for Two-Lane Rural Highway Intersections 6-75