LOW COST SAFETY IMPROVEMENTS
Practitioner Workshop
The Tools – Identification of High Crash
Locations – Session #2
Identification of High Crash Locations
Learning Objectives:
Describe the HSIP Planning Process
Crash Mitigation Process
Identify appropriate Engineering Countermeasures from Crash Patterns
HSIP Planning
Detailed Engineering StudyDocument the Analysis
Project Selection, Implementation, & Evaluation
Document the Evaluation
Network Screening or Site Selection
List of Sites for Review
What is Network Screening? Highway network system
made up of segmentsand intersections
Network screening is a systematic examination of all entities
Purpose: To rank all entities, based on selected criteria, in order to conduct detailed safety studies
Money should go where it achieves the greatest effect in terms Money should go where it achieves the greatest effect in terms of preventing crashes and reducing their severityof preventing crashes and reducing their severity
http://www.ncdot.org//planning/statewide/gis/DataDist/CountyMap.html
Identification of High Crash Locations
The six steps in the crash mitigation process
The six steps in the crash mitigation process
1. Identify Sites2. Collect Crash
Experience3. Gather Field Conditions4. Identify Contributing
Factors and Countermeasures
5. Assess and Select Countermeasures
6. Implement and Evaluate
Identification of High Crash Locations
Step 1: Identify Sites with Potential Safety Problems• Crash data• Traffic Measures• Field Observations• Complaints• Enforcement input• Surrogate measures
Identification of High Crash Locations
Step 1: Identify Sites with Potential Safety Problems• Crash data
Total Number of Crashes Crash Density (Crashes per mile) Crash Rate (Crashes per million vehicle miles) Number Quality Control Rate Quality Control Crash Severity Severity Index Crash Index
Most CommonReporting Errors
Crash locationDirection of travelDriver actionPedestrian conditionVertical road characteristics
Conventional Screening
Conventional techniques of screening in use are known to have difficulties in identifying ‘unsafe’ sites: Crash counts = bias to high volume sites Crash rates = bias to low volume sites Crash rates’ assumption of linearity is invalid Regression-to-mean (RTM) effect if sufficient
allowance is not made for random errors
Identification of High Crash Locations
Roadway Segment:
CR = N / [ADT x 365 x L x 10-6]
where CR is expressed as “Crashes per million vehicle miles (or kilometers)” and
N = Number of crashes per year
ADT = Average Daily Traffic
L = Length of segment (mi or km)
Identification of High Crash Locations
Example: For N = 50 crashes for 3 year period ADT = 3,000 Average Daily Traffic L = 2.8 miles
CR = N / [ADT x 365 x L x 10-6]CR = 50 / [3,000 ADT x 3 yrs x 365 x 2.8
miles x 10-6]CR = 5.43 crashes per million vehicle
miles
Identification of High Crash Locations
Different Hwy Types Experience Different Crash Rates
Identification of High Crash Locations
Step 1: Identify Sites with Potential Safety Problems
*From Mn DOT Traffic Safety Fundamentals Handbook
Identification of High Crash Locations
Intersection:
AR = N / {[Sum (ADT)s /2] x 365 x 10-6}
where AR is expressed as “crashes per million entering vehicles” and
N = Number of crashes per year
Sum (ADT)s = Sum of all Average Daily Traffic entering the intersection
Identification of High Crash Locations
Example: For N = 25 crashes for 3 years ADT (N) = 10,000 ADT (S) = 9,000 ADT (E) = 3,500 ADT (W) = 4,000Sum (ADT)s=(10,000+9,000+3,500+4,000) = 26,500
AR = N / {[Sum (ADT)s /2] x 365 x 10-6}
AR = 25/ {26,500/2] x 3 yrs x365 x 10-6}
AR = 1.72 crashes per million entering vehicles
Identification of High Crash Locations
Collision Types at Rural Intersections
Identification of High Crash Locations
Intersection Crash Rates
• Average of 1.5 crashes per year for Un-Signalized Intersections in rural areas– recent California analysis*
• Average of 2.5 crashes per year in urban areas
*NCHRP 500, Volume 5: A Guide for Addressing Unsignalized Intersection Collisions, 2003.
Identification of High Crash Locations
Step 2: Characterize the Crash Experience
Prepare a Collision Diagram
Step 3: Characterize Field Conditions
Identification of High Crash Locations
Step 3: Characterize Field Conditions
Traffic Data Studies:
Identification of High Crash Locations
Traffic Volume – Turning Movement, ADT
Spot Speeds
Traffic Conflict Study
Sight Distance Evaluation
Non-Crash Based Procedures
Identification of High Crash Locations
Step 4: Identify Contributing Factors and Appropriate Countermeasures
Identification of High Crash Locations
*Table 12, from Missouri HAL Manual
Step 5: Assess Countermeasures and Select Most Appropriate
Identification of High Crash Locations
Probable Causes for Crash Patterns and Engineering Countermeasures
Identification of High Crash Locations
Rear-End Collisions at un-signalized intersection
Identification of High Crash Locations
• Documents list “Crash Reduction Factors” (CRF) and “Accident Modification Factors” (AMF) for specific types of crashes
– CRF :• (+) number is % reduction in crash type• (-) number is % increase in crash type
Identification of High Crash Locations
• “Accident Modification Factors” (AMF) for specific types of crashes– AMF :
• ratio of (after) crashes / (before) crashes• AMF < 1 means a reduction in crash type• AMF > 1 means an increase in crash type
– AMF = 1- CRF
Estimated Crash Reduction Factors
Identification of High Crash Locations
Install Warning Signs
Estimated Crash Reduction Factors
Identification of High Crash Locations
* From MN DOT Traffic Safety Fundamentals Handbook
Traffic Crash Costs:
Identification of High Crash Locations
*NCHRP 440
Traffic Crash Costs:
Identification of High Crash Locations
* From MN DOT Traffic Safety Fundamentals Handbook
Typical Benefit/Cost Ratios
Identification of High Crash Locations
* From MN DOT Traffic Safety Fundamentals Handbook
Step 6: Implement Countermeasures and Evaluate Effectiveness
Identification of High Crash Locations
Improve Sight Distance – 5% Crash Reduction per Quadrant – 20% for all 4 quadrants
Identification of High Crash Locations
Review Questions:
What are the six steps in the crash mitigation process?
Questions?
Identification of High Crash Locations