LOW COST SAFETY IMPROVEMENTS

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


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


Step 1: Identify Sites with Potential Safety Problems• Crash data• Traffic Measures• Field Observations• Complaints• Enforcement input• Surrogate measures


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


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)


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


Different Hwy Types Experience Different Crash Rates


Step 1: Identify Sites with Potential Safety Problems

*From Mn DOT Traffic Safety Fundamentals Handbook


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


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


Collision Types at Rural Intersections


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.


Step 2: Characterize the Crash Experience

Prepare a Collision Diagram

Step 3: Characterize Field Conditions


Step 3: Characterize Field Conditions

Traffic Data Studies:


Traffic Volume – Turning Movement, ADT

Spot Speeds

Traffic Conflict Study

Sight Distance Evaluation

Non-Crash Based Procedures


Step 4: Identify Contributing Factors and Appropriate Countermeasures


*Table 12, from Missouri HAL Manual

Step 5: Assess Countermeasures and Select Most Appropriate


Probable Causes for Crash Patterns and Engineering Countermeasures


Rear-End Collisions at un-signalized intersection


• 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


• “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


Install Warning Signs

Estimated Crash Reduction Factors


* From MN DOT Traffic Safety Fundamentals Handbook

Traffic Crash Costs:


*NCHRP 440

Traffic Crash Costs:



Typical Benefit/Cost Ratios



Step 6: Implement Countermeasures and Evaluate Effectiveness


Improve Sight Distance – 5% Crash Reduction per Quadrant – 20% for all 4 quadrants


Review Questions:

What are the six steps in the crash mitigation process?

Questions?
