NC Local Safety Partnership Evaluation Methods

Workshop Roadmap Program Background and Overview Crash Data Identifying Potential Treatment Locations Preparing Collision Diagrams Selecting Interventions Evaluation Methods Program Implementation and Discussion 2

Module Objective ￭ Understand why evaluation is important ￭ Learn about the two basic kinds of evaluation methods ￭ Learn how to perform a simple before- after analysis 3

How well has a treatment, project, or group of projects reduced crash frequency or severity? Safety Effectiveness Evaluation Basics Evaluation 4

Safety Effectiveness Evaluation Basics Why do safety effectiveness evaluations? ￭ Prove investment effectiveness ￭ Demonstrate program value to decision makers (accountability) ￭ Contribute new scientific knowledge ￭ Improve decisions ￭ Optimize future safety investments

Results in Winston-Salem MetricBeforeAfterChange Targeted Crashes8,5024,078-51% Total Crashes18,65915,129-18% Injuries11,0587,578-31% Property Damage$52.3 mil.$41.9 mil.-19% ￭ Result of 858 Low Cost Treatments in Winston-Salem over past 25 years 6

￭ Goal – Measure true effect of a countermeasure ￭ We want to be sure that the observed change is due to the countermeasure alone ￭ What other factors could cause the change? Safety Effectiveness Evaluation Basics 7

￭ What other factors could cause the change? ￭ Other “treatments” at the same time (e.g., primary seat-belt law at the same time as adding a protected left-turn phase to intersections) ￭ Changes in AADT ￭ Regression to the mean ￭ Underlying trends in crashes (e.g., economy- related changes) ￭ Others ￭ So how do we control for/discount these other “causes”? 8

Two Basic Evaluation Study Designs 1. Before-after studies 2. Cross-sectional studies Choice of method is affected by: Nature of treatment Site type Available data 9

Before-After Study ￭ Examines crash data before and after the treatment is installed ￭ Types of before-after studies ￭ Simple before-after study ￭ Does not account for certain biases ￭ Before-after study with reference/comparison groups ￭ Accounts for changes in volumes and other factors 10

Simple Before-After Study Estimated average expected crash frequency without treatment Measured Is this assumption realistic? TREATMENT ASSUME these are the crashes WITHOUT TREATMENT 11

Before-After Study with Reference/Comparison Group Expected average crash frequency without treatment Measured Expected average crash count without treatment TREATMENT 12

Before-After Study with Reference/Comparison Group Estimated average expected crash frequency without treatment Measured 13

Cross-Sectional Study ￭ Compare crash data for sites with and without treatment over same time period 14

Cross-Sectional Study ￭ Why do a cross-sectional study? ￭ Treatment installation dates unknown ￭ Volumes and crash counts in before period unknown 15

Evaluation Study Type Selection Guide Evaluation Method Treatment SitesNontreatment Sites Before Data After Data Before Data After Data Simple Before-AfterXX Before-After Using Reference/Comparison Group XXXX Cross-Sectional StudyXX 16

EXAMPLE SIMPLE BEFORE- AFTER EVALUATION Module 6 – Evaluation Methods 17

Example Simple Before-After Evaluation ￭ Traffic Signal Installation in Wake Co. ￭ NCDOT Safety Evaluation Unit conducted before-after evaluation ￭ Intersection of SR 1004 (East Garner Road) and SR 2555 (Auburn - Knightdale Road) ￭ Before: Two-way STOP-controlled ￭ After: Traffic signal (actuated) 18

Example Simple Before-After Evaluation 19

Example Simple Before-After Evaluation ￭ Signal installed in October 2003 ￭ Before period: Nov 1998 – Sept 2003 ￭ After period: Dec 2003 – Sept

Example Simple Before-After Evaluation BeforeAfter Percent Reduction (-) Percent Increase (+) Total crashes % Target Crashes345-85% Fatal injury Crashes00N/A Class A injury Crashes20-100% Class B injury Crashes70-100% Class C Injury Crashes94-56% Total Injury Crashes184-78% Total Severity Index % Target Crash Severity Index % Volume8,8009,3006% 21

Example Simple Before-After Evaluation 22

Example Simple Before-After Evaluation 23

EXAMPLES FROM WINSTON- SALEM EVALUATIONS Module 6 – Evaluation Methods 24

Before and After Collision Diagrams 25

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Summary 27

Graph of Poisson Distribution 28

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Take Away Messages ￭ Evaluation helps justify investments and improves decision making ￭ Pros and cons of different types of evaluation ￭ Steps for conducting simple before/after evaluation 33