Data Requirements for Data Requirements for Network Screening and Large Network Screening and Large

Scale Evaluation StudiesScale Evaluation Studies

Calvin J. Mollett, Calvin J. Mollett, M(Eng) M(Eng)

andand

Geni Bahar, P.Eng.Geni Bahar, P.Eng.

Presentation OutlinePresentation Outline

Network Screening

Safety Evaluations

Safety Performance Functions

Data Requirements

Data Format and Quality Issues

Network ScreeningNetwork Screening

Objective: To identify sites with a potential for cost

effective improvements

Conventional screening methods: Crash counts Crash rates

Are inefficient

Network ScreeningNetwork Screening

Inefficiencies: FALSE Positives

Sites incorrectly identified as unsafe Waste resources

FALSE Negatives Sites incorrectly identified as safe Unsafe locations remain untreated

Network ScreeningNetwork Screening

WHY ? Randomness in crash counts Crash counts can fluctuate up or down for no

apparent reason (not related to safety) Often site is identified during random upward

fluctuation

Network ScreeningNetwork Screening

The SOLUTION ? Screening based on potential for safety

improvement: Expected crash frequency Excess number of crashes

Network ScreeningNetwork Screening

Expected crash frequency

ObservedObserved

PredictedPredicted

ExpectedExpected ObservedObserved

ExpectedExpected

PredictedPredicted

A B

Network ScreeningNetwork Screening

Excess crashes

ObservedObserved

PredictedPredicted

ExpectedExpectedSafety Performance FunctionsSafety Performance Functions

Bayes TheoremBayes Theorem

Crash database Crash database

Safety EvaluationSafety Evaluation

Objective: How effective has a measure been in reducing

crashes ? Cost-effectiveness of measure Feasibility for future implementation

Safety EvaluationSafety Evaluation

Procedure

Determine number of Determine number of expected crashes – if expected crashes – if

measure was not measure was not implementedimplemented

Determine actual Determine actual number of crashes – number of crashes – after implementation after implementation

of measureof measure

Safety EffectSafety Effect

Safety EvaluationSafety Evaluation

How to determine expected number of crashes?

Conventional before-after method Average crash count during before period Unreliable due to regression-to-the-mean (RTM)

Safety EvaluationSafety Evaluation

Regression-to-mean

Safety EvaluationSafety Evaluation

Hauer (1997)

Method to estimate the expected number of crashes during the after period

SPFs (for untreated condition) Historical crash counts Actual traffic volumes

Safety Performance Safety Performance FunctionsFunctions

Crash prediction models

Two different types Level 1

E(Ki) = i(AADTi)

Level 2 E(Ki) = i(AADTi)1exp(2shldwidth)

Safety Performance Safety Performance FunctionsFunctions

Data requirements: Reference group Crash data Roadway attribute data Traffic volume data

Data RequirementsData Requirements

Traffic Volumes: Each year of study period Missing volumes to be estimated Links

AADT (both directions) Intersections

Approach AADT on each intersection

Data RequirementsData Requirements

Crash data: Minimum requirements

Location (accurate to within 0.1 mile) Severity (Fatal, Injury, Property Damage) Date

Before : 3 – 5 years After : 1 Year minimum Safety evaluations – identify target crashes

Data RequirementsData Requirements

Roadway attribute data: Identify reference groups (e.g 4 Lane Divided

Urban Freeways) To develop Level 2 (FULL) models

Data RequirementsData Requirements

Treatment Details: List of treatment locations Exact position - e,g route, begin mile point,

end mile point Date/Year of implementation

Data Format and Data Format and QualityQuality

Databases

Location Referencing

Roadway attribute data

Traffic volumes

Crash data

Data Format and Data Format and QualityQuality

Databases: All data – relational database files All files should be linkable – common

referencing system Sufficiency Files

Flat file Combination of different files Created annually

Data Format and Data Format and QualityQuality

Location referencing system: Ideal:

Route, Milepost Also:

Route, Segment, Offset Route, Reference point, Offset Route, Reference marker

Require conversion during data preparation

Data Format and Data Format and QualityQuality

Roadway attributes: Each record – unique identifier

Route number, mile points Intersection identification code

Create sufficiency files Consistency with crash data file

Data Format and Data Format and QualityQuality

Traffic Volumes: Traffic volumes between intersections Volume estimation procedures Development of annual counting programs

Data Format and Data Format and QualityQuality

Crash Data: Crash code changes - updating of historical

data Accuracy of crash location to 0.1 mile Consistency between time of crash and

ambient light Consistency between Impact type and vehicle

directions Uniformity between State databases - MUCC

ConclusionConclusion

Data is a valuable resource

Questions: Do you have the data? Are data of the right format and quality?

Upcoming safety tools SafetyAnalyst Highway Safety Manual

Thank YouThank You