Topic 2

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Topic 2 Network Screening CEE 763
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OBJECTIVES

• Identify locations for further study which have
  both
• A high risk of crash losses
• An economically justifiable opportunity for
  reducing the risk
• Identify countermeasure options and priorities
  which maximize the economic benefits
• It is as much about exclusion of sites from
  consideration as it is about inclusion

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NETWORK SCREENING

• Key tool in a highway safety improvement program
• Definition
• A process which aims to identify locations within
  the road system where correctable crashes are
  found in order to develop appropriate and
  cost-effective treatments to reduce the frequency
  or severity of crashes

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EFFECTIVENESS

• It is important to identify sites with the most
  promise for improvement as engineering studies
  are expensive. Agencies have limited budgets, and
  if a site with potential is not identified, an
  opportunity to substantially improve safety is
  missed.

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SOME TYPICAL NAMES

• High crash location
• High accident potential
• Black spot
• High risk location
• Top 5
• Crash concentration

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Terms Site and Facility

• Site a basic safety study location, e.g., a
  segment (homogeneous), an intersection, and a
  freeway ramp
• Facility a contiguous set of sites
• Freeway (segments, ramps)
• Urban and suburban arterials (segments,
  intersections) divided, undivided, signalized,
  TWSC etc.
• Rural highway (segments) two-lane, multi-lane
• HSM only covers predictive methods for certain
  facility types

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NETWORK SCREENING PROCESS

• Establish focus
• Sites with potential to reduce crash frequency
• Specific crash types or severity
• Identify sites and reference population
• Type of site segments, intersections, ramps
• Sites of similar characteristics
• Select performance measures
• Frequency, rate, severity, etc.
• Select screening method
• Ranking, sliding window, peak searching etc.
• Screen and evaluate results

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ESTABLISH FOCUS
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PERFORMANCE MEASURES

• Crash frequency
• Crash rate
• Quality control
• Excess predicted crash frequency using method of
  moments
• Critical rate
• Crash severity
• Equivalent property damage only (EPDO) crash
  frequency
• Relative severity index
• Level of service of safety
• Excess predicted average crash frequency using
  SPFs
• Probability of specific crash types exceeding
  threshold proportion
• Excess proportion of specific crash types
• Expected crash frequency with EB adjustment
• Excess expected crash frequency with EB
  adjustment

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CRASH FREQUENCY

• Method
• Rank locations with highest count of crashes for
  investigation
• Benefits
• Simple
• Focuses on areas with most crashes
• limitations
• Does not account for exposure
• Favors high-volume, urban locations
• Engineering fix may not be present

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CRASH RATE

• Method
• Rank locations by rate of crashes
• Benefits
• Accounts for exposure
• Relatively simply
• Efforts focused on potential problem not just
  high volume locations
• Limitations
• Favors low volume, low collision sites
• Cannot compare cross different volumes

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INTERSECTION RATES

• Crashes per million entering vehicles (MEV)

Ri intersection crash rate N number of
crashes in the study period n number of years
in the study period TEV the sum of volumes
entering from all approaches, in Average Daily
Traffic
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EXAMPLE

• Observed 46 crashes in two years. The ADT for
  the minor approach was 3000 and the major
  approach was 6000. Note - volumes includes both
  directions. What is the crash rate?

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SEGMENT RATES

• Crashes per million vehicle miles of travel
  (MVMT)
• Example
• Observed 40 crashes on a 17.5 mile segment in one
  year. The ADT was 5,000.

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CRASH AND VOLUME
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FREQUENCY-RATE CRITERIA

• Method
• Rank by combination of frequency and rate based
  methods
• Various ways to combine rankings for composite
  rankings
• Benefits
• Simple
• Address drawbacks of both the frequency and rate
  methods
• Drawbacks
• Final ranking dependent of combination

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EXAMPLE

• Five intersections have the following crash
  frequency and crash rate.
• If a critical frequency is set at 10, and a
  critical crash rate is set at 1.5, which
  intersection(s) should be ranked as high crash
  locations?

Crash Data Intersections Intersections Intersections Intersections Intersections
Crash Data 1 2 3 4 5
Frequency 7 12 4 14 10
Rate 0.5 1.5 2.1 1.0 1.8
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QUALITY CONTROLRate or Frequency

• Method
• Rank location if the crash rate or frequency at a
  site is statistically significantly higher than a
  predetermined rate or frequency for locations of
  similar characteristics
• Benefits
• Based on Poisson distribution
• Seems to identify locations with possible
  treatments
• Drawbacks
• More data is required
• Categorization is key

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QUALITY CONTROL

• Method
• 1) Select average rate or frequency for similar
  facility
• 2) Calculate the critical rate or frequency
• 3) Compare actual rate or frequency
• 4) Flag or rank if exceeds

RC critical rate or critical frequency Ra the
average rate or frequency for similar facility P
probability constant based on desired level of
significance (1.645 for 95) M millions of VMT
or entering vehicles
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EXAMPLE

• There were 40 observed crashes on a 17.5 mile
  segment in one year. The ADT was 5,000. Given the
  average rate for similar segments is 1.02 MVMT,
  does the subject segment exceed the critical rate
  at 95 confidence?

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SEVERITY

• Method
• Rank locations by weighting the severity of
  crashes
• Benefits
• Adds severity to the frequency method
• Usually relates to benefit/cost selection
• Drawbacks
• Dependent on weighting, may concentrate on fatal
  collisions
• Weights are essentially arbitrary since it
  assigned from global crash costs

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EQUIVALENT PROPERTY DAMAGE ONLY (EPDO) CRASH
FREQUENCY
EPDO Equivalent property damage only crashes fi
weight for crash type I Ni
number of crashes of type i
Severity Cost Weight
Fatal (K) 4,008,900 542
Injury (A,B,C) 82,600 11
PDO (O) 7,400 1
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EXAMPLE

• A location has experienced 2 fatal, 12 injury A,
  30 injury B, 40 injury C, and 140 PDO crashes in
  5 years. What is the EPDO crashes?
• Fatal 3,400,000
• A 260,000
• B 56,000
• C 27,000
• PDO 4,000

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RELATIVE SEVERITY INDEX (RSI)
relative severity index cost for
intersection i RSIj relative severity
index cost for crash type j
Crash Type Number of Crashes Cost per Crash
Rear End 19 13,200
Sideswipe 7 34,000
Angle 5 61,100
Fixed Object 3 94,700
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RSI EXAMPLE

• An intersection has the following crashes.
  Determine the RSI for this intersection

Crash Type Number of Crashes Cost per Crash
Rear End 19 13,200
Sideswipe 7 34,000
Angle 5 61,100
Fixed Object 3 94,700
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SAFETY INDICES

• Method
• Rank locations by creating an index which
  includes a number of factors such as rates,
  frequencies, severities, and possibly site data.
  A weighted average or scores are then combined to
  calculate a composite index. The Relative
  Severity Index discussed earlier is one of these
  types.
• Benefits
• Simple and attempts to combine criteria
• Drawbacks
• Rank is sensitive to weights of scores which are
  usually assigned arbitrarily

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ODOT SAFETY PRIORITY INDEX SYSTEM(SPIS)

• Composite score assigned for frequency, severity,
  and rate
• 3 years data, 0.10 mile sections
• Maximum index is 100
• 25 points max for frequency
• 25 points max rate
• 50 points max severity
• Total score Sum of Indicator values (IV) of
  Frequency, Rate, and Severity

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SAFETY PRIORITY INDEX SYSTEM
Note Max SPIS score is 100
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EXAMPLE

• 0 Fatal, 1 A, 0 B, 3 C, 4 PDO. ADT 14,200.

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EXAMPLE

• 0 Fatal, 1 A, 0 B, 3 C, 4 PDO. ADT 14,200.

Answer SPIS Score 38.27
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POTENTIAL ACCIDENT REDUCTION

• Method
• Rank or flag locations where the difference
  between observed and expected crash experience
  will maximize benefits if their crash history can
  be reduced to the expected value.
• Benefits
• Most uses frequency rather than rates
• Can account for regression to the mean
• Drawbacks
• Data hungry, expected values must be predicted

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EXCESS PREDICTED CRASH FREQUENCY USING METHOD OF
MOMENTS

• Calculate average crash frequency per reference
  population
• Calculate crash frequency variance
• Calculate adjusted observed crash frequency per
  site
• Calculate potential for improvement (PI) per site
• Rank site according to PI (highest to lowest)

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EXAMPLE

• An unsignalized intersection has observed 11
  crashes in a year. Suppose among all the
  unsignalized intersections, the average crashes
  per year is 8, and the standard deviation of
  crash for all the intersections is 3. Calculate
  the PI for this intersection.

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EXCESS PREDICTED CRASH FREQUENCY USING SAFETY
PERFORMANCE FUNCTIONS

• Calculate expected crash frequency using SPF
• Calculate excess predicted average crash
  frequency
• Rank site according to the excess frequency

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EXAMPLE

• An unsignalized intersection has observed 11
  crashes in a year. According to the SPF developed
  for all the unsignalized intersections, the
  predicted crash frequency per year is 8. What is
  the excess predicted crash frequency?

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EMPIRICAL BAYES METHODS
Crash Frequence
E(k) is the predicted value at similar sites, in
crash/year Y is the analysis period in number of
years f is over-dispersion factor
Volume
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SAMPLE DATA
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SAMPLE DATA
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CRASH FREQUENCY WITH EB ADJUSTMENT

• Step 1 Calculate the predicted average crash
  frequency using an SPF
• Step 2 Calculate annual correction factor

Year Predicted Average Correction factor
1 2 3 2.5 2.5 2.7 1.0 1.0 ?
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CRASH FREQUENCY WITH EB ADJUSTMENT

• Step 3 Calculate EB weighting factor,Note
  rely on dispersion factor or variance.

Year Predicted Average
1 2 3 2.5 2.5 2.7
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CRASH FREQUENCY WITH EB ADJUSTMENT

• Step 4 Calculate first year EB adjusted average
  crash frequency.

Year Predicted Average Observed Crashes
1 2 3 2.5 2.5 2.7 11 9 14
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CRASH FREQUENCY WITH EB ADJUSTMENT

• Step 5 Calculate final year EB adjusted average
  crash frequency.
• Step 6 Calculate the variance (optional)
• Step 7 - Rank sites based on the EB adjusted
  expected average crash frequency for the final
  year.

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OTHER CRITERIA

• Level of service safety (LOSS)
• Konokov et al. (Colorado DOT)
• Method of moments
• PIARC manual
• Proportions testing
• Exceeding a particular crash type
• Rank locations bases on the current annual cost
  of crashes based on average cost of crash by
  accident type

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WHICH CRITERIA TO USE?

• Little consensus on methods
• The key issue is how the criteria adopted direct
  the analyst to consider sites which contributes
  to the overall road safety goal, namely the
  maximization of benefits of road safety treatments

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METHOD USAGE

• All of the methods are in use either alone or in
  combination
• In US states
• Crash frequency by 15
• Crash rate or RQC by 15 of agencies
• Crash severities by 50 of agencies
• Indices by 18
• Other by 16

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MORE PRECISE DEFINATION OF SITE

• Three alternatives (Hauer et al., TRR 1784
  Screening the road network for sites with
  promise)
• Based on Section
• Based on a uniform length of a roadway, e.g., 0.1
  mi
• Based on a minimum segment that identifies the
  highest accident frequency while satisfying the
  statistical limits (i.e., CV).

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SEARCHING ALGORITHMS
Expected
Segment average
Segment average does not correspond to the
highest
Expected
Segment average
Segments of different length with the highest
crash
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SLIDING WINDOW0.3-mile window with 0.1 increment
The window that has the highest risk is used to
rank the segment.
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EXAMPLE

• A roadway network has ten segments composed of
  three types of facilities. Using the sliding
  window method and the crash rate to rank Segments
  1 and 2.

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More Data

• Segment 1 starts at mile post 1.2 and ends at
  2.0. Segment 2 starts at mile post 2.0 and ends
  at 2.4.

Segment 1
Segment 2
1.2
2.0
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.1
2.2
2.3
2.4
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SLIDING WINDOW0.3-mi window with 0.1-mi increment
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SLIDING WINDOW0.3-mi window with 0.1-mi increment
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SLIDING WINDOW0.3-mi window with 0.1-mi increment
Sliding Window
Concepts Bridging Three Contiguous Roadway
Segments
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SLIDING WINDOW0.3-mi window with 0.1-mi increment
Sliding Window Concepts Window
Positions at the End of Contiguous Roadway
Segments When Window is Moved Incrementally by
0.1 Miles
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SLIDING WINDOW0.3-mi window with 0.1-mi increment
Sliding Window Concepts
Example of Position and Location of Sliding
Windows and Subsegments
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SLIDING WINDOW0.3-mi window with 0.1-mi increment
Sliding Window Concepts
Ranking Example
limiting value 40 acc/mi/yr
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EXAMPLE

• A segment with 2 lanes, rural
• ADT 6000
• Limiting frequency 10
• SPF
• Intercept-3.63
• ADT coefficient 0.53
• Over dispersion Parameter 0.5

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EXAMPLE
0.043
0.147
0.231
0.231
0.240
0.251
0.251
0.287
0.287
0.287
0.310
0.311
0.325
0.329
0.433
0.434
0.440
0.440
0.440
0.441
0.452
0.454
0.483
0.493
0.533
0.598
0.636
0.636
0.658
0.743
0.806
0.806
0.808
0.822
0.823
0.848
0.862
0.862
0.901
0.948
0.983
Accident locations (mile)
Site A 0-0.4 mile
Site C 0.9-1 mile Non contiguous
Site B 0.4-0.9 mile Contiguous
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PEAK SEARCHING0.1-mile window
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PEAK SEARCHING0.2-mile window
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PEAK SEARCHING0.4-mile window
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EXAMPLE

• A roadway segment is 0.47 miles long. Using a
  window length of 0.1 miles, the following crash
  data were obtained for each sub-segment.
  Calculate the CV for each sub-segment, and
  determine whether the search should continue with
  longer window sizes (assume the limiting CV is
  0.25).

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EXAMPLE-continued
Sub-segment Position Excess Expected Crash Frequency C.V.
B1 0.00-0.20 6.50
B2 0.10-0.30 4.45
B3 0.20-0.40 3.80
B4 0.27-0.47 7.15