Incorporating Safety into the
Highway Design Process
What is Meant by “Safe”?
• Is This Road Safe?
What is Meant by “Safety”?
• Is This Road Safe?
– Is a “Yes” or “No” answer sufficient?
– Would your answer change if you were told...
• The road averages 1 crash in 10 years? or...
• The road averages 100 crashes in 10 years?
Kinds of Safety
• Nominal Safety
– A road that conforms to the agency’s policy,
guidelines, and warrants is “nominally” safe
– A road either is, or is not, nominally safe
• Substantive Safety
– The performance of a roadway, as defined by
its “expected” crash frequency (i.e., long run
average)
– Substantive safety is a continuous variable
– Useful to compare one site with “typical” site
Kinds of Safety
• Safety Comparison
(NCHRP Report 480)
Safety-Conscious Design
• AASHTO Guidance
– “Consistent adherence to minimum [design
criteria] values is not advisable”
– “Minimum design criteria may not ensure
adequate levels of safety in all situations”
– “The challenge to the designer is
to achieve the highest level of
safety within the physical and
financial constraints of a project”
• Highway Safety Design and
Operations Guide, 1997
Highway Crashes
• Contributing Factors
– Driver
• Age, gender, skill, fatigue level, alcohol, etc.
– Vehicle
• Type, age, maintenance, etc.
– Environment
• Light conditions, weather, precipitation, fog, etc.
– Roadway
• Geometric design, traffic control, etc.
• Focus of current research
– Geometric design of the roadway
Quantifying Safety
• Safety Prediction Model
– C = base crash rate × volume × length × AMF
• Accident Modification Factor (AMF)
– AMF used to estimate change in crashes due
to a change in geometry (AMF = Cwith/Cwithout)
– Example:
• AMFadd bay = 0.70
• Cno bay = 10 crash/yr
• Cwith bay = Cno bay × AMFadd bay = 7 crashes/yr
– Crash reduction factor (CRF) = 1 - AMF
Crash Data
• Existing Crash Databases
– Texas Department of Public Safety (DPS)
– Local databases
• Severity Scale
– K: Fatal
– A: Incapacitating injury
– B: Non-incapacitating injury
– C: Possible injury
– PDO: property damage only
Research
focus
• Reporting Threshold
– $1000, informally varies among agencies
Crash Data Variability
• Examination of Crash History
– Annual crash counts: 2, 3, 1, 1, 7, 5, 2...
– Count in any one year is effectively random
– Variability year to year is LARGE
– So large that...
• It is very difficult to determine if the change in count
from year to year is due to a change in geometry,
traffic volume, or traffic control device
• It can frustrate efforts to reduce crashes (a change
was made but crashes increased)
• It can fool us into thinking a change that we made
significantly reduced crashes (when it really did not)
Crash Data Variability
• Questions
Each data point
represents 1 year of
crash data at one site
Crash Frequency, crashes/yr
– What is the true mean crash frequency at
this site?
– Is a 3-year average reliable?
8
6
4
2
0
0
5
10
15
20
Year
25
30
35
Crash Data Variability
• Observations
– The average of 3 years (= 6 crashes)...
• 2.0 crashes/yr
• 0.7 to 4.3 crashes/yr (± 115%)
– The average of 35 years (= 100 crashes)…
– One site rarely has
enough crashes to
yield an average
with a precision of
± 20%
8
Crash Frequency, crashes/yr
• 2.8 crashes/yr
• 2.2 to 3.3 (± 20%)
Upper Limit (95% confidence interval)
6
Running Average
4
2
Lower Limit
0
0
5
10
15
20
Year
25
30
35
Influence of Design
• Question
– 15 intersections have left-turn bays added
– Research shows bays reduce crashes by 20%
Each data point
represents 1 year of
crash data
18
Crash Frequency, crashes/yr
– What crash
frequency do
you expect for
site 4 after the
bay is installed?
16
14
12
Average = 10
10
8
6
Site 4
4
2
Before Bay
After Bay
0
0
5
10
0
15
5
20
Site
10
25
15
30
35
Influence of Design
• Observations
– Random variation makes trend difficult to see
– Most sites show crash reduction
18
Crash Frequency, crashes/yr
– Site 4, and a few
other sites, had
more crashes
– This does not
mean bay won’t
be effective in
long run
Site 4
16
14
12
10
8
6
Site 4
4
2
Before Bay
After Bay
0
0
5
10
5
20
0
15
Site
10
25
15
30
35
Influence of Design
• Observations
– Distribution of crash change for sites with
average of 10 crashes/yr and 20% reduction
0.10
0.09
0.08
0.07
Probability
– When reduction
is small, random
variation will let
crash frequency
increase at
some sites in
the year after
0.06
0.05
32% of sites
experience an
increase in
crashes in the
year after
treatment due to
random variation
0.04
0.03
0.02
0.01
0.00
-15.00
-10.00
-5.00
0.00
5.00
Change in Annual Crash Frequency
10.00
Overcoming Variability
• Large variability makes it difficult to
observe a change in crash frequency
due to change in geometry at one site
• Large variability in crash data may
frustrate attempts to confirm expected
change
• Large databases needed to overcome
large variability in crash data
• Statistics must be used to accurately
quantify effect
Background Research
• National Research Sources
– Safety design guidelines
• NCHRP Report 500: Guidelines for
Implementing the AASHTO Strategic Highway
Safety Plan
–
–
–
–
–
Vol. 5: Unsignalized intersections
Vol. 7: Horizontal curves
Vol. 8: Utility poles
Vol. 12: Signalized intersections
Vol. 13: Heavy trucks
• Volumes can be found at:
http://safety.transportation.org/guides.aspx
Background Research
• National Research Sources
– Safety evaluation tools
•
•
•
•
Interactive Highway Safety Design Model
Safety Analyst (forthcoming)
Highway Safety Manual (forthcoming)
Prediction of the Expected Safety Performance
of Rural Two-Lane Highways
Background Research
• TxDOT Project 0-4703
– “Incorporating Safety into the
Highway Design Process”
– Project Director:
• Elizabeth Hilton
– Main products:
• Roadway Safety Design Synthesis
(Report 0-4703-P1)
• Interim Roadway Safety Design
Workbook (Report 0-4703-P4)
Available at: tcd.tamu.edu, click on
“Products”
Facility Types
• IHSDM
– Two lane highways
• Highway Safety
Manual
– Two lane highways
(& intersections)
– Rural multilane
highways (&
intersections)
– Urban streets (&
intersections)
• TxDOT 0-4703
– Freeways
– Rural highways
• Multilane rural
• Two lane rural
–
–
–
–
Urban streets
Freeway ramps
Urban intersections
Rural intersections
Safety Prediction Procedures
• Overview
– Six steps to procedure
– Evaluate a specific roadway segment or
intersection (i.e., facility component)
– Same basic technique for all methods
(IHSDM, HSM, TxDOT 4703)
• Output
– Estimate of crash frequency for segment or
intersection
Step 1
• Identify Roadway Section
– Define limits of roadway section of interest
• Limits of design project
• Portion of highway with safety issue or concern
– May include one or more components
Step 2
• Divide Section into Components
– Analysis based on facility components
• One intersection or
• One interchange ramp or
• One roadway segment
– Each component analyzed individually in
Steps 3 and 4
1
2
4
3
Homogeneous Segment
• Definition
– A homogeneous segment has the same
basic character for its full length
•
•
•
•
•
•
Lane width
Shoulder width
Number of lanes
Curvature
Grade
Horizontal clearance
Step 3
• Gather Data for Subject Component
– Data may include
• Roadway geometry (lane width, etc.)
• Traffic (ADT, truck percentage, etc.)
• Traffic control devices (stop sign, signal)
– What data do I need?
• It depends on the component…
Step 4
• Compute Expected Crash Frequency
– Use safety prediction model
• Model Components
– Base model
– Accident modification factors
Volume
Expected Crash
Frequency
Lane Width
Base Model
• Relationship
– Cb = base crash rate × volume × length
– Injury (plus fatal) crash frequency
• Calibration
– Analyst can adjust crash rate to local
conditions
• Application
– Crash frequency for “typical” segment
– Typical: 12 ft lanes, 8 ft outside shoulder, etc.
Accident Modification Factors
• Definition
– Change in crash frequency for a specific
change in geometry
– Adapts base model to non-base conditions
– One AMF per design element (e.g., lane width)
• Example: Two-lane highway
– Base condition: 12 ft lanes
– Roadway has 10 ft lanes
– AMF = 1.12
Steps 5 & 6
• Repeat Steps 3 and 4 for Each Component
• Add Results for Roadway Section
– Add crash estimates for all components
– Sum represents the expected crash frequency
for the roadway section
• If there are multiple alternatives, repeat
Steps 1 through 6 for each alternative
Questions?
More Information
• Safety Resources from Project 0-4703
– Workbook
– Synthesis
– Procedures Guide (Coming soon…)
– Texas Roadway Safety Design Software
• Web Address
– http:// tcd.tamu.edu/documents/rsd.htm
– Also link from DES-PD site CROSSROADS
– Check periodically for updates