January/February 2003
Making Two-Lane Roads Safer
by Raymond A. Krammes and Carl Hayden
IHSDM is an invaluable safety evaluation software package for
highway designers and planners.
The social, environmental, and economic context in which today's
highways are designed demands trade-off assessments that require more
explicit and quantitative consideration of safety issues than is possible
with available tools. The Federal Highway Administration's (FHWA)
Interactive Highway Safety Design Model (IHSDM) is a suite of software
analysis tools for evaluating safety and operational effects of geometric
design decisions on two-lane rural highways. IHSDM will provide highway
project planners, designers, and reviewers in State and local departments
of transportation (DOTs) and engineering consulting firms with a suite
of safety evaluation tools to support these assessments.
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Today's citizens demand context-sensitive designs that protect
the environment without compromising safety, such as this
repaired roadway in the Sierra Nevada Mountains of California.
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The 2003 release of IHSDM culminates a multiyear research and development
effort. Highway project decisionmakers now can use IHSDM to check
designs for conformance with design policy, estimate their expected
safety performance, and diagnose potential safety and operational
issues throughout the highway design process.
A Need in Today's Highway Design Environment
Traditionally, designers have relied on compliance with design policy
to assure an acceptable level of safety. In today's highway development
environment, citizens are asking designers for more context-sensitive
designs with broader application of the flexibility afforded by design
policy without compromising safety. Making decisions in this environment
calls for more detailed, quantitative estimates of a design alternative's
expected safety performance.
As Timothy R. Neuman, vice president and chief highway engineer for
CH2M HILL, puts it, "We now operate in an era of increasingly challenging
project choices and decisions involving issues of competing values
and priorities, citizen involvement, and limited resources for projects.
Designers need to understand the explicit safety consequences of their
design plans, and they need to be able to communicate this information
to project stakeholders."
Ronald Erickson, geometrics engineer for the Minnesota DOT, adds,
"Especially with the difficult challenge of replacing technical staff,
the application of IHSDM will be a valuable tool for project engineers
to evaluate and check their designs."
IHSDM 2003
Through IHSDM's quantitative estimates of the expected safety performance
of geometric design, IHSDM will help project planners, designers,
and reviewers make more cost-effective decisions about safety measures
within cost constraints, context, and other considerations.
IHSDM is intended for use throughout the highway design process—from
preliminary planning and engineering through detailed design to final
review. It may be used both for projects to improve existing roadways
and projects to construct new roadways.
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This image of a road through the mountains, along with the
slogan, "Safer Roads Through Better Design," is
part of the logo for IHSDM, a suite of software analysis tools
for evaluating the safety and operational impacts of highway
geometric design.
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The 2003 release of IHSDM for two-lane rural highways has five evaluation
modules: (1) policy review, (2) crash prediction, (3) design consistency,
(4) intersection review, and (5) traffic analysis. Each module provides
different measures of the expected safety performance of an existing
or proposed highway geometric design.
The policy review module automates the current process of checking
a design against applicable, quantitative design guidelines. The crash
prediction module provides quantitative safety performance measures,
including expected crash frequency and severity.
The remaining modules diagnose factors contributing to safety performance.
The design consistency module assesses operating speed consistency.
The intersection review module evaluates design elements that influence
the safety performance of at-grade intersections. The traffic analysis
module evaluates traffic operations on the roadway under current or
projected traffic loads.
Additional capabilities are planned for future releases. Research
is underway to develop capabilities for IHSDM to perform similar evaluations
of multilane rural highways. A sixth evaluation module for two-lane
rural highways, driver/vehicle, is also under development and will
provide measures of vehicle dynamics, including lateral acceleration
as well as rollover and skidding potential.
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Crashes on horizontal curves are overrepresented on two-lane
rural highways, such as the one shown in this photo.
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Policy Review Module
The policy review module checks roadway-segment design elements for
compliance with relevant highway geometric design policies. The module
provides electronic files replicating quantitative policy values specified
by the American Association of State Highway and Transportation Officials
in the 1990, 1994, and 2001 editions of A Policy on Geometric Design
of Highways and Streets and automates checks of design values
against those policy values. IHSDM also provides a tool for inputting
policy tables from other agencies' design policies.
The module organizes checks into four categories: cross section,
horizontal alignment, vertical alignment, and sight distance. Cross-section
checks include through-traveled way width and cross slope, auxiliary
lane width and cross slope, shoulder width and cross slope, cross
slope rollover on curves, clear zone and roadside slope, normal ditch
design, and bridge width. Horizontal alignment checks include radius
of curvature, superelevation rate and transition design, length of
horizontal curve, and compound curve ratio. Vertical alignment checks
include tangent grade and vertical curve length. The policy review
module also can check stopping, passing, and decision sight distance.
This module can be applied at several stages in the highway design
process. During project planning and scoping for improvement projects
on existing roadways, it can provide an initial assessment of how
the existing geometric design compares to current design guidelines.
For all projects, it can facilitate quality assurance checks throughout
detailed design and design review.
Charles Clements, roadway design engineer with the Arkansas State
Highway and Transportation Department, says, "The policy review module
[can] be used to not only check our current designs, but we [can]
also use it to check existing roadways to see what revisions could
be made in their design to bring them up to today's standards."
David Olson of the Washington State DOT adds, "Considering the number
of new engineers working in design, the policy review module will
be invaluable in quickly analyzing how well a design complies with
design policy. Not only is it a great tool for tracking compliance
with policy, it also has an educational element for new designers."
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One-third of all crashes on rural two-lane highways occur
at intersections.
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Crash Prediction Module
The crash prediction module estimates the frequency of crashes expected
on a roadway based on its geometric design and traffic characteristics.
The crash prediction algorithm considers the effect of a number of
roadway variables: lane width, shoulder width and type, horizontal
curve length and radius, presence of spiral transition, superelevation,
grade, driveway density, passing lanes and short four-lane sections,
two-way left-turn lanes, and roadside hazard rating. Intersection
variables considered include skew angle, traffic control, presence
of left- and right-turn lanes, and sight distance.
The algorithm for estimating crash frequency combines statistical
base models and accident modification factors. FHWA derived the base
models using crash data from four States. Base models are available
for roadway segments and for three types of intersections: three-legged
intersections with stop control on the minor-road approach, four-legged
intersections with stop control on the minor-road approaches, and
four-legged signalized intersections.
The accident modification factors adjust the base model estimates
for individual geometric design element dimensions and for traffic
control features. The factors are the product of an expert panel review
of related research findings and consensus on the best available estimates
of quantitative safety effects of each design and traffic control
feature. The algorithm can be calibrated by State or local agencies
to reflect roadway, topographic, environmental, and crash-reporting
conditions. The algorithm also provides an Empirical Bayes procedure
for a weighted averaging of the algorithm estimate with project-specific
crash history data.
The crash prediction module can provide input for scoping improvement
projects on existing roadways, comparing the relative safety performance
of design alternatives, and assessing the safety cost-effectiveness
of design decisions.
Describing how he envisions using this module, Charles Clements,
of the Arkansas State Highway and Transportation Department, says,
"The crash prediction module [can] be used to predict the number of
crashes along a particular design. By using it in this manner, we
could avoid hazardous accidents in design before allowing the project
to be constructed."
Dewayne Sykes, assistant State roadway design engineer with the North
Carolina DOT, observes, "The number of crashes is an indicator of
safety easily understood by the nontransportation engineer. A tool
that can provide this information will be invaluable when evaluating
proposed highway designs and when explaining the relative safety of
these designs to the public as well as others."
IHSDM Technical Working Group
An IHSDM Technical Working Group met annually to review progress
and provide feedback to help ensure that the software met its
intended goal. FHWA also received input through numerous presentations
and demonstrations to State DOTs, engineering consulting firms,
and universities, and at international, national, regional,
and State conferences.
The IHSDM Technical Working Group includes design, safety,
and traffic engineering representatives from seven State DOTs
(Arkansas, California, Illinois, Minnesota, North Carolina,
Washington, and West Virginia), as well as an FHWA Division
Office and Resource Center:
Jerry Champa, Traffic Operations Program, California
DOT
Charles Clements, Engineer of Roadway Design, Arkansas
State Highway and Transportation Department
Ronald Erickson, Geometric Engineer, Minnesota DOT
William Fitzgerald, Safety/Geometric Design Engineer,
Eastern Resource Center, FHWA
Charles (Ray) Lewis, Planning and Research Engineer,
West Virginia DOT
David Olson, Assistant Design Policy, Standards and Safety
Research Engineer, Washington State DOT
Lloyd Rue, Traffic and Safety Engineer, Montana Division,
FHWA
Scott Stitt, Program Development Engineer, Illinois DOT
Dewayne Sykes, Assistant State Roadway Design Engineer,
North Carolina DOT
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Forty-one percent of crashes that involve fatalities occur
on two-lane rural highways. Emergency response units at this
crash site work toward saving lives and rescuing the injured.
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Design Consistency Module
The design consistency module helps diagnose safety concerns at horizontal
curves. Crashes on two-lane rural highways are overrepresented at
horizontal curves, and speed inconsistencies are a common contributing
factor to crashes on curves. This module provides estimates of the
magnitude of potential speed inconsistencies.
The design consistency module uses a speed-profile model that estimates
85th percentile, free-flow, passenger vehicle speeds at each point
along a roadway. (See "Managing Speed" on page 48.) The speed-profile
model combines estimated 85th percentile speeds on curves (horizontal,
vertical, and horizontal-vertical combinations), desired speeds on
long tangents, acceleration and deceleration rates exiting and entering
curves, and an algorithm for estimating speeds on vertical grades.
The model was calibrated using speed data collected at horizontal
curves and their approach tangents in six States. The module identifies
two potential consistency issues: (1) large differences between the
assumed design speed and estimated 85th percentile speed, and (2)
large changes in 85th percentile speeds from an approach tangent to
a horizontal curve.
Design consistency evaluations provide valuable information for diagnosing
potential safety issues on existing highways. They also provide quality
assurance checks of proposed preliminary and final alignment designs.
David Olson of the Washington State DOT explains, "The ability to
evaluate design consistency from a perspective of driver expectancy
is a tremendous addition to our project development toolset. Inattentive
application of design guidelines presents opportunities for designs
that conform to design policies and yet surprise the driver. The design
consistency module is specifically developed to identify surprises
and bring them to the attention of the designer. Once these locations
have been identified, the designer can make modifications and quickly
reevaluate their impacts."
Ron Erickson of the Minnesota DOT adds, "This is my favorite module
because, as a designer, evaluating a project for design consistency
is really difficult and for that reason rarely done. Now the design
consistency module will add another quality check into our design
process."
Intersection Review Module
The intersection review module includes both policy and diagnostic
review capabilities. The policy review component checks the following
intersection design elements for compliance with design policy: corner
radius, turn lane design, intersection angle, and intersection sight
distance triangles.
The diagnostic review component is an expert system that leads the
user through a systematic evaluation of an existing or proposed intersection
geometric design to identify potential safety concerns and possible
treatments to address those concerns. The review considers design
issues including:
- Intersection configuration: multileg intersections, skewed intersections,
offset-T intersections, and more than one minor-road approach on
the same side of the major road
- Horizontal alignment: intersection on horizontal curve, curve
on intersection leg, and approach alignment differing between opposing
approaches
- Vertical alignment: intersection on crest vertical curve, crest
or sag vertical curve on intersection approach, steep grade through
intersection, and continuity of minor-road profile through intersection
- Intersection sight distance
The intersection review module can provide useful input to project
scoping, preliminary engineering, and design review. "Most accidents
occur at intersections, making them key safety concerns," says
Dewayne Sykes of the North Carolina DOT. "The intersection review
module with its diagnostic review component will help designers produce
a better, safer intersection design."
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The intersection review module's diagnostic review window
leads users through a series of queries and responses.
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Traffic Analysis Module
The traffic analysis module uses the TWOPAS traffic simulation model
to estimate traffic quality-of-service measures for an existing or
proposed design under current or projected future traffic flows. The
traffic analysis module facilitates use of TWOPAS by feeding it the
roadway geometry data stored by IHSDM.
TWOPAS is the microscopic traffic simulation model used to develop
the two-lane highway chapter of the Transportation Research Board's
(TRB) Highway Capacity Manual. TWOPAS produces measures including
average speed and percentage of time spent following other vehicles.
The traffic analysis module is particularly useful during project
scoping and preliminary engineering for evaluating the operational
performance of alternatives to two-lane cross sections, including
passing lanes, climbing lanes, and short, four-lane sections.
Integrating IHSDM with Other Software
To facilitate safety evaluations in the highway design process, IHSDM
will be able to import horizontal and vertical alignment and cross-section
geometric data from commercial roadway design software packages. To
achieve this integration, FHWA entered into cooperative research and
development agreements with commercial vendors of roadway design software,
including Bentley Systems' GEOPAK Corporation and CAiCE Software Corporation.
FHWA also is supporting standard design data exchange formats, including
LandXML.
Regarding the impact of integration with IHSDM, Alan Akman, CEO of
CAiCE, notes, "We expect many of our State DOT and municipal clients
to make this a standard part of their engineering design process as
we achieve full integration."
Finally, FHWA Administrator Mary E. Peters, concludes, "These agreements
are ideal examples of how public-private partnerships help bridge
the gap between research and real-world applications. The integration
of these products provides the highway industry with critical access
to a tool that will significantly improve highway safety."
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Seventy-seven percent of America's highways are two-lane
roads similar to this one.
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Raymond A. Krammes is a highway research engineer and roadway
team leader in FHWA's Office of Safety Research and Development. Krammes
manages development of IHSDM. He is a registered professional engineer
(Texas) and received his B.S., M.S., and Ph.D. in civil engineering
from The Pennsylvania State University.
Carl Hayden is a highway engineer and has served in FHWA's
safety group, now the Office of Safety Design, for 26 years in various
positions related to wet weather safety, highway design, safety data
analysis, and traffic records. He is a registered professional engineer
(DC), received his B.S. in civil engineering from the Virginia Polytechnic
Institute and State University, and completed FHWA's Highway Engineering
Training Program.
The 2003 release of IHSDM for two-lane rural highways is distributed
through the IHSDM Web site, www.tfhrc.gov/safety/ihsdm/ihsdm.htm.
See the site for details on registering, downloading the software,
and contacting FHWA for technical support, software maintenance, and
training.
Other Articles in this issue:
Saving Lives: A Vital FHWA Goal
Helping Research Pay Off
Safer Roadsides
Making Two-Lane Roads Safer
Driving After Dark
Reducing Points of Conflict
Life in the Crosswalk
Pushing through the Safety Plateau
Data is Key to Understanding and Improving Safety
Managing Speed