January/February
2001
Improving
Roadside Safety by Computer Simulation
by
Dean L. Sicking and King K. Mak
Adapted
from a paper of the same title written by Drs. Sicking and Mak on
behalf of the Committee on Roadside Safety Features of the Transportation
Research Board (TRB). The original paper was published by TRB in January
2000 as part of Transportation in the New Millennium, State of
the Art and Future Directions, Perspectives From Transportation Research
Board Standing Committees.
The
overall level of safety provided on highways in this country has improved
greatly over the last several decades. The clearest demonstration
of this improvement in roadside safety is the continuing drop in fatality
rates. For example, from 1966 to 1996, the fatality rate in single-vehicle,
run-off-road crashes dropped by more than two-thirds from 1.9 to 0.6
fatalities per 100 million vehicle-miles (161 million vehicle-kilometers)
of travel. Some portion of this reduction can be attributed to improvements
in vehicle design and the increased use of occupant restraints; however,
improved roadside safety design and features also contributed to the
reduction.
The "clear zone" concept is perhaps the most important contributor
to roadside safety design. Based on this concept, roadside hazards
are removed or relocated farther from the traveled way whenever possible.
Cutting down trees and placing utility lines underground are examples
of roadside hazard removal, while extension of culverts and drainage
structures is a good example of relocating hazards farther away from
the roadway. When hazards cannot be removed or relocated, breakaway
devices or protective safety features, such as traffic barriers and
crash cushions, have been used to minimize the danger to motorists.
Before these roadside safety features are accepted for installation
along our highways, they must be tested to ensure proper performance.
It can be quite costly to crash a vehicle into every variation of
every feature, and so, computer simulation of vehicular impacts is
rapidly developing as a reliable alternative to full-scale crash testing.
Problems
to Tackle in the Future
Despite the great strides made in roadside safety over the last few
decades, many major roadside safety issues have yet to be addressed
in any serious manner.
Installation
Details
Although safety features are subjected to a costly array of full-scale
crash tests to ensure acceptable safety performance, significant differences
often exist between the tested and field installations. Virtually
all full-scale crash tests are conducted on flat ground, while very
few safety features are actually installed in this situation. In addition,
most ground-mounted devices are tested in a strong soil condition,
whereas field applications may vary from weak soils to portland cement
concrete. Crash test installations are typically
constructed with tight tolerances that are unlikely to be achieved
in actual field constructions. Restricted site conditions can also
present special problems to highway designers.
Nontracking
Impacts
Crash testing of roadside safety devices is typically limited to tracking
impacts. Unfortunately, crash data indicate that approximately half
of all run-off-road accidents involve nontracking vehicles, i.e.,
sliding sideways into an object. Also, nontracking impacts are found
to be more severe than tracking impacts for both barrier systems and
breakaway devices. Roadside safety features successfully tested for
tracking impacts may or may not perform satisfactorily in nontracking
impacts.
Roadside
Geometry
Crash data indicate that roadside geometry, including slopes, embankments,
and ditches, contributes to more than half of all run-off-road accidents
involving serious injury or death. These roadside features are believed
to be the leading cause of rollover in single-vehicle, run-off-road
accidents. The number and type of roadside configurations that can
be evaluated through full-scale crash testing are severely limited
by site restrictions at existing testing facilities.
Future
Vehicle Trends
The safety performance of most roadside safety features has been shown
to be sensitive to vehicle characteristics, including total mass,
height of the center of gravity, and bumper and hood geometry. Because
major changes are made to the vehicle fleet in five- to seven-year
cycles, while most safety features are expected to have serviceable
lives of 20 years or more, the field performance of many safety devices
has been significantly affected.
Solutions
As described above, a number of difficulties remain to be solved in
the continuing effort to improve roadside safety. Roadside safety
problems have traditionally been evaluated primarily through the application
of full-scale crash testing. The high cost associated with full-scale
testing is probably the greatest barrier to solving most of these
problems. It is cost-prohibitive to require full-scale crash testing
of all safety devices for all possible variations in installation
details. Furthermore, although procedures for side-impact testing
of breakaway devices have been developed and implemented, currently
no procedures exist for conducting nontracking impacts that involve
the vehicle rotating at impact. Finally, even though ongoing changes
in the vehicle fleet can be projected into the future to estimate
some of the characteristics of automobiles, it is impossible to build
such a vehicle for crash testing of roadside safety features.
Computer simulation of vehicular impacts using an advanced, nonlinear
finite element code, such as DYNA3D, is the only practical alternative
to full-scale crash testing for the large array of safety performance
evaluations that are needed. Theoretically, this type of simulation
could be used to investigate all of the safety issues summarized above.
Furthermore, after a computer simulation has been developed and successfully
validated against full-scale crash testing, the cost associated with
conducting parametric studies to investigate the effects of installation
details, impact conditions, roadside geometry, and vehicle characteristics
is relatively inexpensive.
Computer simulations also provide a great deal of information that
is frequently unavailable from full-scale crash testing. For example,
finite element modeling provides designers with an accurate picture
of the distribution of stress in critical components of a safety device
throughout the impact event. Unlike full-scale crash tests that normally
only yield pass or fail recommendations for a particular design, computer
simulations can be used to identify areas where a design needs additional
reinforcement or areas where a component has excess capacity.
Current
State of the Art
For computer simulation to solve the wide-ranging problems summarized
above, the procedures need to be widely used and accepted by the safety
community and to have an established record of accurately predicting
crash-test results. Unfortunately, computer simulations of roadside
safety features have yet to meet all of these requirements. The application
of generalized, nonlinear, large-deformation finite element modeling
to the roadside safety field is a relatively recent event, with the
earliest applications dating only to 1992. Although many designers
are now relying heavily on these codes for safety hardware development,
most development efforts are still centered on static and dynamic
testing programs. Even when computer simulation is used to lead development
programs, the codes are most valuable for modeling component and subassembly
testing rather than the evaluation of safety performance through simulation
of full-scale crashes. While the results of computer simulation are
encouraging, relatively few applications have been attempted to predict
the outcome of future full-scale crash tests. Although the Federal
Highway Administration is beginning to use these codes to support
overall policy decisions, computer simulation is not yet an acceptable
means for the final compliance testing of roadside safety features.
Intermediate
Goals
As discussed above, advanced, nonlinear finite element codes are still
not at a stage that allows computer simulations to be used to resolve
the remaining roadside safety problems. The primary goals for the
intermediate future should therefore be associated with advancing
the state of the art for computer simulation.
Finite element simulations of run-off-road crashes involve detailed
models of both the roadside safety features and a vehicle. The finite
element code then uses these models to predict the vehicle kinematics
associated with a run-off-road crash, which, in turn, is used to assess
the risk of injury to which an occupant of the vehicle would be exposed.
Improvements must be made in each of these areas before computer simulation
can play a major role in solving the difficult roadside safety problems
outlined above. Furthermore, some additional knowledge about the expected
distribution of impact conditions and future vehicle characteristics
should be garnered if computer simulation is to reach its full potential
in this field.
Vehicle
Models
Finite
element models for computer simulation of run-off-road impacts must
include detailed descriptions of each structural component on the
vehicle that would be expected to carry a significant load during
an impact. As mentioned, run-off-road crashes frequently involve nontracking
impacts; therefore, an impact could occur at any point around the
circumference of the vehicle, so a general vehicle model must include
all structural components. Although several extremely detailed models
are now available, the number of different types of vehicles that
are represented is extremely limited.
|
Computer-generated
depiction of an SKT-350 Energy-Absorbing Guardrail Terminal
during an end-on collision with a 2,000-kilogram (4,400-pound)
pickup truck traveling at a constant speed of 100 kilometers
per hour (62 miles per hour) prior to the collision. |
|
|
SKT-350
Energy-Absorbing Guardrail Terminal after an end-on crash
test with a 2,000-kilogram (4,400-pound) pickup truck traveling
at a constant speed of 100 kilometers per hour (62 miles
per hour) prior to the collission. |
|
Furthermore,
none of the existing models have been validated against full-scale
crash tests for the wide range of impact conditions that have to be
studied. In addition, the detailed vehicle models now in use still
have some significant limitations, especially in the suspension and
tire representations. These areas of the vehicle models are especially
critical for simulation of run-off-road accidents because of the strong
correlation between tire and suspension damage and vehicle rollover.
Therefore,
significant effort must be directed toward refining existing vehicle-modeling
techniques to provide better tools for analyzing run-off-road impacts.
It also is critical that these models be kept up to date with respect
to current trends in the vehicle fleet.
Safety
Hardware Models
Although a wide range of safety hardware models has been developed
over the past several years, most of these models lack sufficient
validation. Hardware model deficiencies can generally be placed into
two categories: materials limitations and difficulties in modeling
connections.
Materials
such as wood and soil are particularly difficult to model because
of the great inconsistencies from one installation to the next. Characterization
of these materials must begin with the identification of the expected
variability from specimen to specimen or from site to site. Other
types of nonhomogeneous materials, such as portland cement, asphaltic
concrete, and fiber-reinforced plastics, also present significant
problems for materials modelers.
Many types of connections common in roadside safety applications produce
relatively difficult modeling problems. For example, the connection
between a guardrail and a wood block must be carefully modeled to
produce the correct behavior when the bottom W-beam element digs into
the wood block and the post bolt is pulled through the rail.
Roadside safety hardware models must be carefully validated against
detailed component testing to ensure that this sort of unique behavior
is accurately predicted and that the overall impact modeling is reasonably
accurate.
Risk
Assessment
When sufficiently accurate vehicle and hardware models are generated,
computer simulations will be able to correctly identify occupant compartment
kinematics associated with a run-off-road crash. The overall risk
of occupant injury or fatality, however, remains to be determined.
The problem associated with linking vehicle kinematics to occupant
risk is not unique to computer simulations and has plagued full-scale
crash test programs for many years.
The problem is further complicated by the widespread availability
of front and side air bag systems, which may significantly affect
the measures of occupant risk used by the roadside safety community.
Better measures of occupant risk must be developed to address the
more advanced occupant-protection systems now available in the vehicle
fleet.
The computer simulations involving detailed occupant models (including
frontal- and side-impact models) and protection system models (including
air bag and seat belt models) may offer one mechanism for developing
the required links between vehicle kinematics and occupant risk. The
only mechanism for developing such a link, however, is to conduct
detailed investigations into real-world crashes. By reconstructing
run-off-road crashes to identify vehicle and occupant kinematics,
it would be possible to develop better links between existing occupant-risk
measures, such as the head injury criteria or thoracic trauma index
and the probability of injury.
Conclusion
Transportation officials have made great strides in improving roadside
safety along the nation's highways over the last several decades -
a reduction of nearly 70 percent in the run-off-road fatality rate.
To maintain this rate of improvement in reducing injuries and fatalities,
the safety community must begin to address some of the more difficult
roadside safety issues. These issues include sensitivity of safety
features to installation details, problems associated with nontracking
impacts, contributions of roadside geometry to serious accidents,
and the ongoing effort to identify future vehicle trends and their
effects on roadside safety.
The high cost of full-scale crash testing precludes a dramatic expansion
of existing programs to address these issues. Computer simulation
appears to be the only practical means for addressing these problems
in the near future.
To achieve the objective of investigating these difficult roadside
safety issues, significant effort must be devoted toward improving
the capability of computer simulation for modeling run-off-road crashes.
These efforts should focus on developing better vehicle and roadside
safety hardware models and on developing better links between vehicle
kinematics and occupant risk. If a comprehensive effort is directed
toward achieving these overall goals, we can continue our quest to
reduce the injuries and fatalities associated with run-off-road crashes.
Dean
L. Sickingis director of the Midwest Roadside Safety Facility and associate
professor of civil engineering at the University of Nebraska at Lincoln.
Dr. Sicking is also chairman of the Computational Mechanics Subcommittee
of TRB Committee A2A04, "Roadside Safety Features." He has
been involved in highway safety research for more than 20 years and
has worked closely with numerous state departments of transportation,
the Federal Highway Administration, the National Cooperative Highway
Research Program, and a number of private companies. He is a registered
professional engineer in Arizona, Nebraska, and Texas, and his formal
education includes a bachelor's degree in mechanical engineering and
a master's degree and a doctorate in civil engineering from Texas
A&M University.
King
K. Mak is currently a private consultant conducting research and
development efforts in the area of roadside safety, and he is chairman
of TRB Committee A2A04, "Roadside Safety Features." He has
been active in the highway safety area for 30 years, including management
and research positions with the Texas Transportation Institute and
Southwest Research Institute. He has conducted numerous studies for
the Federal Highway Administration, the National Highway Traffic Safety
Administration, the National Cooperative Highway Research Program,
state departments of transportation, and private concerns, and he
is published widely. He has a bachelor's degree in civil engineering
from the University of Hong Kong and master's degrees in transportation
engineering and operations research from the Georgia Institute of
Technology. He is a registered professional engineer in Maryland and
Texas.
Other
Articles in this Issue:
Learning
to Beat Snow and Ice
Safe
Plowing - Applying Intelligent Vehicle Technology
Improving
Roadside Safety by Computer Simulation
Using
the Computer and DYNA3D to save lives
LS-DYNA:
A Computer Modeling Success Story
Preservation
of Wetlands on the Federal-Aid Highway System
Internal
FHWA Partnership Leverages Technology and Innovation
New
Applications Make NDGPS More Pervasive
Center
for Excellence in Advanced Traffic and Logistics Algorithms and Systems
(ATLAS)
National
Work Zone Awareness Week (April 9 to 12) - Enhancing Safety and Mobility
in Work Zones