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CHAPTER 4
RESEARCH APPROACH
INTRODUCTION used in this project to verify and confirm hypotheses devel-
oped from the computer simulation study, as well as to vali-
This chapter discusses the methods of analysis that were date and strengthen the conclusions of this research.
used in this study, including crash and geometric data analy-
ses, computer simulation, and full-scale crash testing.
Existing crash and geometric databases were examined to ANALYSES OF CURB-RELATED SAFETY ISSUES
determine if they could be used to characterize the extent and USING CRASH AND INVENTORY DATA
severity of safety problems associated with curb and curb
Introduction
barrier combinations. The crash databases were also reviewed
to determine if they could provide information regarding the Since the inception of this study, an overall goal has been
nature of impacts involving curbs (e.g., impact speed, angle to use existing databases containing information on crashes,
of impact) in order to develop input for full-scale crash test- roadway inventory, and traffic to better characterize safety
ing and computer simulation studies. problems associated with curb and curbbarrier combinations
Where validated computer models can be developed, com- on higher-speed roadways. Such information was used directly
puter simulation methods are the most versatile approach for in the development of the design guidelines since it can pro-
investigating a wide range of possible impact scenarios (e.g., vide real-world insight into the magnitude of the problem on
vehicle type, curb type, impact condition). Computer simula- various roadway types, the nature of the problem (e.g., how
tion can also be very useful for determining the precise curb impacts are similar or dissimilar to other run-off-road
effects that vehicle-curb interactions have on the stability of collisions), and factors that might be influenced to reduce
various vehicle types and the effects that curbs placed in com- curb impact severity (e.g., to prevent rollover after a curb
bination with roadside safety barriers have on the perfor- impact). In addition to this primary goal of input into design
mance of the barriers. Vehicle dynamics programs and Finite
guidelines, a secondary but related goal of the crash-data
Element Analysis (FEA) are two such methods that were con-
analyses was to provide leads for the crash testing and simu-
sidered for use in this study. Vehicle dynamics programs
lation efforts that were later conducted.
have been used extensively in previous curb-safety-related
The crash-data analyses took place in two phases. Phase I
studies, as indicated in the literature review (Chapter 2). FEA
involved a detailed examination of existing databases to
has been used in several studies involving vehicle impact with
determine which ones might be suitable for use. Based on
roadside safety hardware and has proven to be very effective.
preliminary examination of data and discussions with the proj-
To the knowledge of the authors, however, FEA has not been
ect panel, a final set of crash-data analyses were defined.
used in any study involving curbs or curbbarrier combina-
These analyses were then carried out in Phase II, using the
tions and, therefore, was not discussed in the literature review
selected databases.
section of this report. Since FEA was an important analysis
tool in this research, the effectiveness of the method applied
in the study of roadside barrier crashworthiness is discussed Examination of Databases
in this chapter. A summary of previous studies using FEA to
study vehicle impact with roadside safety barriers is presented As detailed in an interim report, the national databases of
and discussed later in this chapter. interest included the following:
Full-scale crash testing was another method used in this
research. The advantage of full-scale crash tests is that they · Fatality Analysis Reporting System (FARS)
are actual physical impact events in which there is little · National Automotive Sampling System--General Esti-
ambiguity about the results. The disadvantage is that they are mates System
costly, and it is seldom feasible to perform very many tests. · National Automotive Sampling System--Crashworthi-
The testing results, therefore, usually do not address a very ness Data System
wide range of conditions. A full-scale testing program was · FHWA's Highway Safety Information System (HSIS)
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Preliminary examination of each of these databases was under- Data for the 199799 period were used in this study. The
taken to determine whether it would be useful in the overall crashes must involve a vehicle that is towed from the scene.
safety analysis and what types of analyses might be possible Thus, none of the less-severe property-damage-only crashes
with it. (the successes with respect to roadside objects) are included.
The sample, taken from police reports in the same jurisdic-
tions as the NASS-GES sample, is an unequal probability
Fatality Analysis Reporting System (FARS ) sample that is heavily weighted toward more severe crashes.
The data are the highest quality crash data available, since
This database is an annual census of all police-reported fatal- they are based on detailed follow-up investigations by trained
ities in the United States, with data coded and cleaned by a investigators. In order to develop national estimates from the
FARS coder in each state. Data from the 199499 period were data (or estimates related to the overall crash severity distri-
used in this effort. FARS contains data on the presence of a bution), the cases have to be weighted based on the proba-
"Curb" as both First Harmful Event (FHE) and Most Harmful bility of being selected.
Event (MHE), and data on "Rollover" separate from the event CDS includes a virtually unlimited "Sequence of Events,"
codes. FARS does not contain data on the full "Sequence of with "Curb" as one of the objects that can be struck. Like the
Events" in a crash (e.g., curb strike, then guardrail impact, then GES sample, there are no details of curbs or barriers, and
overturn). FARS does not contain any information on curb only limited data on roadway geometrics; the data cannot be
design parameters. Finally, since it is based on fatal crashes, linked to supplemental roadway or traffic inventories. Unlike
FARS data could not be used to examine differences in injury the GES data, the margin of error is rather large when one is
severities with and without curbs; only the fatal crash failures exploring an issue with relatively few severe crashes per year
are present. (like curb-related crashes), since the sample for such crashes
is quite small.
Enhanced CDS data were developed at TTI for NCHRP
National Automotive Sampling System-- Project 17-11, "Determination of Safe/Cost Effective Road-
General Estimates System (NASS-GES) side Slopes and Associated Clear Distances." NASS crash
investigators collected additional data at selected CDS crash
The GES was established by NHTSA to allow national sites, and TTI reconstructed encroachment speed, angle, and
estimates of safety issues. It contains annual files for 1988 tracking information where possible, including a confidence
and later. Data from the 1995-99 period were used in this rating for the reconstructed data.
analysis. GES is based on an annual random sample of approx-
imately 50,000 police crash reports of all severities (ranging
from no-injury to fatality) pulled each year from 60 areas FHWA's HSIS
(400 police jurisdictions) across the nation. All cases are
manually coded to approximately 90 common data elements. HSIS is the only national data file containing both crash
The coding is based on a review of the computerized codes and roadway inventory elements. It includes linkable files of
on the original form and the narrative and sketch. GES assigns police-reported crashes, roadway geometry inventories, and
a weight for each case that allows one to develop national traffic volumes in eight states (five states in the 198597
estimates; severity is the predominant weighting variable. As period; three additional states in the 199097 period). The files
will be seen later, both weighted and unweighted data were contain data for crashes of all severities on all state-system
used, depending on the nature of the specific analysis. While roadways, i.e., it excludes municipal or county roads not con-
not containing a full sequence-of-events variable that would trolled by the state. Since the current project focused on
allow one to trace the entire crash sequence, GES does con- higher-speed major roadways that the states control, this
tain a number of variables that are of interest in this study, restriction was unimportant. While six of the eight states
including a FHE and a MHE, both of which include striking have some form of both "Object Struck" and "Sequence of
a curb. The GES data do not contain any crash location infor- Events" or "First/Most Harmful Event," only Illinois and
mation. Thus, they cannot be linked to any supplemental Michigan have a "Sequence of Events" variable in which
data, such as roadway inventories, operating speed inven- curb impacts are separated from other objects and where
tories, or Average Annual Daily Traffic (AADT). "rollover" can be extracted as a separate event. Like the GES,
both states also include information in the crash file related
to crash/occupant injury severity and speed limit. Therefore,
National Automotive Sampling System-- the data for these two states were chosen for use in this study.
Crashworthiness Data System (NASS-CDS) To capture the most recent years of data in the HSIS files, the
1996 and 1997 data for each state were used.
The NASS Crashworthiness Data System contains detailed A further advantage of HSIS is the linkable roadway
crash reconstruction data collected on site by expert investi- inventory data. For both Illinois and Michigan, the inventory
gators on approximately 5,000 crashes each year since 1979. file includes not only AADT and speed limit for each section
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of highway on the state system but also an indication of the crash-data analyses to be conducted. Table 7 provides a brief
presence of curb. description of each analysis along with the database used. As
Finally, Michigan provides an additional file not present in noted earlier, the analyses fell into two major groups: those
any other HSIS state: a Guardrail Inventory File that contains conducted to further define and examine the extent of the
information on the location and description of each section curb-related safety problem, and those primarily conducted
(run) of guardrail along each side of the highway (e.g., type, to provide input into the simulation and crash-testing efforts.
purpose, and distance from roadway). Because there can be Since an objective of the overall study effort was to relate
multiple rails at any point on the roadway (e.g., rails on each curb design guidelines to some measure of roadway operat-
side and in the median), the file is very complex and difficult ing speed (and, ultimately, to design speed), the panel was
to work with. Furthermore, it has not been actively main- interested in targeting operating speed in the crash-data
tained by the Michigan DOT since 1992. However, because analyses where possible. Unfortunately, operating speed is
this is the only known guardrail inventory file that can be captured neither in crash data nor in normal roadway inven-
linked with other roadway and traffic data to produce crash tory data. However, in a supplemental analysis, 1998 non-
rates per passing vehicle, it was linked with the Michigan crash speed data were obtained from Michigan DOT and
1992 roadway inventory file and with Michigan 1993 and were used with New York State DOT data to define surrogate
1994 crashes in this study. Details of the complex merging operating speeds for different combinations of functional
effort and data decisions can be found in Appendix E. class and speed limit. These surrogate operating speeds were
then attached to crashes and used in the Michigan severity
modeling effort and in the Michigan and Illinois rollover
Description of Data Analyses analyses. These operating speeds could not be used in other
analyses due either to the nature of the issue (e.g., extreme
Based on the goals of the project and the initial review of crashes are a function of individual vehicle speeds rather than
the available databases, the project panel defined a set of six average roadway speeds) or to the source of the data (e.g.,
TABLE 7 Description of data analyses conducted and databases used
Task title Description Data used
Extent of the U.S. The extent of the national safety problem related to 1994-99 FARS
Curb-Related Safety curbs was documented. Questions addressed included, 1995-99 NASS-GES
Problem "how large is both the fatal and nonfatal crash problem,
and has there been any trend over the past 5 years?" and
"are there differences in the nature of the curb-related
fatal and nonfatal crashes as compared to noncurb
single-vehicle crashes?"
Examination of Given the severity of rollovers in general and the nature 1997-99 NASS-CDS
Curb-Related of the curb, this was a detailed, multifile examination of 1996-97 Michigan
Rollover Risk and the risk and nature of rollover given a curb-related crash. 1996-97 Illinois
Nature Given a To help ensure that the curb was directly related to the
Crash rollover, all three databases chosen include a "sequence
of events" that allowed selection of only rollovers
preceded by a curb impact.
Crash, Injury, and To examine differences in the crash rates and rollover 1992 Michigan
Rollover Rates per rates for guardrails with and without curbs, Michigan Guardrail Inventory
Passing Vehicle for data on guardrail inventory, roadway inventory, traffic and Roadway
Guardrail Sections and crashes on urban freeway and other urban multilane Inventory
with and without roads were used in both contingency table analysis and
Curbs negative binomial models. 1993-94 Michigan
crash data
Curb-Crash Severity To further examine curb-crash severity, Michigan data 1996-97 Michigan
Modeling for SV crashes in which a curb was the first object struck
and SV crashes in which no curb was struck were used
in the development of ordinal regression models to
examine the effect of crash-related variables (e.g.,
rollover, speed limit, weather, vehicle type, operating
speed) on crash severity.
Nature of Curb To provide guidance to crash testing and simulation 1997-99 NASS-CDS
Impacts--Crash efforts, an attempt was made to extract the specific TTI Enhanced
Reconstruction Data nature of curb-related impacts (e.g., angle of impact, CDS data
speed, tracking/nontracking) from both basic NASS-
CDS data and from enhanced CDS data obtained from
the Texas Transportation Institute.
Nature of Curb Extreme and nonextreme (i.e., severe and nonsevere) 1995-99 NASS-GES
Impacts--Analysis curb crashes were compared to define crash conditions 1996-97 Michigan
Of "Extreme" Vs. that differ between the two categories. Such identified 1996-97 Illinois
"NonExtreme" conditions might provide both further basic information
Crashes on curb safety and additional factors for consideration in
simulation and crash-testing efforts.
