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CHAPTER 2
Literature Review
A detailed literature review was conducted to identify stud- dent reports on these crashes. The data are then coded and
ies relevant to the identification of impact conditions for entered into state crash data files. Crash data on the police-
ran-off-road crashes. The review identified numerous studies reported level are generally very limited in detail. Occasionally,
pertaining to ran-off-road crashes that could have some bear- more detailed data are collected on selected crashes, such as
ing on this project. However, upon review, most of these those resulting in fatalities and severe injuries, but such detailed
studies utilized only police-level crash data, which do not have investigations constitute only a small fraction of crashes.
the required details or information to assess the impact con- Most of the collected data elements are intended for identi-
ditions of ran-off-road crashes. An annotated bibliography fication and record-keeping purposes--such as date, time, and
is shown as Appendix A, and a summary of related ongoing location of crash; vehicle(s) and driver(s) involved; damage to
research studies are presented in Appendix B. Only a summary the involved vehicle(s) and other property; injury sustained by
of results of the literature review is presented in this chapter. driver(s) and occupant(s) of vehicle(s); and a brief description
The literature review is presented under four general headings: of what happened in the crash. The crash data may be merged
with other data files for additional information. For example,
1. In-depth crash data collection the Highway Safety Information System (HSIS) combines crash
2. Impact conditions of ran-off-road crashes data with other roadway and vehicle-related data files, such as
3. Data needs for study of ran-off-road crashes roadway inventory, traffic, alignment, bridge inventory, vehicle
4. Reconstruction of ran-off-road crashes identification and registration, etc., to expand the information
database for use in various analyses.
Even with the merged files, crash data on the police-reported
2.1 In-Depth Crash Data Collection
level still lack the detail needed for analysis beyond problem
Crash data collection can be grouped into three general identification and are of little use from the standpoint of
levels of detail: estimating impact conditions of single-vehicle, ran-off-road
crashes or evaluating the impact performance of roadside
1. Police-reported level safety features. Thus, studies pertaining to police-reported level
2. Enhanced police-reported level crash data are not included in the literature review.
3. In-depth level Crash data on the enhanced police-reported level are used
in selected research studies in which additional data elements
More detailed discussions on these three categories of crash are collected to supplement the police-reported data. The sup-
data collection are presented in this section with examples. It plemental data collected vary from study to study depending
should be noted, however, that these examples are intended as on the objective(s) of the study. Most of the supplemental
illustrations only and are by no means all inclusive. There have data pertain to items of specific interest to the studies, such as
been so many studies using crash data over the years that it details of roadside conditions, inventory of a particular road-
would not be feasible to include even a fraction of the studies side object(s), etc. However, there have been a few studies in
in this review. which the investigating officers were asked to provide infor-
The police-reported level is the most common type of crash mation on departure and impact conditions.
data available. State and local police officers are required by law In a study by Garrett and Tharp, the investigating officers
to investigate all reportable crashes and complete police acci- were asked to provide estimates on impact speed and angle on
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324 crashes that occurred on the Ohio Turnpike over a period occupant injury levels. The cost associated with in-depth crash
of five months during the summer and fall of 1967 (8). Simi- investigation is, as may be expected, very high and there have
larly, in a study by Perchonok et al. to assess the relationships only been a few ad hoc studies that incorporated such in-depth
between single-vehicle, ran-off-road crash frequency, severity, crash data, i.e., the data collection was designed specifically for
and roadway and roadside features, data on over 9,000 crashes the study.
were collected from six states (2). The investigating police offi- The most notable study involving in-depth crash data is per-
cers were asked to complete supplemental field forms, includ- haps the study on crashes involving pole support structures (9).
ing data pertaining to impact conditions, such as impact speed A stratified random sample of over 1,000 crashes involving
and angle. utility poles, breakaway and nonbreakaway luminaires, and
While enhanced police-reported level crash data provide sign supports were investigated in-depth, and the crashes were
more detailed information, its utility on estimating impact reconstructed to estimate the impact conditions. The in-depth
conditions is limited by a number of factors: crash data were then analyzed in conjunction with police-
reported level data on all crashes and all pole crashes, enhanced
1. Expertise and experience of the investigating police officers. police-reported level data on unreported crashes, and pole
Most police officers receive some basic training in crash inventory data to address the study objectives. The results of
investigation, but only a small proportion of the officers the study include the extent of the pole crash problem; the
receive the highly specialized training in crash reconstruc- characteristics of pole crash sites, vehicle damage, and occu-
tion needed to accurately estimate impact conditions. The pant injuries; assessments on the performance of various
quality of data collected by police officers without the spe- pole types; and a cost-effectiveness evaluation of the break-
cialized training may be questionable. away modification as a safety treatment.
2. Knowledge of the impact performance of roadside safety Another study of crashes on highway narrow bridges
features. Even for trained officers, reconstruction of single- involved in-depth investigation of 124 crashes that occurred
vehicle, ran-off-road crashes pose special problems unless on bridges (10). Again, the in-depth crash data were analyzed
the person is also knowledgeable of the impact performance in conjunction with police-reported level data on crashes that
of roadside features. Most reconstructions are based on occurred on 11,880 bridges from five states and supplemen-
energy dissipation and balance. For many ran-off-road tal field data on a sample of 1,989 bridges to address the study
crashes, energy dissipated by the struck object constitutes objectives. The results of the study include extent of the narrow
a significant portion of the energy equation and must be bridge crash problem and the associated crash frequencies and
properly accounted for. This in turn will require knowledge rates; relationships between various bridge physical and oper-
on the impact performance of roadside features, which is ational characteristics to crash rates and severities; and the char-
beyond the training received by police officers. acteristics and relationships between crash and injury severity
3. Time and effort required. To properly reconstruct a crash for crashes at bridges.
to estimate its impact conditions would require time and Other studies have utilized data from various in-depth crash
effort beyond those available to an investigating officer. investigation programs conducted by the National Highway
Thus, it is reasonable to expect that estimates of impact Traffic Safety Administration (NHTSA). Since its inception in
conditions would be based mostly on the judgment of the late 1960, NHTSA has sponsored numerous programs to col-
officers and less so on step-by-step reconstruction of the lect in-depth crash data. The programs changed over the years,
crashes. from the multidisciplinary accident investigation (MDAI) pro-
gram in the late 1960s in which a small convenient sample of
In summary, enhanced police-reported level crash data, crashes were studied in great detail to the current NASS CDS
which uses investigating officers to collect supplemental data, that investigates a nationally representative stratified random
could provide more detailed information on the impact con- sample of crashes in lesser detail. However, these in-depth data
ditions of single-vehicle, ran-off-road crashes. However, as collection programs are designed to meet the data needs of
discussed above, there are serious limitations to this approach NHTSA and the emphasis is, therefore, on data pertaining to
that could not be easily overcome. Thus, the use of enhanced the vehicle, occupant, and injury severity. Unfortunately, data
police investigation to estimate impact conditions is not pertaining to roadway and roadside characteristics are mostly
recommended. lacking, which limits the use of the data for highway-related
To properly estimate the impact conditions of single-vehicle, research, such as the current study.
ran-off-road crashes, an in-depth level of crash investigation is In order to make use of the NASS CDS data, supplemental
required. The required data would include detailed data on the data collection is necessary to gather information required for
roadway, vehicle trajectory, object(s) struck and damage sus- the specific study. The supplemental data collection can be
tained, vehicle and damage measurements, and driver and prospective or retrospective in nature. The NASS program
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has a special study subsystem that allows for prospective col- way functional classes and design speeds. Part of the research
lection of supplemental data in addition to the standard data involved clinical analysis of 338 NASS CDS cases from 1997
elements collected under CDS. For instance, three special and 1998. Field data on roadway and roadside characteristics
studies were designed to collect in-depth crash data on longi- of crash sites were collected to supplement the standard NASS
tudinal barriers, pole support structures, and crash cushions CDS data elements.
(11, 12, 13). These special studies were met with different These sampled cases (e.g., police accident reports, field
degrees of success. Nearly 1,200 cases were collected under forms, scaled diagrams, and photographs) were then manually
the Longitudinal Barrier Special Study (LBSS) while only a reviewed to glean additional information beyond the comput-
negligible number of cases were collected under the pole and erized data elements. The crashes were then reconstructed to
crash cushion special studies. The LBSS cases were sub- estimate impact conditions and vehicle trajectories from the
sequently reconstructed using the conservation of energy manual review such as impact sequence, pre- and post-impact
approach and the data were analyzed to examine the severity vehicle trajectories, impact angle, etc.
of barrier length-of-need (LON) crashes versus barrier-end The same retrospective approach and data collection pro-
impacts. Cases involving failure of the barrier system were tocol used in NCHRP Project 17-11 were used in the rollover
reviewed clinically (14). study (17) sponsored by FHWA, except that the cases were
Crashes involving concrete barriers were selected from the sampled from the 1999 NASS CDS data file. The objectives of
LBSS data file for use with an FHWA study on rollovers caused this study were to determine the specific causes of rollover
by concrete barriers (15). Of the 130 crashes involving concrete events associated with the full range of passenger vehicle col-
barriers, 31 resulted in rollovers. In addition to comparing the lisions in which such an event occurred. In fact, the data from
characteristics of crashes resulting in rollovers to those of non- NCHRP Project 17-11 were utilized in this study with addi-
rollovers, the rollover crashes were also clinically analyzed to tional in-depth clinical reconstruction on the 180 rollover
identify potential causes for the rollovers. crashes contained in the database. In addition, new data from
These studies illustrated the potential application of the spe- 175 NASS CDS cases from 1999 were added to the database.
cial studies as well as the problems associated with their con- However, NHTSA recently changed its privacy policy to
duct. This special study approach was not again utilized until discard police accident reports after only one year. This policy
the recent Large Truck Crash Causation Special Study, spon- change effectively eliminates this retrospective approach since
sored by the Federal Motor Carrier Safety Administration the only means of identifying the crash sites was from the
(FMCSA). The purpose of this study was to determine specific police accident reports. The prospective special study is the
causes of large truck (trucks with gross vehicle weight rating of only viable approach for future studies using the NASS CDS
over 10,000 lbs) crashes. These crash causation data will help program.
to identify crash countermeasures the FMCSA can undertake A new emerging technology may provide a totally new and
with regard to interstate motor carriers, their drivers, and their better source of data on impact conditions. Automobile man-
vehicles; and in cooperation with other DOT agencies and state ufacturers have installed Event Data Recorders (EDRs) in
governments with regard to the non-commercial vehicles, selected vehicle lines in recent years. The EDR is designed as
pedestrians, and pedal cycles involved in the crashes. a controller for monitoring airbag deployment and seatbelt
Another approach is to supplement the NASS CDS data ret- usage and recording data pertaining to the crash event in case
rospectively with additional field data collection. Data ele- of a crash. Data elements recorded include crash pulse, seat-
ments of specific interest to the study, but not covered under belt usage, and pre-crash information, such as speedometer
NASS CDS, are identified and collected using supplemental reading and engine performance parameters. In the future,
field data collection. The key limitation of this approach is that EDR data may simplify reconstruction of crashes to estimate
the supplemental data elements should not change over time impact conditions and could become and an invaluable sup-
since the supplemental data are collected one to two years sub- plement to in-depth crash investigation.
sequent to the occurrence of the crashes. This is not a bad NHTSA is currently collecting available EDR data under its
assumption for most data elements pertaining to highway and NASS CDS and Special Investigations (SCI) programs and
roadside characteristics since they typically do not change compiling the data into a national database. While the EDR
except during major construction or reconstruction. technology is relatively new and little actual data are currently
This retrospective approach was utilized in ongoing available, its potential in the future is very promising:
NCHRP Project 17-11, "Recovery-Area Distance Relation-
ships for Highway Roadside" (16). The objective of the study · EDRs are now deployed in all vehicle lines, so more data
is to develop relationships between recovery-area distance, should become available.
roadway and roadside features, vehicle factors, encroachment · The number of data elements and the length of recording
parameters, and traffic conditions for the full range of high- period are somewhat limited now. However, with rapid
