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SUMMARY
Accident Modification Factors for Traffic
Engineering and ITS Improvements
Crash reduction factors (CRFs)--related to accident modification factors (AMFs)--
provide a quick way of estimating crash reductions associated with highway safety
improvements. (The remainder of this report will use the term AMF to be consistent with
other ongoing NCHRP research in this area.) AMFs are used by many states and local juris-
dictions in program planning to make decisions concerning whether to implement a specific
treatment and/or to quickly determine the costs and benefits of selected alternatives. AMFs
are also key components of the latest safety-estimation tools and procedures, including the
Interactive Highway Safety Design Model (IHSDM) and the procedures now being developed
for the Highway Safety Manual (HSM).
Even though accurate AMFs are critically important to states and municipalities in their
attempts to achieve the greatest return on their investment when choosing among safety
treatments, there is no accepted standard set of AMFs. This is because the accuracy and
reliability of many published AMFs is questionable, and no AMFs exist for many impor-
tant safety treatments. This lack of reliability, accuracy, and comprehensiveness has been
documented in this study, in prior work, and in ongoing HSM development efforts. The
sources of the problem include the lack of AMFs for newer ITS treatments and for common
combinations of treatments, the fact that AMFs vary with other factors such as traffic
volume, a publication bias that results in publishing only positive findings, and crash
migration and spillover effects that result from some treatments but are not accounted for
in the AMF. However, the major problems with existing AMFs result from the poor data
and poor evaluation methods used in their development. Often, AMFs are based on sim-
ple before-after studies of high-crash locations, and the results can be very biased toward
overestimating accident reductions.
The goals of this study were to (1) produce AMFs for high-priority treatments or treat-
ment combinations where none currently exist, and/or (2) to increase the level of predictive
certainty for existing AMFs in other high-priority areas. Due to funding limitations, not all
possible treatment strategies could be effectively studied within this single project; therefore,
the emphasis was on high-priority treatment strategies that are broadly implemented by
states and local agencies and that can affect significant numbers of crash-related deaths and
injuries. Reliable AMFs, at a minimum, must meet the following criteria:
· The AMFs are methodologically and statistically valid.
· The applicability of the AMF (i.e., the subset of crashes, crash locations, or crash conditions
to which it is applicable) is known and documented.
· The AMFs reflect improvements or combinations of improvements that are of interest to DOTs.
· The AMFs reflect the impact of the improvement on different crash-severity and crash-type
categories.
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· The AMFs reflect variability by including both the best estimate of the AMF along with some
measure of variability (e.g., ranges, confidence intervals, and standard deviation).
· The AMFs reflect the savings in "total harm" provided by the treatments, where "total harm"
is a combination of frequency and severity.
The identification and development of AMFs that meet most of the above requirements
involved a project effort that included four basic types of analyses:
· Empirical Bayes (EB) Before-After Evaluation. Before-treatment and after-treatment crash data
were acquired for locations where the treatment of interest had been installed. The latest statisti-
cal methodologies (i.e., EB) for conducting before-after studies were applied to produce AMFs.
· Reanalysis of Existing/Supplemental Data. Data from prior before-after evaluations were
acquired and reanalyzed using the more rigorous EB methodology. In many cases, supplemen-
tal data were acquired to enhance the evaluation.
· Analysis-Driven Expert Panel. A panel of knowledgeable researchers and practitioners was con-
vened to review critical research studies and reach a consensus on AMFs for a given treatment.
In some cases, the AMF was developed through further analysis by one of the NCHRP project
teams sponsoring the expert panel meeting.
· Cross-Section Modeling. A new analysis was conducted in which a cross-sectional model was
produced and used to derive AMFs for a specific treatment.
The project team developed an initial list of 78 treatments deemed to be important in safety
decisions. The treatments were divided into four groups: intersection-related, roadway-
segment-related, ITS-related, and other. Additional treatments suggested by 34 state DOT
users, who responded to the project survey, and further refinements by the project team
expanded the list to 100 treatments. A review of the literature discovered no AMFs for 50 of
these treatments. Critical literature reviews were then conducted for the remaining 50 treat-
ments. Of that group, 20 were felt to have a high or medium-high level of certainty. Summary
information for each of these 20 AMFs along with a knowledge matrix for all 100 treatments
was published in NCHRP Research Results Digest 299 as an interim product of this research
study. Each summary includes the AMF(s), the level of predictive certainty, the study
methodology, a description of the sites used in the study, and supplemental comments and
footnotes to describe the study results and applicability.
The results of this initial effort clearly supported the need for additional research to
develop new AMFs and to strengthen those with less than a medium-high level of certainty.
The determination of which of the many possible AMFs should be developed or improved
was based on the following:
· Results of the survey of state DOT users,
· Judgment of the level of predictive certainty of existing AMFs from the literature review,
· Measure of the crash-related harm that might be affected by the treatment,
· Whether there was existing ongoing research that might develop an AMF, and
· Availability of data needed in AMF development or improvement.
Priority was given to conducting as many EB evaluations of the high-priority treatments
as possible. Based on the funding available and on team knowledge of available data, it
was decided that EB analyses would be conducted to develop AMFs for the following high-
priority treatments:
· Installation of a traffic signal at a rural intersection;
· Conversion of an undivided four-lane road to three lanes including a two-way-left-turn lane
(TWLTL)--a "road diet";
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· Increasing pavement friction on intersection approaches;
· Increasing pavement friction on roadway segments;
· Modification of left-turn signal phase (three combinations);
· Replacement of 8-in. signal head with 12-in. head;
· Replacement of single red signal head with dual red signal heads; and
· Conversion of nighttime flashing operation to stop-and-go operation.
The second approach to AMF development/modification involved two analysis-driven
expert panels. While earlier project discussions had noted the possibility of expert panels for
AMFs related to specific focus areas (e.g., roadside crashes and pedestrian treatments), it was
decided by the team and the oversight panel that a more critical need was to assist the project
teams from NCHRP Projects 17-26 and 17-29 in developing AMFs needed for the Highway
Safety Manual safety prediction tools these teams were developing for urban/suburban arteri-
als and rural multilane highways.
Working with the research teams from those two projects, this research team identified
and recruited expert panel members; developed a listing of potential treatments for study;
compiled and distributed copies of relevant research reports to the panel; and arranged and
hosted the panels. Before each meeting, the panels prioritized the potential list of treatments
in order to ensure that the most important ones were discussed, and each panel member was
assigned one-third of the high-priority issues and asked to be prepared to present their opin-
ions and lead the discussion. Following the meeting, the project team funded and conducted
additional limited analyses and documented the findings of each expert panel in a report.
The AMFs developed through consensus from a review of prior research studies or from
further analysis recommended by the panels include the following:
· Add exclusive left-turn lane,
· Add exclusive right-turn lane,
· Prohibit right turn on red,
· Modify left-turn signal phase,
· Add intersection lighting,
· Add two-way left-turn lane,
· Change roadside slope,
· Add/remove on-street parking,
· Add segment lighting, and
· Reduce mean travel speed (not treatment-specific).
In summary, this project has verified, modified, or developed 35 AMFs that are felt to be
of high or medium-high quality. These have been documented in formats that are usable by
both practitioners and researchers. These AMFs are the primary project outputs. However,
the project has also documented a process that can be used with future analysis-driven
expert panels and the detailed discussions of the two expert panels that were part of this
effort. This material should be helpful in future efforts to develop or improve AMFs for
treatments for which no AMF could be developed here. Finally, the project developed and
documented a procedure for ranking needed AMF research, a procedure incorporating not
only state DOT user and researcher opinions and knowledge of the quality of AMFs in
the published literature, but also a method for estimating the crash-related harm that might
be affected by each treatment. An approach combining these factors could also be used in
more global efforts to prioritize roadway safety research needs in general.
