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Results of the Literature Review · User Priority Level. The state DOT survey respondents
rated each treatment in terms of how important it was to
Of the 100 treatments examined, the existing research lit- have an AMF. These ratings were combined to provide the
erature allowed the team to conduct detailed critical reviews user priority ranking. Particular attention was given to the
on 50 treatments. The information derived from these criti- top quartile of these ranked treatments.
cal reviews was used to summarize existing knowledge and · Level of Predictive Certainty. As described above, for each
prioritize research for Phase II of this study. First, the infor- treatment where a critical literature review was possible, an
mation was included in an "AMF Knowledge Matrix" that LOPC was assigned--high, medium-high, medium-low,
provides a status report on the quality of AMFs for these 100 or low. Any treatment for which no prior research study
treatments, as indicated by checkmarks within the cells in was discovered was categorized under "non-existent."
Table 1. In addition to the measure of AMF quality, the ma- Phase II efforts concentrated on the lower three levels.
trix also provides information on the user priority level for · Ongoing/Future Research. Determination of whether there
the 25 highest rated treatments and information on other was ongoing or planned research that might improve the
ongoing or planned research that would potentially increase AMF was based on a review of several research-in-progress
the quality of the AMF. The cells that are shaded within the databases, discussions with other highway safety researchers,
matrix represent the top 25 treatments as rated by the state and conversations with research sponsors such as FHWA
DOT respondents. The rank-order of these top 25 treat- and the Insurance Institute for Highway Safety (IIHS). The
ments is shown in the column labeled "User Priority Level." studies referenced in Table 1 are those that have the greatest
(Note that Table 1 reflects AMF knowledge through 2004. potential for producing AMFs for specific treatments.
Additional AMFs have since been developed in this and Results from these studies should be reviewed in the future
other projects and are included in the final AMF listing in to determine if the LOPC for an AMF has been improved.
Chapter 5 of this report.) · Estimate of Crash-Related Harm Possibly Affected by the
Of the 50 treatments critically reviewed (and the 100 treat- Treatment. The importance of a treatment, and thus its
ments considered), 20 were judged to have a high or AMF, is a function of the size of the safety problem that the
medium-high LOPC. These treatments are summarized in treatment affects and the probability that the treatment will
Table 2. The asterisks in Table 2 denote the treatments in the be implemented. For those treatments ranked high in terms
top quartile of the user ratings. Thus, 11 of the 20 treatments of user priorities, it was assumed that implementation
deemed to have AMFs of acceptable quality were in the users' would be widespread given a sound AMF. Each of these
top 25. Summary information for each of these 20 AMFs high-priority treatments was assigned a high, medium-high,
along with the knowledge matrix were published in NCHRP medium-low, or low crash-harm rating. This was done by
Research Results Digest 299 (5) as an interim product of this assigning a target crash type to the treatment--the crash
research study. Each summary includes the AMF(s), the type or types that would be most affected--and defining the
LOPC, the study methodology, a description of the sites used appropriate rating based on the economic level of national
in the study, and supplemental comments and footnotes to "crash harm" associated with that crash type. The economic
describe the study results and applicability. These same estimates for each of 31 crash types were based on work by
resulting AMFs are presented in Chapter 5 of this report Miller (71). A more detailed discussion of this methodology
along with new AMFs produced by the Phase II efforts of this is presented in Appendix A.
research. · Availability of Needed Research Data. For each treat-
ment being considered, detailed historic data concerning
treatment descriptions and treatment dates from the
Prioritizing Phase II Efforts
implementing agencies were necessary, as well as linkable
to Develop Additional AMFs
historic crash, roadway inventory, and traffic flow data
The results of the literature review and the input from DOT for both treated sites and for comparison/reference sites.
practitioners clearly supported the need for additional research The data either had to be available from the implement-
to develop new AMFs and to strengthen those with less than a ing jurisdictions or from FHWA's Highway Safety Infor-
medium-high LOPC. Only 20 of the 100 treatments being mation System (HSIS), which includes such historic data
studied had AMFs of high or medium-high quality, meaning for nine states.
that 80 had lower quality AMFs (or didn't have them at all), in-
cluding 14 of the DOT users' top-25 treatments. Given project The first four of these factors were then captured for a subset
funding limitations, a decision was made concerning which of the treatments shown in the AMF knowledge matrix--those
treatments should be further researched in the Phase II efforts. that were in the users' top 25 rankings and additional treat-
This decision was based on the following factors: ments of interest to the project oversight panel (see Table 3).
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Table 1. AMF knowledge matrix showing AMF quality, user priority, and ongoing or future research
for 100 intersection, roadway segment, and miscellaneous treatments.
Level of Predictive Certainty
User Ongoing/
Treatment Priority Medium- Medium- Non- Future
Level High Low Work
High Low Existent
Intersection Treatments
Install a roundabout 19 (8)1
Reduce or eliminate intersection skew (9 )
Correct sight distance (9 )
Install offset T's
Install turn lane or bypass lane at T-intersection (10, 11)
Add exclusive left-turn lane 1 (12)
Install double left-turn lane (change from single)
Create positive offset for opposing left-turn lanes (13) (14)
Add exclusive right-turn lane 4 (12)
Add channelization for right-turns 11 (15, 16)
Install median acceleration lane
Add raised/painted median islands (12,17)
Install a traffic signal 2 (18)2
Remove a traffic signal (19)3
Add a left-turn phase (protected or
8 (20, 21)
protected/permissive)
Modify signal change interval (22)
Add all-red phase
Change cycle length
Change from incandescent to LED signals
Add signal heads (23)
Increase signal head size (24)
Add backplates
Install red-light cameras (25,26)
Install red-light hold systems (27)4
Install dynamic advance warning flashers "Red 17 (28)
Signal Ahead"
Install overhead flashing beacon (29)
Convert to all-way stop (30)
Remove all-way stop
Convert stop-control to yield-control (31)
Prohibit left turns
Install rumble strips on approach to intersection (32,33)
Install intersection lighting 13 (37)
(34,35,36)
Close driveways near intersections 22
Install marked crosswalk (38)5
Add pedestrian signals or pedestrian phase (39,40)
Install curb extensions (bulbouts)
Install raised crosswalks
Install raised/tabled intersection
Reduce turn radius (shorten pedestrian crossing)
Remove parking near intersection (41)
Roadway Segment Treatments
Add a travel lane 8 (42)
Convert two-lane road to multilane road
Reduce number of lanes (road diet) (43,44)6
Narrow lane widths to add lanes (45)7
Narrow urban lanes to install turn lane
Add two-way left-turn lane (TWLTL) (9)8 (46)
Replace a TWLTL with median/left-turn bays
Add passing lanes (two-lane roads) (9 ) (47)
Widen median (48,49)
Install raised median 20 (8,9,45)
Increase lane width 21 (9,51)9
Change shoulder width and/or type 15 (9)9
Flatten horizontal curve 12 (9,52)10
Improve curve superelevation 18 (9 )
Reduce grade (9)
Flatten vertical curve
Add static curve warning signs and/or pavement
markings
Add dynamic curve warning sign
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Table 1. (Continued).
Level of Predictive Certainty
User Ongoing/
Treatment Priority Medium- Medium- Non- Future
Level High Low Work
High Low Existent
Add shoulder rumble strips 5 (53)11 (54,55)
Add edgeline rumble strips 14 (56)12 (14)
Add centerline rumble strips (two-lane roads) 10 (57) (54,14)
Remove roadside obstacle 3 (9) (58)
Flatten sideslope 22 (59) (52)
Install/upgrade guardrail 22 (60)
Install median barriers 6 (60) (61)13
Relocate utility poles (62)
Use shoulder on freeways/expressways for bus lane
Remove parking
Eliminate left-turns at driveways 16
Add delineation
Install roadway segment lighting 23 (63)
Use dynamic message sign
Use variable speed limit (64)
Use automated speed enforcement
Install reversible roadways/lane control
Reduce speed limit (60) (65,66)
Use differential speed limit
Add sidewalk/walkway
Stripe bicycle lane
Add midblock pedestrian signal
Install raised crosswalks (non-intersection)
Install mid-block pedestrian crossing
Miscellaneous Treatments
Lengthen acceleration lane
Consolidate driveways (9)14
Traffic calming
Provide signal coordination 7 (67)
Increase pavement friction (68)
Provide pedestrian refuge (69,70)
Install raised medians at crosswalk (39)
Install pedestrian countdown signals
Install crosswalk in-pavement lighting
Install automatic pedestrian detectors
Fog/wind/weather detection and warning systems
Install ramp metering
Use safety service patrols
Implement 511/traveler information
Implement integrated public safety/transportation
communications
Use drone radars
Install truck rollover warning system
Install truck height warning
1
Numbers in parentheses refer to the references for the best available AMF(s) or the ongoing/planned research effort(s).
2
AMF was developed from urban intersection dataset; no AMF exists for rural intersections.
3
AMF is for one-way streets in an urban environment.
4
Yellow-light-hold study.
5
For unsignalized intersections only; no AMF for signalized intersections.
6
There have been two studies recently conducted using different methodologies that have arrived at different conclusions regarding the magnitude of the
safety effect. Both were reanalyzed as part of this research study; see results in Chapter 3.
7
Freeways only, no AMFs for other road types.
8
Information available for two-lane roads only.
9
AMFs available for rural two-lane and multilane roads; no AMF available for urban/suburban arterials.
10
AMF available for rural two-lane roads; no AMF available for rural multilane or urban/suburban arterials.
11
AMF available for freeways only; no AMF available for other road classes.
12
There has been some work on profiled pavement markings that have some similarities to edge-line rumble strips (71).
13
The project team is aware of a TRB paper under review which should increase the knowledge level significantly.
14
AMF available for rural two-lane roads only; no AMF available for rural multilane roads or urban/suburban arterials.
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Table 2. Treatments with AMFs that have a high necessary to identify segments of roadways or intersections
or medium-high level of predictive certainty. that were similar to the treated locations, but were untreated
during the before-treatment and after-treatment periods.
Treatment Level of Predictive Certainty
Intersection Treatments
Such historical data are available in HSIS for nine states. Use
*Install a roundabout High of those data would require that the treatment being consid-
*Add exclusive left-turn lane High ered was implemented in one of those nine states and that
*Add exclusive right-turn lane High treatment details can be found. As part of the user survey,
*Install an urban traffic signal High the project team also asked states to provide a listing of treat-
Remove an urban traffic signal High
Modify signal change interval Medium-High ment evaluations they had conducted in the past. If a state had
Convert to all-way stop control Medium-High evaluated one of the higher-ranked treatments of interest,
Convert stop-control to yield-control Medium-High follow-up conversations were held to determine if historic
Install red-light cameras High
crash, inventory, and traffic data were available for both treat-
Roadway Segment Treatments
ment sites and possible comparison/reference sites. Based on
Narrow lane widths to add lanes Medium-High
Add passing lanes (two-lane roads) Medium-High information about available data from both sources, the proj-
Add two-way left-turn lane (TWLTL) Medium-High ect team then developed a listing of proposed AMF develop-
*Increase lane width Medium-High ment strategies for each treatment in Table 3 for which the
*Change shoulder width and/or type Medium-High overall priority for future work was high or medium-high.
*Flatten horizontal curve Medium-High
*Improve curve superelevation Medium-High Table 4 shows the AMFs proposed for development or im-
*Add shoulder rumble strips on freeways Medium-High provement and presents suggested study methodologies for
*Add centerline rumble strips Medium-High each--an EB evaluation based on new data, an analysis-
*Install/upgrade guardrail Medium-High driven expert panel, or a reanalysis of prior study data.
Miscellaneous Treatments
Based on their review of these suggestions and on a series of
Install raised medians at crosswalks Medium-High
*Treatments in top quartile of the DOT users' priority listing.
follow-up discussions with the project team, the oversight
panel recommended that priority should be given to conduct-
ing as many EB evaluations of these high-priority treatments as
These four factors were combined by the project team into possible, that expert panels were acceptable as long as they were
an overall ranking (high, medium-high, medium-low, or analysis-driven (i.e., based on review of past research findings
low) for possible additional research as shown in the last col- with limited additional analyses where needed), and that the
umn of Table 3. It is noted that even though only a limited project efforts should continue to be closely coordinated with
number of these higher-ranked treatments could be further ongoing work to develop safety prediction tools for urban/
researched in Phase II of this project effort, the rankings in suburban arterials (NCHRP Project 17-26) and rural multilane
this table could also be used in future decisions concerning highways (NCHRP Project 17-29) for use in the HSM. Based
funding for treatment evaluations. on the funding available and on team knowledge of available
For those treatments with a high or medium-high ranking data, it was decided that EB analyses would be conducted to
for possible additional research, the project team then com- develop AMFs for the following high-priority treatments:
pleted their exploration of possible available data (the final
factor in the above list of decision factors). As noted above, · Installation of a traffic signal at a rural intersection (new EB
data requirements for a sound evaluation (such as an EB before-after evaluation);
before-after analysis) included both treatment information · Conversion of an undivided four-lane road to three lanes
(i.e., treatment details, date of installation, and location) and including a two-way left-turn lane (TWLTL)--a "road
historic crash, roadway inventory, and traffic data before and diet" (reanalysis of data from two previous studies);
after the treatment installation date for both the treatment · Increasing pavement friction on intersection approaches
sites and for comparable reference sites. The latter require- (reanalysis of previous study data); and
ment is most difficult to meet. State DOTs often have details · Increasing pavement friction on roadway segments
of treatments and sometimes have conducted a simple before- (reanalysis of previous study data).
after study, which means that before-treatment and after-
treatment crash counts were documented. State DOTs are less In addition, in order to maximize the number of AMFs
likely to have retained information on traffic volume changes produced through EB evaluations, project staff conducted a
during the before-treatment and after-treatment periods for detailed analysis of data provided by Winston-Salem, North
the treatment site. While some states do retain historic traffic Carolina. There, the Director of Transportation had docu-
volume, average annual daily traffic (AADT) data, virtually no mented installation records for over 70 treatment types
state retains historic files of roadway inventories. The latter is implemented since the 1980s. There were multiple sites for
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Table 3. Factors used and final rankings for additional research on intersection,
segment, and miscellaneous treatments.
Overall
User Level of Ongoing/ Crash Priority
Treatments Priority Predictive Future Harm for
Ranking Certainty Work1 Rating2, 3 Future
Work2
Intersection Treatments
Install a roundabout 19 Medium High H (4,5) L
Reduce or eliminate intersection skew Medium Low ML* (4) ML
Install offset T's Non-Existent ML* (5) ML
Add exclusive left-turn lane 1 High MH (10) L
Create positive offset for opposing left-
Non-Existent H (5) ML
turn lanes
Add exclusive right-turn lane 4 High L* (27) L
Add channelization for right-turns 11 Non-Existent L (27) MH
Install a traffic signal High (urban) L (urban)
2 H (4)
None (rural) H (rural)
Add a left-turn phase (protected or
8 Medium Low H (5) MH
protected/permissive)
Install dynamic advance warning
17 Low MH (10) MH
flashers "Red Signal Ahead"
Install overhead flashing beacon Non-Existent H (4) ML
Convert to all-way stop Medium High H (4) ML
Prohibit left turns Non-Existent MH* (5) MH
Install intersection lighting 13 Low MH* (4,9) ML
Close driveways near intersections 22 Non-Existent ML (16) ML
Install marked crosswalk Medium Low L* (7) L
Road Segment Treatments
Add a travel lane 8 Low MH (11) MH
Reduce number of lanes (road diet) Medium Low MH* (1) MH
Add passing lanes (two-lane roads) Medium High H (6) L
Widen median Low ML* (11,6) MH
Install raised median 20 Non-Existent L* (6) L
Increase lane width (two-lane) 21 Medium High H (6) L
Increase lane width (multilane) 21 Non-Existent L (6) L
Change shoulder width and/or type
15 Medium High H (2,3) L
(two-lane)
Change shoulder width and/or type
15 Non-Existent L* (2,3) ML
(multilane)
Flatten horizontal curve (two lane) 12 Medium High H (2,3) L
Flatten horizontal curve (multilane) 12 Non-Existent L* (2,3) ML
Improve curve superelevation 18 Medium High H (2,3) ML
Add static curve warning signs
Non-Existent H (2,3) H
and/or pavement markings
1 2
Add shoulder rumble strips (freeways) 5 Medium High H (2,3) L
Add shoulder rumble strips (multi-lane
5 Medium Low L* (2,3) ML
divided)
Add shoulder rumble strips (two lane) 5 Non-Existent H (2,3) MH
Add edgeline rumble strips 14 Non-Existent H (2,3) MH
Add centerline rumble strips (two-lane
10 Medium High H (6) L
roads)
Remove roadside obstacle 3 Medium Low H (2) MH
Flatten sideslope 22 Low H (3) MH
Install/upgrade guardrail 22 Medium High H (2,3) ML
Install median barriers 6 Medium Low L* (6) L
Eliminate left turns at driveways 16 Non-Existent ML (16) MH
Install roadway segment lighting 23 Medium Low MH MH
Add sidewalk/walkway Non-Existent ML* (1) ML
Add midblock pedestrian signal Non-Existent MH* (1) MH
Install raised crosswalks (non-intersection) Non-Existent MH* (1) MH
Install mid-block pedestrian crossing Non-Existent MH* (1) ML
Miscellaneous Treatments
Consolidate driveways Medium Low MH (10,20) ML
Provide signal coordination 7 Non-Existent MH (10,12) MH
Increase pavement friction Low MH* (2,3) MH
Install crosswalk in-pavement lighting Non-Existent H (1) ML
1
Checkmarks reflect treatments for which there is ongoing or planned future work to develop AMFs.
2
H = High, MH = Medium-High, ML = Medium-Low, L = Low.
3
Primary crash types are shown in parentheses with the crash harm rating. See Appendix A for discussion of the
crash types. Those ratings that were adjusted are designated with an asterisk.
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Table 4. Possible study methodologies for high-priority AMFs.
High Priority Treatments Overall Possible Study Methodologies
(Grey shading indicates high ranking Priority
in user survey) for EB Expert Reanalysis of
Future Evaluation Panel Prior Study
Work Data
Intersection AMFs
Install or remove a signal H (Rural)
Add a left-turn phase MH
(permissive/protected or protected-only)
Channelize right turns MH
Install dynamic advance warning flashers MH
"Red Signal Ahead"
Provide signal coordination MH
Prohibit left turns MH
Increase pavement friction on approaches H
Segment AMFs
Add a travel lane MH
Remove roadside obstacles (including MH
urban)
Add shoulder rumble strips (two- MH
lane/others)
Add edgeline rumble strips MH
Eliminate left turns at driveways MH
Flatten sideslopes MH
Install roadway segment lighting MH
Add advance curve warning signs/on- H
pavement markings
Increase pavement friction MH
Add midblock pedestrian signal MH
Raise crosswalks (non-intersection) MH
Reduce number of lanes (road diet) MH
Widen median MH
*
H = High, MH = Medium-High, ML = Medium-Low, L = Low.
most of the treatment types. He had consistently conducted · Replacement of a single red signal head with dual red signal
simple before-after studies of the effects of these treatments heads, and
on both total crashes and "target" crashes (e.g., angle · Conversion of nighttime flashing operation to steady oper-
crashes for stop-to-signal conversions). These studies usu- ation.
ally contained 3 to 5 years of both before-treatment and
after-treatment crash data. The documented data did not The first of these, the left-turn phase treatment, is included
contain AADT data across the study years, and there were in Table 4. While the latter three treatments are not included
no computerized data files that would allow the develop- as high-priority treatments in that table, examination of ear-
ment of a reference group for use in EB analyses. However, lier studies in the Phase I literature review indicated that the
if these data could be obtained from other sources, then, if AMFs existing for the second and third treatments (replacing
carefully chosen, multiple treatments could be analyzed 8-in. signal head with 12-in. head and replacing single red
using the same reference group. The project team then signal head with dual red signal heads) were rated as medium-
examined the data to identify treatment types evaluated in low in predictive certainty. The final one (converting night-
recent years that had sufficient sample sizes in the before- time flashing operation to steady operation) had no AMF. In
treatment and after-treatment periods to allow statistical addition, this work allowed the researchers to develop AMFs
significance tests. The team also identified "clean" treat- for treatments of high interest to local (urban) traffic engi-
ment sites with each treatment type, where no additional neers. Finally, as noted above, the use of a single reference
treatments had been applied during the before-treatment group greatly lowered the evaluation cost per treatment.
and after-treatment periods (a major undertaking since The other approach to AMF development/modification
many of the sites had undergone more than one treatment). involved two analysis-driven expert panels. While earlier
Based on these preliminary analyses, a decision was made to project discussions and information in Table 4 suggested the
evaluate the following treatments at signalized intersections: possibility of expert panels for AMFs related to specific focus
areas (e.g., roadside crashes and pedestrian treatments),
· Modification of a left-turn signal phase (three combinations), it was decided by the team and the oversight panel that a more
· Replacement of an 8-in. signal head with 12-in. head, critical need was to assist the research teams for NCHRP
