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34 CHAPTER 5 Compilation of Recommended AMFs Introduction Analysis-Driven Expert Panel. A panel of knowledgeable researchers and practitioners was convened to review crit- This chapter includes a detailed description of each AMF ical research studies and reach a consensus on AMFs for a verified, modified, or developed in this research effort. given treatment. In some cases, the AMF was developed Table 18 includes the listing of AMFs and the source of each through further analysis by one of the NCHRP project AMF. The possible sources include the following: research teams sponsoring the expert panel meeting. Cross-Sectional Model. A new analysis was conducted in Literature Review. Completed research was discovered which a cross-sectional model was produced and used to and critically reviewed. The assessment revealed that AMFs derive AMFs for a specific treatment. existed for the given treatment with an LOPC of either high or medium high. AMF Summaries EB Before-After Evaluation. Before-treatment and after- treatment crash data were acquired for locations where the For each AMF listed in Table 18, a summary of the research treatment of interest had been installed. The latest statisti- from which the AMF was developed is given below. Each sum- cal methodologies (i.e., EB) for conducting before-after mary includes the AMF mean estimate(s) with standard errors studies were applied to produce AMFs. shown in parentheses, the LOPC, the study methodology, a Reanalysis of Existing/Supplemental Data. Data from description of the sites used in the study, and supplemental prior before-after evaluations were acquired and reanalyzed comments and footnotes to describe the study results and using the more rigorous EB methodology. In many cases, applicability. Table 19 presents a glossary of acronyms used in supplemental data were acquired to enhance the evaluation. the AMF summaries.

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Table 18. List of AMFs developed or modified in NCHRP Project 17-25.* Treatment Source of AMF Intersection Treatments Install Roundabout Literature Review Add Exclusive Left-Turn Lane Literature Review/Expert Panel Add Exclusive Right-Turn Lane Literature Review/Expert Panel Install Traffic Signal at Urban Intersection Literature Review Install Traffic Signal at Rural Intersection EB Before-After Evaluation Remove Traffic Signal (Urban Environment) Literature Review Modify Signal Change Interval Literature Review Prohibit Right Turn on Red Expert Panel/Further Analysis Modify Left-Turn Phase Expert Panel/Reanalysis of Existing/Supplemental Data Replace 8-in. Signal Heads with 12-in. Signal Heads Reanalysis of Existing/Supplemental Data Replace Single Red Signal Head with Dual Red Signal Reanalysis of Existing/Supplemental Data Heads Convert Nighttime Flash Operation to Steady Reanalysis of Existing/Supplemental Data Operation Convert to All-Way Stop Control Literature Review Convert Stop Control to Yield Control Literature Review Install Red-Light Cameras Literature Review Add Intersection Lighting Expert Panel Increase Pavement Friction on Intersection Approach Reanalysis of Existing Data Roadway Segment Treatments Narrow Lane Widths to Add Lanes Literature Review Add Passing Lanes (Two-Lane Roads) Literature Review Add Two-Way Left-Turn Lane (TWLTL) Literature Review/Expert Panel Change Lane Width Literature Review Change Shoulder Width and/or Type Literature Review Flatten Horizontal Curve Literature Review Improve Curve Superelevation Literature Review Add Shoulder Rumble Strips Literature Review Add Centerline Rumble Strips Literature Review Install/Upgrade Guardrail Literature Review Convert Undivided Four-Lane Road to Three-Lane and Reanalysis of Existing Data TWLTL (Road Diet) Increase Pavement Friction on Roadway Segment Reanalysis of Existing Data Change Median Width Cross-Sectional Model Change Roadside Sideslope Expert Panel Add/Remove On-Street Parking Expert Panel Add Roadway Segment Lighting Expert Panel Miscellaneous Install Raised Medians at Crosswalks Literature Review Reduce Mean Travel Speed Reanalysis of Existing Data *AMFs are listed in order of their presentation in this report. Table 19. Glossary of acronyms for AMF summaries. AADT Average annual daily traffic ADT Average daily traffic AMF Accident modification factor B/A Before/after EB Empirical Bayes HOV High-occupancy vehicle HSIS Highway Safety Information System LTL Left-turn lane LOPC Level of predictive certainty MH Medium high PDO Property damage only RTL Right-turn lane RTOR Right turn on red SD Superelevation deficiency SPF Safety performance function TWLTL Two-way left-turn lane vpd Vehicles per day

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TREATMENT: Install Roundabout AMF Level of Predictive Certainty: High METHODOLOGY: Empirical Bayes Before-After CRASH TYPE STUDIED AND ESTIMATED EFFECTS REFERENCE: Rodegerdts et al. - 2007 ( 8 ). No. of Single Lane - Urban/Suburban Improved AMF (prior control - two-way stop-controlled) STUDY SITES: Sites All Crashes 0.44 (0.06) Treatment sites included 55 intersections that were converted to 16 Injury Crashes 0.22 (0.07) roundabouts (36 were previously two-way stop-controlled, 10 were all-way Single Lane - Rural stop-controlled, and 9 were controlled by signals). (prior control - two-way stop-controlled) All Crashes 0.29 (0.04) The roundabouts were in rural, suburban, and urban environments. 9 Injury Crashes 0.13 (0.03) Single-lane and multilane roundabouts were included; traffic volumes at Multilane - Urban/Suburban the treatment sites in the after condition ranged from 2,668 vpd to 58,800 (prior control - stop sign) vpd. All Crashes 0.82 (0.08) 11 Injury Crashes 0.28 (0.09) COMMENTS: Single/Multilane - Urban/Suburban A non-significant increase of 3% was found for 10 sites which were all- (prior control - signal) way stop-controlled prior to conversion to a roundabout. All Crashes 0.52 (0.05) 9 Injury Crashes 0.22 (0.06) The authors were not able to determine the safety effects for pedestrians and bicyclists, but refer the reader to the positive results that have been All Sites A found in Scandinavian evaluations. All Crashes 0.65 (0.03) 55 Injury Crashes 0.24 (0.03) No evidence was found to indicate roundabouts result in more difficulties for older drivers. FOOTNOTES: A Ulf and Jrgen - 1999 ( 78 ).

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TREATMENT: Add Exclusive Left-Turn Lane AMF Level of Predictive Certainty: High* METHODOLOGY: Empirical Bayes Before-After/Analysis-Driven Expert CRASH TYPE STUDIED AND ESTIMATED EFFECTS Panels REFERENCE: Harwood et al. - 2002 ( 12 ); NCHRP Project 17-25 research No. of AMF Total Intersection Accidents results (all severity levels, all accident types) Improved One BothA STUDY SITES: Sites Approach Approaches Rural Stop-Controlled Intersection (four legs) 25 0.72 (0.03) 0.52 (0.03) Included rural and urban sites located in eight states Illinois, Iowa, Rural Stop-Controlled Intersection (three legs) 36 0.56 (0.06) -- Louisiana, Minnesota, Nebraska, North Carolina, Oregon, and Virginia Rural Signalized Intersection (four legs) 0.82D 0.67D 199 treatment sites where a left-turn lane (LTL) was added, as well as 300 D Rural Signalized Intersection (three legs) 0.85 -- similar intersections that were not improved during the study period and used for comparison and reference sites. Urban Stop-Controlled Intersection (four legs) 9 0.73C (0.03) 0.53C (0.04) All improvements were made during the years 1989 through 1998. Mean Urban Stop-Controlled Intersection (three legs) 8 0.67 (0.12) -- duration of before and after periods were 6.7 years and 3.9 years, Urban Signalized Intersection (four legs) 39 0.90 (0.01) 0.81 (0.13) respectively. Urban Signalized Intersection (three legs) 0.93D -- COMMENTS: Fatal and Injury Intersection Accidents (all accident types) The study applied two alternative evaluation approaches (B/A with yoked Rural Stop-Controlled Intersection (four legs) 24 0.65 (0.03) 0.42 (0.04) comparisons and B/A with a comparison group) and recommended that the Rural Stop-Controlled Intersection (three legs) 11 0.45C (0.08) -- EB evaluation results be used if statistically significant. If not, it was Urban Stop-Controlled Intersection (four legs) 9 C C 0.71 (0.04) 0.50 (0.06) recommended that statistically significant comparison group results be used, Urban Stop-Controlled Intersection (three legs) 0.65F -- followed by statistically significant yoked comparison results. The authors Urban Signalized Intersection (four legs) 39 0.91 (0.01) 0.83 (0.02) note that results from either comparison method may be "overly optimistic." Stop-controlled locations had stop signs on the minor road approaches. Urban Signalized Intersection (three legs) 0.94F -- B Mean total entering ADT for rural stop-controlled, rural signalized, urban Project-Related Accidents (all severity levels) stop-controlled, and urban signalized improved sites were 9,700 vpd, 17,800 Rural Stop-Controlled Intersection (four legs) 23 0.63 (0.07) 0.40 C vpd, 15,500 vpd, and 26,800 vpd, respectively. Rural Stop-Controlled Intersection (three legs) 35 0.38 (0.15) -- All tests of statistical significance in this report were performed at the 5% Urban Stop-Controlled Intersection (four legs) 7 0.74 (0.07) 0.55 significance level (95% confidence level). Only statistically significant Urban Signalized Intersection (four legs) 35 0.87E (0.03) 0.76E results are shown. *LOPC considered to be MH for AMFs derived by analysis-driven expert panels. FOOTNOTES: A AMF (both approaches) = AMF (one approach) x AMF (one approach). B Project-Related Accidents - All accidents involving one or more vehicles that had made, were making, or intended to make the specific left-turn maneuver(s) for which the left-turn lane(s) being evaluated were installed. C AMF based on comparison group evaluation. D Recommended AMF based on analysis-driven expert panel results (rural two-lane roads) from Harwood et al. - 2000 ( 9 ). E AMF based on yoked comparison evaluation. F Recommended AMF based on analysis-driven expert panel results from NCHRP 17-25/17-26 expert panel on urban/suburban arterials.

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TREATMENT: Add Exclusive Right-Turn Lane AMF Level of Predictive Certainty: High* METHODOLOGY: Empirical Bayes Before-After/Analysis-Driven CRASH TYPE STUDIED AND ESTIMATED EFFECTS Expert Panel REFERENCE: Harwood et al. - 2002 (12 ); NCHRP Project 17-25 No. of AMF Total Intersection Accidents research results (all severity levels, all accident types) Improved One BothA STUDY SITES: Sites Approach Approaches Rural Stop-Controlled Intersection (four legs) 28 0.86 (0.05) 0.74 Included rural and urban sites located in eight states Illinois, Iowa, Rural Signalized Intersection (four legs) 0.96B (0.02) 0.92B Louisiana, Minnesota, Nebraska, North Carolina, Oregon, and Virginia. Urban Signalized Intersection (four legs) 18 0.96 (0.02) 0.92 108 treatment sites where a right-turn lane (RTL) was added, as well as C 300 similar intersections that were not improved during the study period Urban Signalized Intersection (three legs) 0.96 -- C and used for comparison and reference sites. Urban Stop-Controlled Intersection (four legs) 0.86 0.74C All improvements were made during the years 1989 through 1998. Fatal and Injury Intersection Accidents (all accident types) Mean duration of before and after periods were 6.7 years and 3.9 years, Rural Stop-Controlled Intersection (four legs) 29 0.77D (0.07) 0.59D respectively. Rural Signalized Intersection (four legs) B 0.91 (0.03) 0.83B Urban Signalized Intersection (four legs) 17 0.91 (0.03) 0.83 C Urban Signalized Intersection (three legs) 0.91 -- Urban Stop-Controlled Intersection (four legs) 0.77C 0.59C *LOPC considered to be MH for AMFs derived by analysis-driven expert panels. COMMENTS: The study applied two alternative evaluation approaches (B/A with yoked comparisons and B/A with a comparison group) and recommended that the EB evaluation results be used if statistically significant. If not, it was recommended that statistically significant comparison group results be used, followed by statistically significant yoked comparison results. The authors note that results from either comparison method may be "overly optimistic." Stop-controlled locations had stop signs on the minor road approaches. Mean total entering ADT for rural stop-controlled, rural signalized, urban stop-controlled, and urban signalized improved sites were 9,700 vpd, 17,800 vpd, 15,500 vpd, and 26,800 vpd, respectively. All tests of statistical significance in this report were performed at the 5% significance level (95% confidence level). Only statistically significant results are shown. FOOTNOTES: A AMF (both approaches) = AMF (one approach) x AMF (one approach). B Authors recommend that the AMFs for urban signalized intersections be applied to rural signalized intersections. C Recommended AMF based on analysis-driven expert panel results from NCHRP 17-25/17-26 panel on urban/suburban arterials. D AMF based on comparison group evaluation.

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TREATMENT: Install Traffic Signal at Urban Intersection AMF Level of Predictive Certainty: High METHODOLOGY: Empirical Bayes Before-After CRASH TYPE STUDIED AND ESTIMATED EFFECTS REFERENCE: McGee, Taori, and Persaud - 2003 ( 18 ) No. of Three-Leg Intersections Improved AMF STUDY SITES: Sites All Crashes 0.86 (0.32) Included sites located in five states California, Florida, Maryland, Virginia, Right-Angle Crashes 22 0.66 (0.45) Wisconsin and Toronto. Rear-End Crashes 1.5 (0.51) Three-leg intersection data included 22 treatment sites (converted from stop to Four-Leg Intersections signal control) and 118 reference group sites (99 stop-controlled and 19 All Crashes 0.77 (0.22) signalized intersections). Four-leg intersection data included 100 treatment sites (converted from stop to Right-Angle Crashes 100 0.33 (0.20) signal control) and 295 reference group sites (96 stop-controlled and 199 Rear-End Crashes 1.38 (0.39) signalized intersections). An additional reference group was developed from the HSIS California urban data and included 1,418 stop-controlled and 799 signalized intersections. A Minor street traffic volumes for the treatment sites ranged from 911 to 3,952 vpd; major street volumes ranged from 11,739 to 24,584 vpd. COMMENTS: AMFs are for crashes involving fatalities and injuries only; property-damage-only (PDO) crashes were excluded from the analysis. AMFs were developed using data from urban intersections. The authors do not recommend that these results be applied to rural intersections. The study notes that the results could be adapted (i.e., reversed) to assess the safety of removing a traffic signal. The authors of the study do not have as much confidence in using the results in this way. FOOTNOTES: A The Highway Safety Information System (HSIS) is a multistate safety database that contains accident, roadway inventory, and traffic volume data for a select group of states and is sponsored by the FHWA.

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TREATMENT: Install Traffic Signal at Rural Intersection AMF Level of Predictive Certainty: High METHODOLOGY: Empirical Bayes Before-After CRASH TYPE STUDIED AND ESTIMATED EFFECTS No. of Three-Leg and Four-Leg Intersections REFERENCE: NCHRP Project 17-25 research results Improved AMF Combined Sites STUDY SITES: All Crashes 0.56 (0.03) Right-Angle Crashes 0.23 (0.02) Included sites located in Minnesota and California. Data were acquired from the 45 Highway Safety Information System.A Rear-End Crashes 1.58 (0.14) Three-leg intersection data included six treatment sites (converted from stop to Left-Turn Crashes 0.4 (0.05) signal control) and 1,927 stop-controlled reference group sites. ECONOMIC ANALYSIS Four-leg intersection data included 39 treatment sites (converted from stop to Three-Leg and Four-Leg Intersections signal control) and 1,661 stop-controlled reference group sites. AMF Combined An additional reference group was developed using 84 signalized intersections All Crashes 0.27 (0.001) to develop a more sophisticated procedure for evaluating the potential safety effects of a contemplated signal conversion. Minor street traffic volumes for the treatment sites ranged from 101 to 10,300 vpd; major street volumes ranged from 3,261 to 29,926 vpd COMMENTS: The authors of the study do not recommend that the results be adapted (i.e., reversed) to assess the safety of removing a traffic signal. The treatment benefits are greater on higher volume intersections and are greater where the ratio of expected right-angle crashes to rear-end crashes is higher. There is little difference between the effects on three-leg vs. four-leg sites or on sites with two lanes on the major vs. four lanes. Thus, the overall crash frequency AMFs can be assumed to apply to all rural site types. Economic analysis was conducted to determine if the increase in rear-end crashes negated the decrease in other, generally more severe, collision types. The economic analysis may be used to develop AMFs for total crashes, which account for the differences in injury severity that occur with different collision types. The AMF for all crash severities would be 0.27. FOOTNOTES: A The Highway Safety Information System (HSIS) is a multistate safety database that contains accident, roadway inventory, and traffic volume data for a select group of states and is sponsored by the FHWA.

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TREATMENT: Remove Traffic Signal (Urban AMF Level of Predictive Certainty: High Environment) METHODOLOGY: Empirical Bayes Before-After CRASH TYPE STUDIED AND ESTIMATED EFFECTS REFERENCE: Persaud et al. - 1997 ( 19 ) No. of Type of Collision Improved AMF (all severities) STUDY SITES: Sites All Crashes 0.76 (0.38) 199 treatment sites and 71comparison sites in Philadelphia. Right-Angle and Turning Crashes 0.76 (0.35) Treatment sites were unwarranted signals and mostly changed from signal Rear-End Crashes 199 0.71 (0.06) control to all-way stop control between 1979 and 1988. Pedestrian Crashes 0.82 (0.12) All intersections were at one-way streets in non-arterial streets in an urban environment. Fixed-Object Crashes 0.69A Crash data were acquired for the years 1978 through 1992. Light Condition (all severities) Traffic volumes were often estimated from upstream and downstream Day 0.78 199 AADTs due to the sparse volume data available. Night 0.70 Injury Severity (all collision types) Severe 0.47 (0.10) 199 Minor 0.76 (0.18) COMMENTS: The authors note the inability to account for year-to-year variation in traffic volumes, but nonetheless express confidence in the results. It is important to note that this study was for one-way streets in an urban environment. There are no comparable studies for two-way streets or for intersections in rural environments. FOOTNOTES: A The AMF for fixed object crashes was based on the classical estimate (i.e., expected number of crashes in the after period is based on count of crashes in the before period as opposed to the EB estimate of before-period crashes).

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TREATMENT: Modify Signal Change Interval AMF Level of Predictive Certainty: Medium-High METHODOLOGY: Before-After with Control Group CRASH TYPE STUDIED AND ESTIMATED EFFECTS No. of REFERENCE: Retting, Chapline, and Williams - 2002 (22 ) Accident Type Treated AMF (all severities) STUDY SITES: Sites All Crashes 0.92 (0.09) Included crash data from 40 treatment intersections and 56 control Multiple-Vehicle Crashes 0.95A intersections in Nassau County and Suffolk County, New York. Rear-End Crashes 40 1.12A (0.16) All intersections were standard four-leg junctions. Right-Angle Crashes 0.96A (0.18) Pedestrian/Bicyclist Crashes 0.63 The treatment sites were randomly selected for the signal timing change, Accident Type eliminating the site-selection bias. (injury crashes only) All Crashes 0.88 (0.09) Six years of crash data were used in the analysis (October 1991 through Multiple-Vehicle Crashes 0.91 October 1997), with 3 years each in the before and after periods. Rear-End Crashes 40 1.08A (0.17) Analysis included only "reportable" crashes, which require an injury or a Right-Angle Crashes 1.06A (0.22) minimum of $1,000 in property damage in New York. Pedestrian/Bicyclist Crashes 0.63 COMMENTS: IMPORTANT NOTE - Both the yellow change interval and the red clearance interval were adjusted at the treatment sites to conform to the Institute of Transportation Engineers Determining Vehicle Change Intervals: A Proposed Recommended Practice (95 ). In some cases, this meant an increase in the interval, while in others, the interval was decreased. Thus the AMFs do not reflect the effects of increasing only the change and clearance intervals. AMFs are based on the odds ratios. Yellow change intervals at the treatment sites ranged from 3 to 4 seconds in the before period and 2.6 to 5.4 seconds in the after period. Red clearance intervals ranged from 2 to 3 seconds in the before period and 1.1 to 6.5 seconds in the after period. Authors acknowledge that the results do not account for variables such as geometry, traffic volume, and other signal parameters such as cycle length and number of phases FOOTNOTES: A Results were not significant at a 90% confidence level (P > 0.10). AMF of 1.0 recommended for these accident types.

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TREATMENT: Prohibit Right Turn on Red AMF Level of Predictive Certainty: Medium-High METHODOLOGY: Analysis-Driven Expert Panel ACCIDENT MODIFICATION FUNCTION REFERENCE: NCHRP Projects 17-25 and 17-26 research results COMMENTS: AMF = (0.984)n where: Expert panel on urban/suburban arterials considered this AMF function n = number of signalized intersection approaches where RTOR is to be the best estimate for the prohibition of right turn on red (RTOR). prohibited The AMF was derived from a simple before-after analysis of Note: AMF applies to total intersection crashes. intersections in Alabama and South Carolina after the passage of laws in both states that permitted RTOR.A The results were presented in terms of the effect on total crashes at an intersection if RTOR was permitted (AMF = 1.067). Making an assumption that most of the intersections FOOTNOTES: A were four-leg locations, the AMF for each approach becomes 1.016. Clark, Maghsoodloo, and Brown - 1983 ( 79 ) The inverse of the Clark AMF was derived to reflect the prohibition of RTOR (1/1.016 = 0.984).

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TREATMENT: Modify Left-Turn Phase AMF Level of Predictive Certainty: Medium-High METHODOLOGY: Empirical Bayes Before-After/Analysis-Driven CRASH TYPE STUDIED AND ESTIMATED EFFECTS Expert Panel REFERENCE: NCHRP Project 17-25 research results No. of Accident Type AMF STUDY SITES: Sites Change from Permissive or Permissive/Protected to Protected-Only Phasing The treatment sites included 12 signalized intersections in Winston- Left-Turn Crashes 0.01 (0.01)** Salem, NC. Among those 12 sites, the left-turn phase was changed from 12 Total Crashes 0.99A (0.07) permissive to protected-only at eight sites and from permissive/protected to Change from Permissive to Permissive/Protected Phasing protected-only at four sites. B Left-Turn Crashes 0.84 (0.02) 35 COMMENTS: Total Crashes 1.00B FOOTNOTES: There was evidence that non-left-turn crashes increased following the ** statistically significant at the 95% confidence level. A change to protected-only left-turn phasing. Further research is necessary to AMF of 0.99 was not statistically significant; AMF of 1.00 recommended. B determine the specific reasons for the effect on non-left-turn crashes. Recommended AMF based on analysis-driven expert panel results from NCHRP However, it seems reasonable to speculate that introducing a protected left- Projects 17-25/17-26 panel on urban/suburban arterials. Primary source of turn phase will tend to increase mostly rear-end crashes (which are in information was the study by Lyon et al. - 2005 (21 ), which included 35 four-leg general less severe compared to left-turn crashes) because of the increased intersections in Toronto. number of phases (and therefore dilemma zone opportunities) and the increase in queues that results from reduced green time available for all traffic not protected by the introduced phase. This also implies that the measure would be most effective overall where there is a relatively high frequency of left-turn crashes.

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TREATMENT: Flatten Horizontal Curve AMF Level of Predictive Certainty: Medium-High METHODOLOGY: Analysis-Driven Expert Panel ACCIDENT MODIFICATION FUNCTION REFERENCE: Harwood et al. - 2000 (9 ) 80.2 1.55L C 0.012S AMF R COMMENTS: 1.55L C AMF applies to total accidents on the curved roadway segment. where: LC = length of horizontal curve (miles); does not include spiral curve length AMF was derived from the regression model developed by Zegeer et al.A R = radius of curvature (ft) S = 1 if spiral transition curve is present and 0 if no such transition exists The AMF is applicable to rural two-lane roads only. FOOTNOTES: A Zegeer et al. - 1992 (90 ). TREATMENT: Improve Curve Superelevation AMF Level of Predictive Certainty: Medium-High METHODOLOGY: Analysis-Driven Expert Panel Superelevation Deficiency (SD) AMF REFERENCE: Harwood et al. - 2000 (9 ) < 0.01 1.00 0.01 < SD < 0.02 1.00 + 6(SD - 0.01) COMMENTS: > 0.02 1.06 + 3(SD - 0.02) AMF applies to total accidents occurring on curved roadway segments. Expert panel noted there was no safety effect until the superelevation reached 0.01. AMF was derived from the results of Zegeer et al.A FOOTNOTES: A Zegeer et al. - 1992 (90 ). The AMF is applicable to rural two-lane roads only.

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TREATMENT: Add Shoulder Rumble Strips AMF Level of Predictive Certainty: Medium-High METHODOLOGY: Before-After with Comparison Sites CRASH TYPE STUDIED AND ESTIMATED EFFECTS REFERENCE: Griffith - 1999 (53 ) No. of All Freeways (Rural and Urban) Improved AMF Sites STUDY SITES: All Single-Vehicle Run-Off-Road Crashes 0.82 (0.07) 55 Injury Single-Vehicle Run-Off-Road Crashes 0.87 (0.12) Included 55 treatment sites and 55 matched comparison sites from rural Rural Freeways and urban freeways in Illinois. All Single-Vehicle Run-Off-Road Crashes 0.79 (0.10) 29 Injury Single-Vehicle Run-Off-Road Crashes 0.93 (0.16) The treatment sites covered 196 miles of rural freeway and 67 miles of COMMENTS: urban freeway. Results for all freeways based on yoked comparison analysis; results for rural The treatment sites were not selected on the basis of accident history; freeways based on comparison group method using 29 of the treatment sites. Results thus, there was no selection bias. could not be developed for urban sites separately. An analysis of multi-vehicle accidents showed the rumble strips to have no effect on such accidents. The AMF is not applicable to other road classes (two-lane or multilane).

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TREATMENT: Add Centerline Rumble Strips AMF Level of Predictive Certainty: Medium-High METHODOLOGY: Empirical Bayes Before-After CRASH TYPE STUDIED AND ESTIMATED EFFECTS REFERENCE: Persaud, Retting, and Lyon - 2003 (57 ) No. of Accident Type Improved AMF (all severities) Sites STUDY SITES: All Crashes 0.86 (0.05) Frontal/Opposing-Direction Sideswipe 98 0.79 (0.12) Crash and traffic volume data were collected for 98 treatment sites, Crashes consisting of 210 miles, where centerline rumble strips had been installed Accident Type on rural two-lane roads in the states of California, Colorado, Delaware, (injury crashes) Maryland, Minnesota, Oregon, and Washington. All Crashes 0.85 (0.08) Frontal/Opposing-Direction Sideswipe 98 0.75 (0.15) The average length of the treatment sites was 2 miles, and the traffic Crashes volumes ranged from 5,000 to 22,000 vpd. COMMENTS: The reference group of sites was developed from HSIS data for the states The authors note that the results cover a wide range of geometric conditions, of California, Washington, and Minnesota.A Additional data were acquired including curved and tangent sections and sections with and without grades. from Colorado for SPF calibration for the Colorado sites. The results include all rumble strip designs (milled-in, rolled-in, formed, and raised thermo-plastic) and placements (continuous versus intermittent) that were present. The AMF is not applicable to other road classes (multilane). FOOTNOTES: A The Highway Safety Information System (HSIS) is a multistate safety database that contains accident, roadway inventory, and traffic volume data for a select group of states and is sponsored by the FHWA.

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TREATMENT: Install/Upgrade Guardrail AMF Level of Predictive Certainty: Medium-High METHODOLOGY: Meta-Analysis CRASH TYPE STUDIED AND ESTIMATED EFFECTS REFERENCE: Elvik and Vaa - 2004 ( 60 ) Run-Off-Road Accidents AMF STUDY SITES: Fatal Injury Crashes 0.56 (0.10) All Injury Crashes 0.53 (0.05) 20 studies were evaluated, including 12 U.S. studies (6 of which were conducted in 1982 or later). COMMENTS: The results apply to the installation of guardrail along an embankment. The studies were not differentiated by roadway class. The analysis also included an estimate for the change in accident rate, but the results were not significant. Results were also included for changing to softer guardrails. However, specifics on the type of change in hardware were not indicated, and not all results were significant. Therefore, they are not included here.

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TREATMENT: Convert Undivided Four-Lane Road AMF Level of Predictive Certainty: High to Three-Lane and TWLTL (Road Diet) METHODOLOGY: Empirical Bayes Before-After CRASH TYPE STUDIED AND ESTIMATED EFFECT REFERENCE: NCHRP Project 17-25 research results Accident No. of STUDY SITES: State/Site Characteristics AMF Type Treated Sites 15 urban locations in Iowa with a mean length of 1.02 miles, a Iowa minimum and maximum length of 0.24 and 1.72 miles. AADT after Predominately U.S. and state routes within Total 15 0.53 (0.02) conversion ranged from 3,718 to 13,908. small urban areas (average population of Crashes 15 miles 17,000) 30 urban locations from Washington and California studied California/Washington previously with a mean length of 0.84 miles, a minimum and Predominately corridors within suburban areas Total 30 0.81 (0.03) maximum length of 0.08 and 2.54 miles. AADT after conversion surrounding larger cities (average population of Crashes 30 mi ranged from 6,194 to 26,376. 269,000) Total 45 All Sites 0.71 (0.02) Crashes 40 mi FOOTNOTES: A Huang, Stewart, and Zegeer - 2002 (43 ). B Pawlovich et al. - 2006 (44 ). COMMENTS: The study conducted was a reanalysis of data from two prior studies. A,B The reanalysis of the Washington/California data indicated a 19% decrease in total crashes. The reanalysis of the Iowa data showed a reduction of 47% in total crashes. If the characteristics of the treated site can be defined on the basis of road and area type (as shown above), the AMFs of 0.53 and 0.81 should be used. Otherwise, it is recommended that the aggregate AMF of 0.71 be applied.

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TREATMENT: Increase Pavement Friction on Roadway AMF Level of Predictive Certainty: High Segment METHODOLOGY: Empirical Bayes Before-After CRASH TYPE STUDIED AND ESTIMATED EFFECTS No. of Accident Type Treated AMF REFERENCE: NCHRP Project 17-25 research results Sites All Crashes 0.76 (0.02) STUDY SITES: Wet-Road Crashes 0.43 (0.02) Rear-End Crashes 36 miles 0.83 (0.04) The treatment data for this analysis were from the Skid Accident Rear-End Wet-Road Crashes 0.58 (0.06) Reduction Program (SKARP) developed by NY State DOT in 1995. A Single Vehicle Crashes 0.7 (0.04) Data were collected from New York State for 36.3 miles of treated COMMENTS: segments and 1,242.4 miles of reference segments. Locations were in both The treatment generally involved a 1.5-in. resurfacing or a 0.5-in. microsurfacing urban and rural locations. using non-carbonate aggregates. The segments are in close proximity to treated intersections, which are the primary targets of the treatment. Table 10 in Chapter 3 of this report provides additional AMFs by categories for Sites are selected for treatment based on both a high proportion of wet- number of lanes and urban versus rural locations, although not all are statistically road accidents and low friction numbers. significant. FOOTNOTES: A Bray - 2001 (80 ).

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TREATMENT: Change Median Width AMF Level of Predictive Certainty: Medium-High METHODOLOGY: Cross-Sectional Model ACCIDENT MODIFICATION FACTORS REFERENCES: NCHRP Project 17-25 research results Full Access Control STUDY SITES: Ten years of data from 1993 to 2002 on divided roadway sections in California were obtained from the Highway Safety Information System (HSIS). The dataset included 500 miles of rural and urban roadways with parital or no access control and 1,400 miles with full access control. There were no median barriers on any of these roadway segments, meaning the barriers were traversable. Median widths for the segments included in the analysis ranged from 4 ft to 100 ft. Over the 10-year period, the partial/no-access control sections experienced approximately 41,000 total crashes and 5,000 cross-median crashes. The full-access control sections experienced approximately 125,000 total crashes and 5,000 cross-median crashes. Partial or No Access Control COMMENTS: NCHRP Projects 17-25/17-26 expert panel reviewed several studies of the effects of median width on crashes and reached a recommendation to either reanalyze data from one of those efforts or conduct a more robust analysis.

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TREATMENT: Change Roadside Sideslope AMF Level of Predictive Certainty: Medium-High METHODOLOGY: Expert Panel ACCIDENT MODIFICATION FACTORS REFERENCES: NCHRP Project 17-25 research results COMMENTS: Original study conducted by Zegeer et al.A used log linear regression models to develop estimates of the effects of sideslope on single-vehicle crashes and total crashes on rural two-lane roads. The AMFs shown were derived from these models. The NCHRP Projects17-25/17-29 expert panel on rural multilane highways concluded that the AMFs derived were valid and the best available for both rural two-lane roads and rural multilane highways. FOOTNOTES: A Zegeer et al. - 1988 ( 84 ).

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TREATMENT: Add/Remove On-Street Parking AMF Level of Predictive Certainty: Medium-High METHODOLOGY: Analysis-Driven Expert Panel ACCIDENT MODIFICATION FUNCTION REFERENCE: Bonneson, Zimmerman, and Fitzpatrick - 2005 (91) AMF = 1 + Ppk (Bpk - 1) where: COMMENTS: Ppk = proportion of curb length with on-street parking (= 0.5Lpk/L) Lpk = curb length with on-street parking (mi) Expert panel on urban/suburban arterials considered this AMF function L = roadway segment length (mi) to be the best estimate for the addition or removal of on-street parking. Bpk = (1.10 + 0.365Iu2 + 0.609Pb/o)[(fap/pp 1.0)Pap + 1.0] AMF was derived from a negative binomial regression model where: (Bonneson and McCoy)A and from other prior study data (McCoy et al.).B Iu2 = cross-section indicator variable (two-lane street = 1; otherwise = 0) Pb/o = proportion of street with parking that has business or office as Value for the ratio of crashes on streets with angle parking to crashes adjacent land use on streets with parallel parking (fap/pp) derived by Bonneson et al.C to be fap/pp = ratio of crashes on streets with angle parking to crashes on 2.34 on the basis of data from McCoy et al.B and Box.D streets with parallel parking (= 2.34; see comment) Pap = for that part of the street with parking, the proportion of the street with angle parking FOOTNOTES: A Bonneson and McCoy - 1997 ( 92). B McCoy et al. - 1990 ( 93). C Bonneson, Zimmerman, and Fitzpatrick - 2005 (91). D Box - 2002 ( 94 ).

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TREATMENT: Add Roadway Segment Lighting AMF Level of Predictive Certainty: Medium-High METHODOLOGY: Meta-analysis/Expert Panel CRASH TYPE STUDIED AND ESTIMATED EFFECTS REFERENCE: Elvik and Vaa - 2004 (60 ); NCHRP 17-25 Final Report; Nighttime Crashes AMF NCHRP 17-26 Final Report STUDY SITES: Total Crashes 0.80 All Injury Crashes 0.71 38 studies were evaluated as part of the meta-analysis, including 14 U.S. studies.A All Crashes AMF Distributions of crashes by injury severity and time of day were obtained Total Crashes 0.94 from the HSIS data for the states of Minnesota and Michigan. All Injury Crashes 0.92 COMMENTS: The meta-analysis results produced AMF estimates for reductions in fatal, injury and property-damage-only accidents of 0.36, 0.72, and 0.83, respectively.A The NCHRP 17-25/17-26 expert panel on urban/suburban arterials recommended that the meta-analysis results be applied to roadway segments and that the fatal and injury results be combined into a single AMF for all levels of injury. The NCHRP 17-26 Final Report includes a distribution of crashes by time of day and injury severity for several roadway classes. AMFs shown represent the mean estimates for all roadway classes and were derived on the basis of these distributions and the meta-analysis AMFs. FOOTNOTES: A Elvik and Vaa (60 ) TREATMENT: Install Raised Medians at Crosswalks AMF Level of Predictive Certainty: Medium-High METHODOLOGY: Matched Comparison CRASH TYPE STUDIED AND ESTIMATED EFFECTS REFERENCE: Zegeer et al. - 2001 ( 38 ) No. of Total Pedestrian Accidents Median AMF (all severities) STUDY SITES: Sites Marked Crosswalks* 0.54 2,000 sites were included in the study to evaluate the effect of marked vs. Unmarked Crosswalks* 173 0.61 unmarked crosswalks (1,000 matched pairs of each type), * Applicable to urban and suburban multilane roads (up to eight lanes) with traffic volumes greater than 15,000 vpd. 260 of these sites were on multilane roads and had raised medians. COMMENTS: On average, 5 years of crash data were collected for each site, as well as traffic data and pedestrian volume estimates. The AMFs were computed from the pedestrian crash rates (pedestrian crashes per million crossings) for sites with medians versus the sites without medians.

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TREATMENT: Reduce Mean Travel Speed AMF Level of Predictive Certainty: Medium-High METHODOLOGY: Reanalysis of Existing Data ACCIDENT MODIFICATION FACTORS REFERENCES: NCHRP Project 17-25 research results COMMENTS: Original study conducted by Elvik and colleagues used the Power Model to develop estimates of the effectiveness of changes in mean travel speeds. Data included mean speed change and the related crash-frequency change from 97 published international studies containing 460 results. Each result contained information on mean speed and crash frequency before treatment and mean speed and crash frequency after treatment. A The NCHRP Projects 17-25/17-26 expert panel reviewed the original study and requested supplemental analysis to explore the validity of the results and to develop AMFs. NOTE: This series of AMFs is related to any treatment that is associated with a changed mean speed (e.g., changes in enforcement or installation of traffic calming measures). FOOTNOTES: A Elvik, Christensen, and Amundsen - 2004 (74 ).