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Intersection Crash Prediction Methods for the Highway Safety Manual (2021)

Chapter: Chapter 8. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Single-Point Diamond Interchanges

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Suggested Citation:"Chapter 8. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Single-Point Diamond Interchanges." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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Suggested Citation:"Chapter 8. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Single-Point Diamond Interchanges." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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Suggested Citation:"Chapter 8. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Single-Point Diamond Interchanges." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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Suggested Citation:"Chapter 8. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Single-Point Diamond Interchanges." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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Suggested Citation:"Chapter 8. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Single-Point Diamond Interchanges." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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Suggested Citation:"Chapter 8. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Single-Point Diamond Interchanges." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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Suggested Citation:"Chapter 8. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Single-Point Diamond Interchanges." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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Suggested Citation:"Chapter 8. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Single-Point Diamond Interchanges." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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Suggested Citation:"Chapter 8. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Single-Point Diamond Interchanges." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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Suggested Citation:"Chapter 8. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Single-Point Diamond Interchanges." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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Suggested Citation:"Chapter 8. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Single-Point Diamond Interchanges." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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Suggested Citation:"Chapter 8. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Single-Point Diamond Interchanges." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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Suggested Citation:"Chapter 8. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Single-Point Diamond Interchanges." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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Suggested Citation:"Chapter 8. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Single-Point Diamond Interchanges." National Academies of Sciences, Engineering, and Medicine. 2021. Intersection Crash Prediction Methods for the Highway Safety Manual. Washington, DC: The National Academies Press. doi: 10.17226/26153.
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183 Chapter 8. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Single-Point Diamond Interchanges This section describes the development of crash prediction models for crossroad ramp terminals at single-point diamond interchanges (SPs). Single-point diamond interchanges are implemented in urban areas. Their crossroad ramp terminals are characterized by one intersection through which all at-grade traffic movements are made (Leisch, 2005). Section 8.1 describes the site selection and data collection processes for developing crash prediction models for crossroad ramp terminals at single-point diamond interchanges. Section 8.2 provides descriptive statistics of the databases used for model development. Section 8.3 presents the statistical analysis and resulting SPFs for crossroad ramp terminals at single-point diamond interchanges. Section 8.4 discusses the CMFs recommended for use with the SPFs. Section 8.5 addresses the outcomes of the analysis to develop SDFs for crossroad ramp terminals of single-point diamond interchanges. Section 8.6 provides recommendations for incorporating the new crash prediction models for crossroad ramp terminals at single-point diamond interchanges in the second edition of the HSM. 8.1 Site Selection and Data Collection A list of potential single-point diamond interchanges was developed by searching databases and satellite imagery in five states: • Arizona (AZ) • Missouri (MO) • Nevada (NV) • Tennessee (TN) • Utah (UT) Data collection activities for these sites included gathering geometric design attributes of the interchanges as well as traffic and crash data. Geometric attributes were collected from aerial imagery in Google Earth®, as well as Google Street View®. Table 91 lists the geometric attributes collected (and respective definitions and permitted values) for each single-point diamond interchange. Table 91. Site characteristic variables collected for crossroad ramp terminals at single-point diamond interchanges Variable Definition Range or Permitted Values General Intersection Attributes Intersection configuration (i.e., number of legs and type of traffic control) Indicates the number of legs and type of traffic control 4SG Area type Indicates whether the intersection is in a rural or urban area Urban Presence of intersection lighting Indicates if overhead lighting is present at the intersection proper Yes, no Crossroad over or under freeway Indicates whether the crossroad passes over or under the freeway Over or under Construction year Estimated year when the interchange was constructed Range: 1992 to 2014

184 Table 91. Site characteristic variables collected for crossroad ramp terminals at single-point diamond interchanges (Continued) Approach Specific Attributes Route name or number Specifies the route name or number of the approach Location at intersection Side of the intersection the approach is located Primary, secondary Presence of left-turn lanes The number of approaches with one or more left-turn lanes 4 Number of left-turn lanes Number of left-turn lanes provided for turning movements to/from each freeway ramp 0, 1, 2, 3 Left-turn protected only Number of approaches with protected only left-turn options 4 Presence of right-turn lane Number of approaches with one or more right-turn lanes 0,1,2,3,4 Number of right-turn lanes Number of right-turn lanes provided for turning movements to/from each freeway ramp 0, 1, 2, 3 Number of through lanes Number of through lanes present on each crossroad approach to the crossroad ramp terminal 1, 2, 3, 4 Presence of frontage roads Indicates the presence of frontage roads at the interchange, where a through movement is added between the exit and entrance ramps Yes, no Presence of crosswalk Indicates the presence of crosswalks at the crossroad ramp terminal Yes, no Presence of bike lane Indicates the presence of a bike lane on the crossroad at the crossroad ramp terminal Yes, no Median width Width of median (in feet) on each crossroad approach to the crossroad ramp terminal Range: 0 to 47 ft Median type Type of median present on each crossroad approach to the crossroad ramp terminal Raised, flush, depressed, none Skew angle The intersection skew angle of the freeway mainline and crossroad Range: 0 to 90 degrees Number of driveways Number of driveways located within 250 ft of the crossroad stop bars/lines Range: 0 to 10 Presence of public street approach Indicates if an unsignalized public street approach is present within 250 ft of a crossroad stop bar/line Yes, no Presence of railroad crossing Indicates the presence of a railroad crossing on the crossroad Yes, no Freeway posted speed limit The posted speed limit on the freeway mainline Range: 45 to 75 mph Crossroad posted speed limit The posted speed limit on the crossroad Range: 30 to 55 mph Terminal length The distance measured along the crossroad between the outermost ramp terminal boundaries Range: 468 to 2274 ft Traffic control type for right turns Type of traffic control for right-turn movements Signal, stop, yield, none U-turns allowed Indicates if a U-turn is allowed between exit ramps and entrance ramps Yes, no Distance to right-turn approach Distance from the center of the crossroad ramp terminal to the center of the right turn approach Range: 100 to 1654 ft The “construction year” was estimated using the “Clock” feature in Google Earth® as the earliest year with the interchange present in aerial imagery. Some single-point diamond interchanges in the database were built during the study period and therefore had fewer years of data available for analysis. Additional information about the interchange configuration was used to exclude sites with uncommon or inconsistent geometric conditions, such as the lack of a crossroad approach or ramp approach. Traffic data collection activities primarily involved accessing publicly available traffic volumes and statistics.

185 Crash data were obtained from state DOTs. The crash data generally included details about the crash location (geographic coordinates), as well as attributes describing the crash, people involved in the crash, and the road and environmental conditions at the location and time of the crash. Identifying crashes associated with the ramp terminal required a clear definition of a ramp terminal-related crash based on geographic location and crash attributes. To maintain a level of consistency with the ramp terminal models in NCHRP Project 17-45, these crashes were selected using the following criteria: • Crashes occurring on the crossroad within the ramp terminal boundary, defined as a point 100 ft from the gore or curb return of the outermost ramp connection, and having one of the following attributes: - at intersection - intersection-related - at driveway - driveway-related - involving a pedestrian or bicyclist • Crashes occurring on a ramp with at least one of the following attributes: - at intersection; - intersection-related; - involving a pedestrian or bicyclist, or - located on an exit ramp and manner of collision is rear-end. This definition departs from the NCHRP Project 17-45 ramp terminal definition, using a different distance reference to define the crossroad ramp terminal boundary. The NCHRP Project 17-45 definition used 250 ft from the crossroad ramp terminal, measured from the center of the intersection. The definition implemented for the crossroad ramp terminals of single-point diamond interchanges is based on the American National Standards Institute (ANSI) D16.1-2007 (Manual on Classification of Motor Vehicle Traffic Accidents) definition of an interchange crash. According to the ANSI definition, an interchange crash is a crash in which the first harmful event occurs within a boundary defined by a point 100 ft from the gore or curb return of the outermost ramp connection. Figure 69 illustrates the boundaries for defining ramp terminal crashes at a single-point diamond interchange. This ramp terminal boundary adjustment was necessary for this application due to the size of a typical crossroad ramp terminal at a single-point diamond interchange and its main characteristic of operating as one intersection. Figure 70 shows an example of a single-point diamond interchange with a crossroad terminal size/length approximately equal to the average of terminal sizes/lengths at sites in Arizona and Utah. At this location, the maximum distance between the center of the interchange and the outermost ramp connection is approximately 330 ft, more than the 250 ft used in the NCHRP Project 17-45 definition. As a result, using the 250 ft boundary would have resulted in missing crashes associated with right turn movements at the entrance and exit ramps. It would have also resulted in missing part of the longer left-turn lanes on the cross street that are common to crossroad ramp terminals at single-point diamond interchanges. The ramp terminal boundary that is based on the ANSI definition and implemented in this research

186 extends 100 ft beyond the outermost ramp connections, capturing crashes associated with the right-turn movements and the left-turn lanes. Figure 69. Single-point diamond interchange ramp terminal boundaries for defining ramp terminal crashes (adapted from Bonneson et al., 2012) Figure 70. Example of a single-point diamond interchange with the implemented ramp terminal boundary identified along the crossroad (Source: ArcMap)

187 All of the collected data (i.e., site characteristics, crashes, and traffic volumes) were assembled into one database for the purposes of model development. After initial database development and quality assessments, interchanges in Arizona and Utah were selected for model development due to a higher level of confidence in accurately and reliably locating and identifying terminal- related crashes in those states. This decision resulted in 70 potential crossroad ramp terminals for model development. This list of interchanges was further reduced due to unusual geometric attributes and missing traffic data. Specifically, 12 sites were excluded due to missing ramp volumes on at least one ramp approach, four were excluded for unusual ramp terminal configurations (e.g., exit ramp integration with nearby intersections or streets), and two were excluded for unusual crossroad configurations (e.g., missing crossroad approach, resulting in a three-leg variation of a single-point diamond interchange). With 52 potential sites remaining for model development, cumulative residual (CURE) plots for preliminary models indicated three potential outliers were present in the database. These locations generally had an excessive number of PDO crashes relative to their reported traffic volumes on the crossroads and ramps, resulting in unusually large residuals. The final database excluded these three sites, resulting in 49 crossroad ramp terminals at single-point diamond interchanges for model development. 8.2 Descriptive Statistics of Database A total of 49 crossroad ramp terminals at single-point diamond interchanges were used for crash prediction model development. The selected sites were from two states: Arizona and Utah. To remain consistent with the standards for development of the intersection predictive models in the first edition of the HSM, the goal of this research was to develop crash prediction models with a minimum of 200 site-years of data, and preferably 450 site-years of data or more. Traffic Volumes and Site Characteristics Traffic volumes and crash data from years 2011 through 2015 were used for analysis. Table 92 provides summary statistics for traffic volume at the study sites used for model development. Study period (date range), number of sites and site-years, and traffic volume descriptive statistics are shown by state. Table 92. Crossroad and ramp AADT statistics at single-point diamond interchange crossroad ramp terminals State Date Range Number of Sites Number of Site- Years Crossroad AADT (veh/day) Ramp AADT (sum of all four ramps) (veh/day) Min Max Mean Median Min Max Mean Median AZ 2011-2015 28 140 14,934 70,790 36,169 36,302 16,556 64,648 40,113 39,308 UT 2011-2015 21 99 13,445 47,295 29,255 29,315 14,069 80,030 42,326 38,075 All states 2011- 2015 49 239 13,445 70,790 33,305 33,800 14,069 80,030 41,030 39,169

188 Interchange geometric characteristics were collected using Google Earth® and Google Street View® (Table 91). The key variables of interest for modeling were: • Terminal length (measured along crossroad) - Min = 605 ft, Max = 1236 ft, Mean = 829 ft • Number of through lanes on crossroad approaches - Min = 1, Max = 4, Mean = 2.53 • Number of left-turn lanes - Exit (from freeway) and entrance (to freeway) movements: Min = 1, Max = 3, Mean = 1.94 • Number of right-turn lanes - Entrance (to freeway) movements: Min = 1, Max = 2, Mean = 1.05 - Exit (from freeway) movements: Min = 1, Max = 2, Mean = 1.43 - All movements: Min = 1, Max = 2, Mean = 1.24 • Traffic control type for right turns - To entrance ramp: • Both signalized (frontage roads): 7 sites • Both no control: 42 sites - From exit ramp • Both signalized: 19 sites • Both yield control: 19 sites • Both no control (free right): 2 sites • 1 signalized, 1 stop control: 3 sites • 1 signalized, 1 yield control: 1 site • 1 stop control, 1 yield control: 1 site • 1 signalized, 1 no control: 1 site • 1 stop control, 1 no control: 1 site • 1 yield control, 1 no control: 1 site The findings with respect to some of these site characteristics are discussed in Section 8.3 on SPF development. Crash Counts All 49 interchanges included in the study experienced crashes. The average number of single- and MV crashes per terminal was 124.6 crashes (approximately 25.0 crashes per terminal per year), and the average number of vehicle-pedestrian plus vehicle-bicycle crashes per intersection was 2.1 over the entire study period (approximately 0.4 pedestrian and bicycle crashes per terminal per year). Table 93 shows all, SV, and MV crash counts by crash severity and time of day for each state over the entire study period. Crash counts are tallied by collision type and manner of collision across all states in Table 94.

189 Table 93. All crashes combined, single- and MV, and pedestrian and bicycle crash counts by crash severity—single-point diamond interchange crossroad ramp terminals State Date Range Number of Sites Number of Site- Years Time of Day All Crashes Combined SV Crashes Multiple-Vehicle Crashes Pedestrian Crashes Bicycle Crashes Total FI PDO Total FI PDO Total FI PDO FI FI AZ 2011-2015 28 140 All 4071 1079 2992 287 83 204 3723 941 2782 18 43 UT 2011-2015 21 99 All 2133 504 1629 53 15 38 2040 454 1586 16 24 All states 2011- 2015 49 239 All 6,204 1,583 4,621 340 98 242 5,763 1,395 4,368 34 67 Table 94. Crash counts by collision type and manner of collision and crash severity at single-point diamond interchange crossroad ramp terminals Collision Type Total FI PDO Single-Vehicle Crashes Collision with animal 0 0 0 Collision with fixed object 288 74 214 Collision with other object 8 3 5 Collision with parked vehicle 0 0 0 Other SV collision 44 21 23 Total SV crashes 340 98 242 Multiple-Vehicle Crashes Head-on collision 83 54 29 Angle collision 573 205 368 Rear-end collision 4485 1056 3429 Sideswipe collision 579 63 516 Other MV collision 43 17 26 Total MV crashes 5763 1395 4368 Nonmotorized Crashes Pedestrian 34 34 0 Bicycle 67 67 0 Total nonmotorized crashes 101 101 0 Total Crashes 6204 1594 4610 8.3 Safety Performance Functions—Model Development SPFs for the crossroad ramp terminal of a single-point diamond interchange were initially developed using Equation 56: 𝑁 = 𝑒𝑥𝑝 𝑎 + 𝑏 × ln(𝐴𝐴𝐷𝑇 ) + 𝑐 × ln 𝐴𝐴𝐷𝑇 + 𝑑 × 𝑒𝑥𝑖𝑡_𝑓𝑟𝑒𝑒_𝑟𝑖𝑔ℎ𝑡 (Eq. 56) Where: Nspf int = predicted average crash frequency of a crossroad ramp terminal at a single-point diamond interchange with base conditions (crashes/year) AADTxrd = AADT on the crossroad (veh/day) AADTramp = um of ramp AADTs (veh/day) exit_free_right = number of exit ramps with free-flow right turns (0, 1, or 2) a, b, c, and d = estimated regression coefficients

190 All SPFs were developed using a NB regression model based on all sites combined. Based on a review of the number of states, sites, site-years, and crashes for the database assembled, data for all sites were used for model development to maximize the sample size rather than using a portion of the data for model development and a portion for model validation. Separate models using data from Arizona and Utah were initially explored and showed relatively consistent model coefficients. This increased confidence in the approach to pool all data for model development. STATA 12.1 was used for modeling. The final SPFs based on Equation 57 for crossroad ramp terminals at single-point diamond interchanges are shown in Table 95. Table 95 shows the estimated model coefficients and overdispersion parameter (estimate), their standard error, and associated p-values (or significance level) for each severity level. Figures 71-73 graphically present the SPFs shown in Table 95 for various crossroad and ramp AADTs. SPFs for vehicle-pedestrian and vehicle-bicycle crashes at crossroad ramp terminals of single- point diamond interchanges could not be developed as pedestrian and bicycle volumes were not available. The SPFs in Table 95 predict the average crash frequency at the crossroad ramp terminal for all crash types (i.e., multi-vehicle, SV, pedestrian, and bicyclist) for total, FI, and PDO severity levels. The estimated SPFs use both the crossroad AADT and sum of AADTs on all ramps connected to the interchange. The coefficients for these terms are positive and statistically significant (at greater than 99% confidence level) in each SPF, although their magnitudes fluctuate between the FI and PDO models. The estimated coefficient for crossroad AADT was lower for PDO crashes than for FI crashes. The estimated coefficient for ramp AADT was higher for PDO crashes than for FI crashes and greater than unity. This is associated with the larger number of rear-end PDO crashes occurring on the ramps at the study sites with larger ramp volumes. Multiple models were tested considering the effects of different geometric attributes, including the interchange length, number of turn lanes (right and left), number of through lanes, and number of approaches with a particular right turn control type. Only the right turn control type was found to have a statistically significant effect. However, the type of right turn control also coincides with a particular state (i.e., Arizona uses more yield control, Utah uses more signal control and free-flow right turns), limiting the ability to estimate the effect of the right turn control variable without confounding effects. The free-right turn on exit ramps variable was included in the model because it was statistically significant, and its coefficient was relatively consistent between models. This variable is capturing not only the differences in right turn capacity and its effect on rear-end exit ramp crashes, but also the removal of conflict points within the defined ramp terminal area. The free-flow right turns at the study locations are accommodated by an auxiliary lane along the crossroad (thereby removing the need for right- turning vehicles to merge within the terminal area). Rather than presenting the free-flow right- turn effects as CMFs, separate SPFs were developed in the form of Equation 57 based on number of exit ramps with free-flow right turns to the crossroad – 0, 1, or 2. The final adjusted values for the estimated parameters are presented in Table 96. There are no additional base conditions for the SPFs.

191 𝑁 = 𝑒𝑥𝑝 𝑎 + 𝑏 × ln(𝐴𝐴𝐷𝑇 ) + 𝑐 × ln 𝐴𝐴𝐷𝑇 (Eq. 57) Table 95. SPF coefficients for crossroad ramp terminals at single-point diamond interchanges (based on Equation 56) Crash Severity Parameter Estimate Standard Error Pr > F Significance Level Total Crashes Intercept -15.31 1.70 -- -- ln(AADTxrd) 0.69 0.17 0.000 Significant at 99% level ln(AADTramp) 1.08 0.18 0.000 Significant at 99% level exit_free_right -0.60 0.11 0.000 Significant at 99% level Overdispersion 0.10 0.02 -- -- FI Crashes Intercept -16.71 2.06 -- -- ln(AADTxrd) 0.88 0.20 0.000 Significant at 99% level ln(AADTramp) 0.88 0.21 0.000 Significant at 99% level exit_free_right -0.58 0.13 0.000 Significant at 99% level Overdispersion 0.11 0.03 -- -- PDO Crashes Intercept -15.60 1.72 -- -- ln(AADTxrd) 0.61 0.17 0.000 Significant at 99% level ln(AADTramp) 1.15 0.18 0.000 Significant at 99% level exit_free_right -0.60 0.11 0.000 Significant at 99% level Overdispersion 0.10 0.02 -- -- Base Conditions: 0, 1, and 2 are valid values for the number of exit ramps with free-flow right turns to the crossroad. There are no additional base conditions. Figure 71. Graphical representation of the SPF for total crashes at crossroad ramp terminals at single-point diamond interchanges

192 Figure 72. Graphical representation of the SPF for FI crashes at crossroad ramp terminals at single-point diamond interchanges Figure 73. Graphical representation of the SPF for PDO crashes at crossroad ramp terminals at single-point diamond interchanges

193 Table 96. SPF coefficients for crossroad ramp terminals at single-point diamond interchanges (based on Equation 57) Crash Severity Number of Free-Flow Right Turns from Exit Ramp to Crossroad SPF Coefficient Dispersion Parameter a b c Total crashes 0 -15.31 0.69 1.08 0.10 1 -15.91 0.69 1.08 0.10 2 -16.51 0.69 1.08 0.10 Fatal-and- injury crashes 0 -16.71 0.88 0.88 0.11 1 -17.29 0.88 0.88 0.11 2 -17.87 0.88 0.88 0.11 Property- damage- only crashes 0 -15.60 0.61 1.15 0.10 1 -16.20 0.61 1.15 0.10 2 -16.80 0.61 1.15 0.10 There are no additional base conditions. Tables 97 and 98 provide proportions for crash severity levels and collision types and manner of collision, respectively, for crashes at crossroad ramp terminals of single-point diamond interchanges. These proportions were calculated based on the observed data from both states combined. Table 97. Distributions for crash severity level at crossroad ramp terminals at single-point diamond interchanges Crash Severity Level Percentage of Total Crashes Percentage of FI Crashes Fatal 0.16 0.6 Incapacitating injury 1.19 4.7 Non-incapacitating injury 7.09 27.8 Possible injury 17.07 66.9 Total fatal plus injury 25.52 Property-damage-only 74.48 Total 100.0 100.0 Table 98. Distributions for collision type and manner of collision at crossroad ramp terminals at single-point diamond interchanges Collision Type Percentage of Total Crashes FI PDO Single-Vehicle Crashes Collision with animal 0.0 0.0 Collision with fixed object 4.6 4.6 Collision with other object 0.2 0.1 Collision with parked vehicle 0.0 0.0 Other SV collision 1.3 0.5 Multiple-Vehicle Crashes Head-on collision 3.4 0.6 Angle collision 12.9 8.0 Rear-end collision 66.2 74.4 Sideswipe collision 4.0 11.2 Other MV collision 1.1 0.6 Nonmotorized Crashes Pedestrian 2.1 0.0 Bicycle 4.2 0.0 Total Crashes 100.0 100.0 Following the development of the crash prediction models for crossroad ramp terminals at single-point diamond interchanges, compatibility testing of the new models to confirm that the

194 new models provide reasonable results over a broad range of input conditions and that the new models integrate seamlessly with existing intersection crash prediction models in the first edition of the HSM was conducted. The graphical representations of the crash prediction models in Figures 71-73 provide some sense of the reasonableness of the new models for crossroad ramp terminals at single-point diamond interchanges. Nothing from these figures suggests that the models provide unreasonable results. Comparison of the crash prediction models for crossroad ramp terminals at single-point diamond interchanges to the crash prediction models for tight diamond interchanges is presented in Section 9.3. Regarding seamlessly integrating the new crash prediction models for crossroad ramp terminals at single-point diamond interchanges with existing crash prediction models in Chapter 19 of the HSM, the primary issue that needs to be clearly addressed is the approach for defining crashes associated with the crossroad ramp terminals at single-point diamond interchanges. As stated in Section 8.1, crashes associated with the crossroad ramp terminals at single-point diamond interchanges are defined as follows: • Crashes occurring on the crossroad within the ramp terminal boundary, defined as a point 100 ft from the gore or curb return of the outermost ramp connection, and having one of the following attributes: - at intersection - intersection-related - at driveway - driveway-related - involving a pedestrian or bicyclist • Crashes occurring on a ramp with at least one of the following attributes: - at intersection - intersection-related - involving a pedestrian or bicyclist - located on an exit ramp and manner of collision is rear-end This definition departs from the NCHRP Project 17-45 and HSM approach for defining ramp terminal crashes, using a different distance reference to define the crossroad ramp terminal boundary, so this needs to be clearly stated in the second edition of the HSM. No other issues were identified concerning integrating the new crash prediction models for crossroad ramp terminals at single-point diamond interchanges with existing crash prediction models in Chapter 19 of the HSM.

195 8.4 Crash Modification Factors During the development of the crash prediction models for crossroad ramp terminals at single-point diamond interchanges, three potential sources of CMFs for use with the SPFs were considered: • CMFs developed as part of this research based on a cross-sectional study design and regression modeling • CMFs already incorporated into the first edition of the HSM and applicable to crossroad ramp terminals at single-point diamond interchanges • High-quality CMFs applicable to crossroad ramp terminals at single-point diamond interchanges developed using defensible study designs (e.g., observational before-after evaluation studies using SPFs—the EB method), as referenced in FHWA’s CMF Clearinghouse with four or five-star quality ratings or based on a review of relevant intersection safety literature Based on a review of the CMFs already incorporated in the first edition of the HSM and other potential high-quality CMFs developed using defensible study designs, no CMFs were identified that were adaptable to the predictive models for crossroad ramp terminals at single-point diamond interchanges. New potential CMFs were explored during regression modeling, but only the right turn configuration from the exit ramps to the crossroad (i.e., free-flow versus yield/stop/signal control) showed consistent and statistically significant safety effects. As noted in Section 8.3, instead of presenting the free-flow right-turn effects as CMFs, separate SPFs are recommended for three exit ramp to crossroad right turn configurations (defined by the number of exit ramps with free-flow right turns to the crossroad—0, 1, or 2). No additional base conditions or CMFs are recommended for use with the SPFs for crossroad ramp terminals as single-point diamond interchanges. The lack of other effects is not necessarily surprising. Crossroad ramp terminals at single-point diamond interchanges are relatively similar in some of their major features (e.g., one intersection through which all at-grade traffic movements are made, signal timing, presence of exclusive left-turn lanes). Sample sizes did not allow any differences in safety performance to be detected at finer levels of detail (e.g., number of exclusive left-turn lanes, number of cross street through lanes). 8.5 Severity Distribution Functions Development of SDFs was explored for crossroad ramp terminals at single-point diamond interchanges using methods outlined in Section 2.2.3 of this report. SDFs were not used in the development of crash prediction methods in the first edition of the HSM but were subsequently used in the Supplement to the HSM for freeways and ramps (AASHTO, 2014). The database used to explore SDFs for crossroad ramp terminals at single-point diamond interchanges consisted of the same crashes and crossroad ramp terminals as the database used to estimate the SPFs, but restructured so that the basic observation unit (i.e., database row) was a crash instead of a ramp terminal. No traffic or geometric variables showed consistent and statistically significant effects in the SDFs for crossroad ramp terminals at single-point diamond interchanges.

196 8.6 Summary of Recommended Models for Incorporation in the HSM In summary, several crash prediction models were developed for crossroad ramp terminals at single-point diamond interchanges for consideration in the second edition of the HSM. The final models for FI and PDO severity levels presented in Table 96 are recommended for inclusion in the second edition of the HSM, consistent with existing methods in HSM Chapter 19. Separate SPFs are presented in the form of Equation 57 based on number of exit ramps with free-flow right turns to the crossroad (– 0, 1, or 2). Attempts to develop SDFs for crossroad ramp terminals at single-point diamond interchanges proved unsuccessful for the reasons explained in Section 4.6. The SPFs by severity for crossroad ramp terminals at single-point diamond interchanges provided in Table 96, combined with the severity distributions provided in Table 97, are recommended for addressing crash severity at these intersection types, without use of SDFs. The SPFs predict FI and PDO crashes separately. Additional disaggregation of FI crashes into fatal, incapacitating injury, non-incapacitating injury, and possible injury crashes can be accomplished using the severity distributions provided in Table 97. Appendix A presents recommended text for incorporating the final recommended models for crossroad ramp terminals at single-point diamond interchanges into Chapter 19 of the HSM.

Next: Chapter 9. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Tight Diamond Interchanges »
Intersection Crash Prediction Methods for the Highway Safety Manual Get This Book
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The first edition of the Highway Safety Manual (HSM), in 2010, included Safety Performance Functions (SPFs) for roadway segments and intersections. However, not all intersection types are covered in the first edition of the HSM.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 297: Intersection Crash Prediction Methods for the Highway Safety Manual develops SPFs for new intersection configurations and traffic control types not covered in the first edition of the HSM, for consideration in the second edition of the HSM.

Supplemental to the Document is recommended draft text for the second edition if the HSM, a worksheet for Chapter 10, a worksheet for Chapter 11, a worksheet for Chapter 12, a worksheet for Chapter 19, and a presentation.

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