Application of Crash Modification Factors for Access Management, Volume 1: Practitioner's Guide (2021)

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10 Access management is the process that provides (or manages) access to land development while preserving the flow of traffic on the surrounding road network in terms of safety, capacity, and speed. Access management provides important benefits to the transportation system. These benefits have been increasingly recognized at all levels of government, and a growing number of states, cities, counties, and planning regions are managing access by requiring driveway permit applications and establishing where new access should be allowed. These agencies are also closing, consolidating, or improving driveways, median openings, and intersections as part of their access management implementation strategy. The second edition of TRB’s Access Management Manual (Williams et al. 2014) calls for the application of the following access management principles: • Provide a specialized roadway system (i.e., a hierarchical functional system), • Promote intersection hierarchy, • Locate signals to favor through movements, • Preserve the functional area of intersections and interchanges, • Limit the number of conflict points, • Separate conflict areas, • Remove turning vehicles from through-traffic lanes, • Use non-traversable medians on major roadways, • Provide a supporting street network, and • Provide unified access and circulation systems. Overview of Strategies The application of access management strategies is based on the functional classification of a roadway and the context of the area where the strategies would be applied. Irrespective of whether there is a formal access classification, the roadway function and the area context should be used to establish the level of allowable access. Access should vary for roadways of different levels of importance in the highway system, including access-related standards that identify where and how much access can be allowed and spacing standards. Tables 1 through 4 provide lists of access management strategies that are discussed in the sec- tions throughout this chapter. The four tables correspond to four primary categories of access management strategies: access spacing, roadway cross section, intersection, and property access. For each category, the table presents the applicable strategies, indicates how each strategy relates to basic access management principles, and identifies the applicable analysis scale(s) (e.g., site, intersection, segment, or corridor level). The final column of the table (Analysis Scale) helps to guide analysts to the appropriate chapter for detailed analysis. Chapter 4 presents methods for C H A P T E R   3 Safety Effects of Access Management

Safety Effects of Access Management 11   Table 4. Overview of property access strategies. Strategy Applicable Principles Analysis Scale Manage driveway design elements • Limit number of conflict points • Separate conflict areas Site Provide sight distance at unsignalized access • Preserve functional area of intersection Site, Intersection Provide frontage/backage road • Limit number of conflict points • Remove turning vehicles from through lanes • Provide a supporting street network Segment, Corridor Provide shared driveways and internal cross connectivity • Limit number of conflict points • Remove turning vehicles from through lanes • Provide supporting street network • Provide unified access and circulation Site, Segment, Corridor Strategy Applicable Principles Analysis Scale Establish unsignalized access density and spacing criteria • Limit number of conflict points • Separate conflict areas Corridor Establish signal density and spacing criteria • Locate signals to favor through movements • Limit number of conflict points • Separate conflict areas Corridor Establish functional area and corner clearance criteria • Preserve functional area of intersection • Separate conflict areas Intersection Establish spacing criteria for interchange crossroads • Limit number of conflict points • Separate conflict areas • Preserve functional area of interchange Intersection, Corridor Table 1. Overview of access spacing strategies. Strategy Applicable Principles Analysis Scale Provide non-traversable median and accommodate left turns and U-turns • Limit number of conflict points • Separate conflict areas • Remove turning vehicles from through lanes Corridor Establish spacing criteria for median openings/crossovers • Limit number of conflict points • Separate conflict areas Corridor Provide two-way left-turn lane (TWLTL) • Remove turning vehicles from through lanes Segment, Corridor Convert two-way street to one-way operation • Limit number of conflict points • Provide a specialized roadway system Corridor Table 2. Overview of roadway cross-section strategies. Strategy Applicable Principles Analysis Scale Provide left-turn lane • Remove turning vehicles from through lanes Intersection Provide right-turn lane • Remove turning vehicles from through lanes Intersection Alternative intersection designs • Limit number of conflict points Intersection, • Remove turning vehicles from through lanes Corridor Table 3. Overview of intersection strategies.

12 Application of Crash Modification Factors for Access Management quantifying segment- and intersection-level safety performance. Chapter 5 presents methods for quantifying corridor-level safety performance. The remainder of this chapter describes each strategy, summarizes the safety impacts, and presents the higher-quality and most-applicable CMFs when available. The CMFs presented in Chapter 3 can be applied to the observed, predicted, or expected crashes with varying degrees of reliability, as discussed in Appendix B. As a reminder, applying CMFs to expected or predicted crashes is typically more reliable than applying CMFs to observed crashes. In some instances, the CMFs presented in this chapter can be applied in the segment- and intersection-level predictive methods presented in Chapter 4; however, prior to applying any CMFs that were not developed specifically for use with the predictive method presented in Chapter 4, there is a need to consider the applicability of the CMFs with respect to crash type, crash severity, and base condition as well as the potential for double-counting crash reductions with each additional CMF. Refer to Appendix B for further discussion on how to select and apply CMFs. When CMFs or crash-based predictive methods are not available for the strategy of interest, consider applying the Safe System approach or other similar method to compare safety performance. Access Spacing This section describes strategies related to access spacing, including unsignalized access den- sity and spacing, traffic signal spacing, functional area and corner clearance, and interchange crossroad spacing. Unsignalized Access Density and Spacing (Intersections and Driveways) Unsignalized access, commonly referred to as “driveways” or “street intersections,” intro- duces conflict points and friction into the flow of traffic along a roadway. Vehicles entering and leaving the roadway often disrupt and slow the movement of through traffic, and the difference in speeds between through traffic and turning traffic increases the potential for crashes. As indi- cated in the TRB Access Management Manual, second edition (Williams et al. 2014), research has consistently shown that crash rates increase as access density increases. AASHTO’s A Policy on Geometric Design of Highways and Streets (i.e., the “Green Book”) indicates that driveway design and location merit special consideration (AASHTO 2018) in order to avoid the negative impacts of poorly managed access. Where the need for access is acknowledged by the driveway permitting agency, the access location should be selected to minimize its adverse effects on roadway safety and traffic flow. Increasing the spacing between access locations through proper planning of future access loca- tions and closing or consolidating existing access locations, improves traffic flow and safety along the roadway by achieving the following: • Reducing the number of conflict points per mile, • Providing a greater distance for motorists to anticipate and recover from turning maneu- vers, and • Providing opportunities for the construction of acceleration lanes, deceleration lanes, or exclusive left-turn or right-turn lanes. Figure 2 illustrates the spacing distance between two adjacent unsignalized driveways, where the distance is measured from the nearest edges of each driveway. Some agencies choose to measure the spacing distance from the centerlines of the adjacent driveways.

Safety Effects of Access Management 13   The Highway Safety Manual (1st Edition) provides separate models for multivehicle-driveway and non-driveway crashes per mile, considering the traffic volume, number and type of drive- ways, and median type (divided or undivided) (AASHTO 2010). Table 5 presents CMFs inferred from the Highway Safety Manual (1st Edition), assuming a traffic volume of 20,000 vehicles per day and eight combinations of driveway density, driveway type, and median type (Persaud et al. 2011). While the preferred approach is to use the Part C Predictive Method to estimate the change in safety performance for the specific scenario of interest, these results indicate that CMFs for reducing driveway density are lower (i.e., indicating a greater safety benefit) for undivided arterials compared to divided arterials and for commercial driveways compared to residential driveways. There is minimal change in the CMF values across traffic volume. While the Highway Safety Manual (1st Edition) could similarly be used to infer CMFs for changing driveway type (e.g., commercial to residential), this is not relevant to access management because transpor- tation agencies generally do not have any authority to modify land use. Further, the Highway Safety Manual (2nd Edition–forthcoming) will not distinguish among driveway types in the Part C Predictive Method; it will only indicate the number of driveways within the segment. Based on a review of the CMF Clearinghouse in December 2019, there were 36 CMFs rated three stars or higher that apply to unsignalized access density or spacing on urban and sub- urban arterials. Tables 6 through 8 present nine of those CMFs along with the general applica- bility, CMF ID (linked to the CMF Clearinghouse in the PDF of this publication, which can be found on www.trb.org by searching on “NCHRP Research Report 974”), and base condition. Source: FHWA. Figure 2. Unsignalized driveway spacing. Strategy Crash Severity CMF Applicability Change driveway density from 40 to 30 per mile All 0.83 Major commercial driveways on undivided arterials 0.93 Major commercial driveways on divided arterials 0.87 Major residential driveways on undivided arterials 0.96 Major residential driveways on divided arterials Change driveway density from 20 to 10 per mile All 0.74 Major commercial driveways on undivided arterials 0.92 Major commercial driveways on divided arterials 0.82 Major residential driveways on undivided arterials 0.95 Major residential driveways on divided arterials Notes: These CMFs apply to multivehicle driveway-related crashes on four-lane urban and suburban arterials with traffic volume of 20,000 vehicles per day. Source: AASHTO 2010. Table 5. CMFs for driveway density on four-lane urban and suburban arterials inferred from the first edition of the Highway Safety Manual Part C Predictive Method.

14 Application of Crash Modification Factors for Access Management Strategy Crash Severity CMF CMF ID Applicability Change driveway density from X to Y driveways per mile All exp(0.0096*(Y−X)) 2459 • Median divided • 30–45 mph posted speed • 29,320 to 96,080 vehicles/day Injury exp(0.0059*(Y−X)) 2462 PDO exp(0.0071*(Y−X)) 2463 All exp(0.0090*(Y−X)) 2507 • TWLTL • 30–45 mph posted speed • 4,883 to 71,280 vehicles/day Injury exp(0.0026*(Y−X)) 2512 PDO exp(0.0094*(Y−X)) 2513 Note: The base condition is X driveways per mile, and the condition of interest is Y driveways per mile. Source: FHWA n.d. Table 6. CMFs for driveway density on urban and suburban arterials. Strategy Crash Severity CMF CMF ID Applicability Change unsignalized intersection density from X to Y per mile All exp(0.0126*(Y−X)) 2501 • TWLTL • 30–45 mph posted speed • 4,883 to 71,280 vehicles/day Injury exp(0.0207*(Y−X)) 2506 Note: The base condition is X unsignalized intersections per mile, and the condition of interest is Y unsignalized intersections per mile. Source: FHWA n.d. Table 8. CMFs for unsignalized intersection density on urban and suburban arterials. These tables present CMFs for total crashes and CMFs by crash severity, unless otherwise noted in the “Applicability” column. Refer to the link to the CMF Clearinghouse to check for addi- tional CMFs by crash type. NCHRP Project 03-120 developed transit and truck-related CMFs for access density for segments with traversable, including two-way left-turn lane (TWLTL), and non-traversable medians (Butorac et al. 2018a, 2018b). The study defined unsignalized access as commercial driveways, office driveways, and public street approaches. The results indicated that higher access density is associated with higher transit and truck-related crashes. Table 9 presents the CMFs from this effort for urban and suburban arterials with traversable medians (including TWLTLs) and non-traversable medians where the base condition is X access points per mile and the condition of interest is Y access points per mile. The access density is based on the count of commercial driveways, office driveways, and public street approaches. Traffic Signal Density and Spacing Establishing traffic signal density and spacing criteria for arterial roadways is one of the most important and basic access management strategies. These criteria apply to both signalized drive- ways and signalized public roadway intersections. The proper spacing of traffic signals is based on frequency and uniformity. Frequency refers to the number of traffic signals for a given length of roadway and is sometimes referred to as Table 7. CMFs for driveway spacing on urban and suburban arterials. Strategy Crash Severity CMF CMF ID Applicability Change driveway spacing from X to Y feet All exp(-0.0004154*(Y−X)) 8201 Driveway-related crashes Note: The base condition for driveway spacing is X feet, and the condition of interest is Y feet. Source: FHWA n.d.

Safety Effects of Access Management 15   “signal density.” It is typically expressed as the number of signals per mile. Uniformity refers to the variation in the distances between individual traffic signals along a given length of roadway. It is desirable to minimize this variation and to space traffic signals at uniform distances, as shown in Figure 3. Closely spaced traffic signals can result in increased crash frequency and in an excessive number of stops even under moderate traffic volume conditions (Gluck et al. 1999). For example, if a 2-mile segment of roadway would require four traffic signals (i.e., a signal density of two signals/mile), it is generally more desirable to space the signals at a uniform distance along the roadway (e.g., every ½ mile), rather than space them irregularly (e.g., 1 mile, ¼ mile, ½ mile, and ¼ mile). Properly spaced traffic signals allow for the efficient progression of motor vehicle and pedestrian traffic, as well as provide an agency with greater flexibility in developing signal timing plans to reduce crashes. Based on a review of the CMF Clearinghouse in December 2019, there were 11 CMFs rated three stars or higher that apply to signalized intersection density or spacing on urban and sub- urban arterials. Tables 10 and 11 present four of those CMFs along with the general applicability, CMF ID (linked to the CMF Clearinghouse in the PDF of this publication, which can be found on www.trb.org by searching on “NCHRP Research Report 974”), and base condition. These tables only present CMFs for total crashes and CMFs by crash severity. Refer to the link to the CMF Clearinghouse to check for additional CMFs by crash type. Table 9. CMFs for access density on urban and suburban arterials. Strategy Crash Severity CMF Applicability Change total access density from X to Y per mile All exp(0.0153*(Y – X)) • Transit-related crashes • Traversable medians (including TWLTLs) with access density from 0 to 98 access points per mile • Non-traversable medians with access density from 0 to 73 access points per mile Change full access density from X to Y per mile All exp(0.0401*(Y – X)) • Truck-related crashes • Traversable medians (including TWLTLs) with access density from 0 to 98 access points per mile • Non-traversable medians with access density from 0 to 73 access points per mile Change partial access density from X to Y per mile All exp(0.0293*(Y – X)) • Truck-related crashes • Traversable medians (including TWLTLs) with access density from 0 to 98 access points per mile • Non-traversable medians with access density from 0 to 73 access points per mile Note: These CMFs apply to transit- and truck-related crashes as noted in the applicability. The base condition is X access points per mile, and the condition of interest is Y access points per mile. Access points include commercial driveways, office driveways, and public street approaches. Full access points include those where all turning movements are permitted. Partial access points include those where some turning movements are prohibited. Source: Butorac et al. 2018a and 2018b.

16 Application of Crash Modification Factors for Access Management Source: Adapted from Gluck et al. 1999, Figure 5, p. 23. Figure 3. Comparison of uniform and non-uniform signal spacing. Strategy Crash Severity CMF CMF ID Applicability Change signal spacing from X to Y feet (in thousands) All exp(−0.1276*(Y−X)) 2454 • Median divided • 30–45 mph posted speed • 29,320 to 96,080 vehicles/day All exp(−0.1144*(Y−X)) 2497 • TWLTL • 30–45 mph posted speed • 4,883 to 71,280 vehicles/day PDO exp(−0.1201*(Y−X)) 2500 Note: The base condition is a signal spacing of X feet (in thousands) and the condition of interest is a signal spacing of Y feet (in thousands). Source: FHWA n.d. Table 11. CMFs for signalized intersection spacing on urban and suburban arterials. Strategy Crash Severity CMF CMF ID Applicability Change signal density from X to Y signals per mile All exp(0.919*(Y−X)) 2217 Not specified Note: The base condition is X traffic signals per mile and the condition of interest is Y traffic signals per mile. Source: FHWA n.d. Table 10. CMFs for signalized intersection density on urban and suburban arterials.

Safety Effects of Access Management 17   Functional Area and Corner Clearance Protecting the functional integrity of intersections is extremely important from the safety and operations perspectives. One strategy to help accomplish this is to provide corner clear- ance by locating driveways outside the functional area of an intersection. As shown in Figure 4, the intersection functional area extends beyond the physical intersection limits to include the upstream approaches where deceleration, maneuvering, and queuing take place, as well as the downstream departure area beyond the intersection where driveways could introduce conict points and generate queues backing up through the intersection. As noted in AASHTO’s A Policy on Geometric Design of Highways and Streets, driveways should not be located within the functional area of an intersection or in the inuence area of an adjacent driveway (AASHTO 2018). Source: Adapted from Transportation Research Circular 456, March 1996, Figure 4, p.16. Figure 4. Intersection physical area versus functional area.

18 Application of Crash Modification Factors for Access Management Based on a review of the CMF Clearinghouse in December 2019, there were no CMFs rated three stars or higher that apply to the functional area of an intersection. There were 28 CMFs rated three stars or higher that apply to corner clearance at signalized intersections on urban and suburban arterials. Tables 12 and 13 present eight of those CMFs along with the general applicability, CMF ID (linked to the CMF Clearinghouse in the PDF of this publication, which can be found on www.trb.org by searching on “NCHRP Research Report 974”), and base con- dition. These tables only present CMFs for total crashes and CMFs by crash severity. Refer to the link to the CMF Clearinghouse to check for additional CMFs by crash type. While Table 12 suggests that driveways within 50 feet of the upstream corner are expected to have fewer crashes compared to the base condition of no driveways within 50 feet, the research team does not rec- ommend installing driveways on the upstream corner as a safety strategy. Further, these corner clearance distances are based on the underlying research study and do not reflect a desired minimum corner clearance. Again, AASHTO’s A Policy on Geometric Design of Highways and Streets notes that driveways should not be located within the functional area of an intersection (AASHTO 2018). Interchange Crossroad Spacing Freeway interchanges provide the means of moving traffic between freeways and intersecting crossroads. Although direct property access is prohibited on the freeway itself, safety and opera- tional problems can arise when driveways and intersections along the crossroad are located too close to the interchange ramp termini. Heavy weaving volumes, complex traffic signal operations, frequent crashes, and recurrent congestion could result. In addition, driveways and median breaks that are provided for direct access to properties along the crossroad compound these problems. Strategy Crash Severity CMF CMF ID Applicability Presence of driveways within 50 feet of 1 downstream corner on the major road All 1.33 9736 • 4-legged intersections • 10,406 to 93,000 vehicles/day on the major road • 500 to 48,000 vehicles/day on the minor road Fatal and injury 1.29 9740 Presence of driveways within 50 feet of 2 downstream corners on the major road All 1.76 9737 Fatal and injury 1.68 9741 Note: The base condition is no driveways within 50 feet of the downstream corners to the intersection. Source: FHWA n.d. Table 13. CMFs for downstream corner clearance at signalized intersections on urban and suburban arterials. Strategy Crash Severity CMF CMF ID Applicability Presence of driveways within 50 feet of 1 upstream corner on the major road All 0.82 9734 • 4-legged intersections • 10,406 to 93,000 vehicles/day on the major road • 500 to 48,000 vehicles/day on the minor road Fatal and injury 0.79 9738 Presence of driveways within 50 feet of 2 upstream corners on the major road All 0.67 9735 Fatal and injury 0.62 9739 Note: The base condition is no driveways within 50 feet of the upstream corners to the intersection. Source: FHWA n.d. Table 12. CMFs for upstream corner clearance at signalized intersections on urban and suburban arterials.

Safety Effects of Access Management 19   Managing access on crossroads in the vicinity of interchanges protects the longevity of both the interchange and the intersecting crossroad by reducing crash rates, minimizing congestion, and simplifying driving tasks. Improperly managing access on the crossroad near the inter- change may cause congestion and potential crashes, thereby shortening the life cycle of the interchange. In addition, it may cause significant impairment of crossroad and freeway mainline safety and operations. For these reasons, access management should be applied to interchange crossroads such that unsignalized access, including both driveways and intersections, is suffi- ciently separated from freeway interchange ramp termini. Based on a review of the CMF Clearinghouse in December 2019, there were three CMFs rated three stars or higher that apply to interchange crossroad spacing on urban and suburban arte- rials. Table 14 presents one of those CMFs along with the general applicability, CMF ID (linked to the CMF Clearinghouse in the PDF of this publication, which can be found on www.trb.org by searching on “NCHRP Research Report 974”), and base condition. This table only presents the CMF for total crashes (there were no CMFs available by crash severity). Refer to the link to the CMF Clearinghouse to check for additional CMFs by crash type. In addition to CMFs from the CMF Clearinghouse, the NCHRP Project 17-74 research team developed CMFs for the distance to a nearby ramp terminal at urban and suburban arterial intersections (See Volume 2 of NCHRP Research Report 974). Table 15 presents the CMFs from this effort. The applicable CMF is applied to the total estimated crashes, excluding vehicle- pedestrian and vehicle-bicycle crashes. The research indicates that predicted crashes increase at stop-controlled intersections where a ramp terminal is present within 1,500 feet; however, the Presence of ramp terminal within 1,500 feet Strategy Crash Severity CMF Applicability All 2.12* • Stop-controlled intersections • All crashes except vehicle-pedestrian and vehicle-bicycle crashes 1.00 • Signalized intersections • All crashes except vehicle-pedestrian and vehicle-bicycle crashes Note: The base condition is no ramp terminal within 1,500 feet of the intersection. *Observed variability suggests this strategy could result in an increase, decrease, or no change in crashes. Source: Gross et al. 2021. Table 15. CMFs for distance to ramp terminal at urban and suburban intersections. Strategy Crash Severity CMF CMF ID Applicability Change signalized ramp terminal spacing from X to Y feet All exp(0.014308*(Y−X)) 3060 • Diamond interchange • 35–45 mph posted speed • 4,200 to 50,850 vehicles/day on the major road • 2,000 to 24,800 vehicles/day on the minor road Note: The base condition is a ramp terminal spacing of X feet, and the condition of interest is a ramp terminal spacing of Y feet. Source: FHWA n.d. Table 14. CMF for signalized ramp terminal spacing on urban and suburban arterials.

20 Application of Crash Modication Factors for Access Management results are not statistically signicant, even at the 90-percent condence level (i.e., CMF = 2.12 with standard error = 0.91). As such, the observed variability suggests that this strategy could result in an increase, decrease, or no change in crashes. For signalized intersections, there is no indication of a change in safety performance when a ramp terminal is present within 1,500 feet. Roadway Cross Section is section describes strategies related to roadway cross section, including non-traversable median treatments, median opening spacing and design, TWLTL median treatments, and con- verting two-way streets to one-way operation. Median Treatment—Non-Traversable Median Installations of non-traversable (i.e., raised) medians with provisions for median openings to accommodate le turns and U-turns have proven to be among the most eective strategies for reducing conict points and improving trac operations along roadways. e installation of a non-traversable median reduces the number of conict points by restricting driveways (not located at median openings) to right-in/right-out movements and directing le-turn and U-turn movements to designated median openings, as shown in Figure 5. is reduces the number of conict points from 32 at a conventional four-legged intersection of two-lane roads to four at a similar intersection with a non-traversable median. At a conventional three-legged intersection of two-lane roads, the number of conflict points is reduced from nine to two. In either case, there will be two added conict points downstream (one diverge and one merge) at a median opening that facilitates U-turns. Allowing unrestricted le-turn movements to and from all access driveways increases the number of vehicular conict points with other vehicles, pedestrians, and bicyclists. Le-turning Source: Adapted from Gluck et al. 1999, Figure 30. Figure 5. Allowable trafc movements before and after raised median installation.

Safety Effects of Access Management 21   vehicles have been shown to account for approximately 72 percent of crashes at a driveway (FHWA 2010). A non-traversable median with designated median openings to allow for left-turn and U-turn movements offers the following advantages over other types of roadway cross sections: • Vehicles traveling in opposite directions are physically separated, reducing the propensity for head-on crashes. • When properly designed, the physical space provided for the deceleration and storage of left- turning and U-turning vehicles occurs outside the through-traffic lanes, and the resulting reduction in speed differential between the turning and through vehicles improves traffic operations and reduces the potential for crashes. • At a full median opening, the width of the non-traversable median provides a refuge area for passenger cars, making a two-stage left turn from a side street (i.e., crossing traffic approaching from the left, and then turning left and merging with traffic approaching from the right) or traveling straight across the roadway. • The number of left-turn conflict points with vehicles, pedestrians, and bicyclists is reduced. • The non-traversable median provides a refuge area for pedestrians crossing the roadway at intersections. • Midblock pedestrian crossings can be provided and signalized without interfering with traffic progression (i.e., by stopping traffic approaching from the left first, and then stopping traffic from the right). • Locations for making left turns and U-turns are clearly identifiable to the driver, thus reducing driver workload. • Roadways with a non-traversable median have been found to be safer than those with a TWLTL (Williams et al. 2014). The Highway Safety Manual (1st Edition) and NCHRP Project 17-62, “Improved Prediction Models for Crash Types and Crash Severities” provide models to estimate the safety perfor- mance of arterials with and without different types of medians (as discussed in Chapter 4 of this guide); however, the results should not be used to infer CMFs. In some cases, it may be reason- able to use the predictions from two different models to estimate the change in safety (i.e., infer CMFs) for converting from one condition to another. In other cases, it is not reasonable to use the results from cross-sectional models to infer CMFs. Specifically, when cross-sectional models do not account for all differences in safety between the two site types, the comparison of results from two different cross-sectional models may not produce reasonable and reliable CMFs. Further, it is necessary to calibrate cross-sectional models to the same spatial and temporal conditions before using the models to infer CMFs. The CMFs from the CMF Clearinghouse, presented later in this section, are more reliable for estimating the safety impacts of this strategy. Refer to the last section of this chapter, “Counterintuitive Results,” for further discussion of the inferred CMFs for this strategy. Based on a review of the CMF Clearinghouse in December 2019, there were numerous CMFs rated three stars or higher that apply to non-traversable medians on urban and suburban arte- rials. There are various types of non-traversable median conversions, including the conversion from undivided, TWLTL, and traversable median (non-TWLTL). Tables 16 through 19 present 37 related CMFs along with the general applicability, CMF ID (linked to the CMF Clearinghouse in the PDF of this publication, which can be found on www.trb.org by searching on “NCHRP Research Report 974”), and base condition. These tables only present CMFs for total crashes and CMFs by crash severity. Refer to the link to the CMF Clearinghouse to check for additional CMFs by crash type.

22 Application of Crash Modification Factors for Access Management Strategy Crash Severity CMF CMF ID Applicability Install non-traversable median on undivided road Fatal and injury 0.61 21 2-lane roads Injury 0.78 22 Multilane roads PDO 1.09 23 Multilane roads All 0.29 2219 1,390 to 51,200 vehicles/day All 0.86 3935 2-lane roads < 45 mph posted speed Note: The base condition is an undivided road. Source: FHWA n.d. Table 16. CMFs for converting undivided road to non-traversable median on urban and suburban arterials. Strategy Crash Severity CMF CMF ID Applicability Replace TWLTL with non-traversable median All 0.53 7771 Not specifiedFatal and injury 0.67 7772 All 0.77 2514 • 35–40 mph posted speed • 4,883 to 96,080 vehicles/day Injury 0.79 2519PDO 0.67 2520 All 0.70 5044 • 2-, 4-, and 6-lane roads • 10,500 to 57,000 vehicles/day Fatal and injury 0.66 5043 Injury 0.66 5041 PDO 0.74 5040 All 0.63 5110 • 6-lane roads • 26,224 to 57,000 vehicles/day Fatal and injury 0.58 5114 Injury 0.58 5112 PDO 0.68 5111 All 0.68 5130 • 4- and 6-lane roads • 35–40 mph posted speed • 35,120 vehicles/day Fatal and injury 0.65 5133 Injury 0.64 5132 PDO 0.71 5131 All 0.74 5148 • 4- and 6-lane roads • 45–55 mph posted speed • 37,578 vehicles/day Fatal and injury 0.70 5152 Injury 0.70 5150 PDO 0.78 5149 Note: The base condition is a road with a TWLTL. Source: FHWA n.d. Table 17. CMFs for converting TWLTL to non-traversable median on urban and suburban arterials. Strategy Crash Severity CMF CMF ID Applicability Replace traversable median (non-TWLTL) with non-traversable median All 0.61 3034 10,000 to 55,000 vehicles/day Fatal and serious injury 0.56 3035 Note: The base condition is a road with a traversable median (non-TWLTL). Source: FHWA n.d. Table 18. CMFs for converting traversable median (non-TWLTL) to non-traversable median on urban and suburban arterials.

Safety Effects of Access Management 23   Median Opening Spacing and Design The Florida Department of Transportation’s Median Handbook indicates that “a restrictive median with well-designed median openings is one of the most important tools” to improve the safety and efficiency of the highway system (Florida DOT 2014). A median opening is an opening in a non-traversable median that provides for crossing and turning traffic. A “full” median opening allows all turning movements, whereas a “partial” median opening allows only specific movements and physically prohibits all other movements. To realize the safety benefits, median openings should not encroach on the functional area of another median opening or intersection (see Figure 4 for an illustration of functional area). A median opening that allows all turns has numerous conflict points. As described in the previous section, “Median Treatment—Non-Traversable Median,” a conventional four-legged intersection of two-lane roads has 32 conflict points, and a conventional three-legged intersec- tion of two-lane roads has nine conflict points. One way to limit the number of conflict points is through the design of median openings. Figure 6 illustrates the reduction in conflict points from converting a full movement, three-legged intersection on a four-lane undivided roadway (11 conflict points) to a directional median on a divided roadway (six conflict points). The graphic on the right shows a “directional” median opening serving a side street that allows for left turns from the major street but prohibits left turns from the minor street. According to the Median Handbook (Florida DOT 2014), this is a design that greatly reduces the conflict points by limiting the number of allowed turning movements. Through use of restrictive medians, Strategy Crash Severity CMF CMF ID Applicability Replace direct left turns with right turn followed by U-turn All 0.80 351 • 4- to 8-lane roads • 0 to 34,000 vehicles/day Fatal and injury 0.64 353 PDO 0.89 352 All 0.49 357 • 4-lane roads • 0 to 34,000 vehicles/day Fatal and injury 0.38 359 PDO 0.56 358 All 0.86 360 • 6-lane roads • 0 to 34,000 vehicles/day Fatal and injury 0.69 362 PDO 0.95 361 Note: The base condition is a segment that allows direct left turns. Source: FHWA n.d. Table 19. CMFs for replacing direct left turns with right turn followed by U-turn on urban and suburban arterials. Source: Florida DOT 2014. Figure 6. Reduce conflict points using directional median openings.

24 Application of Crash Modification Factors for Access Management most driveways along the corridor become right-in/right-out driveways; however, the left-turn movements that would have been allowed with a full median opening are now shifted to a down- stream U-turn location. As such, it is important to consider the net safety performance when managing turning movements at a specific location. Based on a review of the CMF Clearinghouse in December 2019, there were numerous CMFs rated three stars or higher that apply to median opening spacing and design on urban and sub- urban arterials. There are various types of median opening designs, including full median openings, restricted median openings (e.g., left-turn from major road only), and median openings with and without left-turn lanes. Tables 20 through 23 present 14 related CMFs along with the general applicability, CMF ID (linked to the CMF Clearinghouse in the PDF of this publication, which can be found on www.trb.org by searching on “NCHRP Research Report 974”), and base condi- tion. These tables only present CMFs for total crashes and CMFs by crash severity. Refer to the link to the CMF Clearinghouse to check for additional CMFs by crash type. Strategy Crash Severity CMF CMF ID Applicability Change median opening density from X to Y median openings per mile All exp(0.0481*(Y−X)) 2492 • Median divided • 30–45 mph posted speed • 29,320 to 96,080 vehicles/day Injury exp(0.0513*(Y−X)) 2495 PDO exp(0.0456*(Y−X)) 2496 Note: The base condition is X median openings per mile, and the condition of interest is Y median openings per mile. Source: FHWA n.d. Table 20. CMFs for median opening density on urban and suburban arterials. Strategy Crash Severity CMF CMF ID Applicability Increase separation distance by 10% between driveway (with restricted left turn) and downstream U-turn All 0.97 2216 • 6-lane roads • 50–55 mph posted speed • 18,200 to 86,300 vehicles/day Source: FHWA n.d. Table 21. CMF for increasing distance between driveway exit and downstream U-turn on urban and suburban arterials. Strategy Crash Severity CMF CMF ID Applicability Convert full median opening to directional median opening All 0.93 5457 • 4- to 6-lane roads • Median divided • 40–55 mph posted speed • 27,000 to 96,000 vehicles/day Fatal, serious, and minor injury (KAB on KABCO scale) 0.77 5453 Fatal and serious injury (KA on KABCO scale) 0.76 5452 Serious injury (A on KABCO scale) 0.82 5454 Possible injury (C on KABCO scale) 0.82 5455 PDO 1.13 5456 Note: The base condition is full median opening on a median-divided road. Source: FHWA n.d. Table 22. CMFs for converting full median opening to directional median opening on urban and suburban arterials.

Safety Effects of Access Management 25   Median Treatment—Two-Way Left-Turn Lane Like installations of non-traversable medians on formerly undivided highways, installations of TWLTLs offer safety and operational benefits. Compared to undivided highways, TWLTLs allow the deceleration and storage of left-turning vehicles outside of the through-traffic lanes. The resulting reduction in speed differential between the turning and through vehicles improves traffic operations and reduces the potential for crashes and crash severity. A cross section with a TWLTL offers the following advantages over an undivided roadway cross section: • TWLTLs reduce the frequency of crashes as compared to undivided roadways (Gluck et al. 1999). • Vehicles traveling in opposite directions are separated, reducing the potential for head-on crashes. • The TWLTL provides a refuge area for passenger cars making a two-stage left turn from a side street or driveway (i.e., crossing traffic approaching from the left, waiting in the TWLTL, and then merging with traffic approaching from the right). As reported in the second edition of the TRB Access Management Manual (Williams et al. 2014), crash models developed for and discussed in NCHRP Report 395: Capacity and Opera- tional Effects of Midblock Left-Turn Lanes (Bonneson and McCoy 1997) indicated that road- ways with a TWLTL and traffic volumes of 17,500 vehicles per day or more are expected to have safety performance (e.g., number of crashes per year) that is similar to that of an undivided roadway. The TRB Access Management Manual also indicated that TWLTLs do not provide the same safety benefits as non-traversable medians, which help to physically separate opposing traffic (Williams et al. 2014). NCHRP Report 420, after presenting the results of safety analyses comparing non-traversable medians and TWLTLs, indicates that literature compiled since the 1980s reflects the safety benefits of non-traversable medians over TWLTLs (Gluck et al. 1999). Specifically, four-lane and six-lane divided roadways with non-traversable medians (and pro- tected left-turn lanes) have shown better safety performance (lower average crash rates) than five-lane and seven-lane roadways with a TWLTL. A few studies have shown benefits based on before-and-after studies of the same roadway; however, most use a cross-sectional comparison of crash rates for the two basic types of roads. While NCHRP Report 420 (p. 76) concluded that roadways with non-traversable medians appear safer than similar roadways with TWLTLs, care should be exercised in selecting the appropriate design. Specifically, for roadways with non-traversable medians, there is a need to provide adequate capacity and design at signalized intersections to counteract the potential for crash migration (i.e., the shift in crashes from one location to another) and to mitigate congestion-related collisions. The Highway Safety Manual (1st Edition) and NCHRP Project 17-62, “Improved Prediction Models for Crash Types and Crash Severities” provide models to estimate the safety performance Table 23. CMFs for converting full median opening to left-in-only median opening on urban and suburban arterials. Strategy Crash Severity CMF CMF ID Applicability Convert full median opening to left-in-only median opening All 0.95 5464 • 4- to 6-lane roads • Median divided • 40–55 mph posted speed • 45,000 to 75,000 vehicles/day Fatal, serious, and minor injury (KAB on KABCO scale) 0.93 5460 Possible injury (C on KABCO scale) 0.80 5462 PDO 1.13 5463 Note: The base condition is full median opening on a median-divided road. Source: FHWA n.d.

26 Application of Crash Modification Factors for Access Management of arterials with and without TWLTLs, as discussed in Chapter 4 of this guide; however, the results should not be used to infer CMFs. In some cases, it may be reasonable to use the predic- tions from two different models to estimate the change in safety (i.e., infer CMFs) for converting from one condition to another. In other cases, it is not reasonable to use the results from cross- sectional models to infer CMFs. Specifically, when cross-sectional models do not account for all differences in safety between the two site types, the comparison of results from two different cross-sectional models may not produce reasonable and reliable CMFs. Further, it is necessary to calibrate cross-sectional models to the same spatial and temporal conditions before using the models to infer CMFs. The CMFs from the CMF Clearinghouse, presented later in this section, are more reliable for estimating the safety impacts of this strategy. Refer to the last section of this chapter, Counterintuitive Results, for further discussion of the inferred CMFs for this strategy. A review of the CMF Clearinghouse, in December 2019, indicated that there were numerous CMFs rated three stars or higher that apply to TWLTLs on urban and suburban arterials. There are various types of conversions, including the conversion from two to three lanes and four to five lanes. There are also conversions that involve removing through lanes to accommodate a TWLTL (i.e., road diets). Tables 24 through 26 present 28 related CMFs along with the general applicability, CMF ID (linked to the CMF Clearinghouse in the PDF of this publication, which can be found on www.trb.org by searching on “NCHRP Research Report 974”), and base con- dition. These tables present CMFs for total crashes and CMFs by crash severity, unless other- wise noted in the “Applicability” column. Refer to the link to the CMF Clearinghouse to check for additional CMFs by crash type. Convert Two-Way Streets to One-Way Operation As indicated in FHWA’s Access Management in the Vicinity of Intersections: Technical Summary (FHWA 2010), one-way couplets are often found in urban areas and provide access management benefits. One-way streets limit the number of conflicting movements at each intersection. They also limit driveways to right-in-right-out-only or left-in-left-out-only turning maneuvers. Thus, they reduce the number and types of conflict points that occur at each driveway. One-way streets can also be beneficial for pedestrians crossing the street, requiring them to look for oncoming traffic in one direction only. They also provide additional opportunities to use available road- way width to provide auxiliary lanes for right-turn and/or left-turn movements, reducing con- flict points between through and turning vehicles. One-way operation has additional benefits related to traffic signal progression. Strategy Crash Severity CMF CMF ID Applicability Install TWLTL on 2-lane undivided road All 0.80 2341 • All area types (rural and urban) • 500 to 25,577 vehicles/day Fatal and injury 0.74 2346 All 0.96 2353 • Urban roads in Arkansas • 810 to 21,057 vehicles/day All 1.03 2355 • Urban roads in California • 5,307 to 23,800 vehicles/day All 0.91 2357 • Urban roads in Illinois • 4,391 to 14,867 vehicles/day All 1.05 2359 • Urban roads in North Carolina • 500 to 25,577 vehicles/day Notes: The base condition is a two-lane undivided road. The results for individual states are shown to indicate the potential for differential effects (i.e., TWLTLs may be less effective in urban areas compared to rural areas). Source: FHWA n.d. Table 24. CMFs for converting two-lane to three-lane road with TWLTL on urban and suburban arterials.

Safety Effects of Access Management 27   Strategy Crash Severity CMF CMF ID Applicability Install TWLTL on 4-lane undivided road All 0.48 9416 • 6,800 to 27,304 vehicles/day• Non-intersection crashes All 0.36 9417 • 18,748 to 20,098 vehicles/day • Population of < 2,500 people • Non-intersection crashes All 0.56 9418 • 6,800 to 24,880 vehicles/day • Population of 5,000 to 25,000 people • Non-intersection crashes All 0.66 9419 • 22,262 to 24,880 vehicles/day • Population of 50,000 to 100,000 people • Non-intersection crashes All 0.21 9420 • 24,880 to 27,304 vehicles/day • Population of 100,000 to 200,000 people • Non-intersection crashes All 0.43 9421 • 24,880 to 27,304 vehicles/day • Population of 200,000 to 500,000 people • Non-intersection crashes All -0.0006*DD1 + 0.0631*DD – 1.1783 9422 • 6,800 to 27,304 vehicles/day • 30 to 66 driveways per mile • Non-intersection crashes All 1.32*CDR2 – 1.1975*CDR + 0.5766 9423 • 6,800 to 27,304 vehicles/day • Proportion of commercial driveways from 0.25 to 1.0 • Non-intersection crashes Note: The base condition is a four-lane undivided road. 1 DD = driveway density (number of driveways per mile). 2 CDR = commercial driveway ratio (ratio of commercial driveways to total driveways in segment). Source: FHWA n.d. Table 25. CMFs for converting four-lane to five-lane road with TWLTL on urban and suburban arterials. Strategy Crash Severity CMF CMF ID Applicability Convert 4-lane undivided road to 3 lanes with TWLTL All 0.53 2841 • Suburban All 0.75 5553 • Urban • 2,030 to 15,350 vehicles/day All 0.71 199 • Urban minor arterials All 0.95 4709 • 3,510 to 17,020 vehicles/day All 1.05 4719 • Commercial areas • 3,510 to 17,020 vehicles/day All 1.06 4717 • Mixed-use areas • 3,510 to 17,020 vehicles/day All 0.77 4715 • Residential areas • 3,510 to 17,020 vehicles/day All 0.91 4711 • High-crash areas (> 26 crashes/mile) • 3,510 to 17,020 vehicles/day All 0.98 4713 • Low-crash areas (< 26 crashes/mile) • 3,510 to 17,020 vehicles/day All 0.81 4723 • High driveway density (> 32/mile) • 3,510 to 17,020 vehicles/day All 1.07 4721 • Low driveway density (< 32/mile) • 3,510 to 17,020 vehicles/day All 0.63 874 • Urban Fatal and injury 1.00 875 • Urban PDO 0.54 876 • Urban Note: The base condition is a four-lane undivided road. Source: FHWA n.d. Table 26. CMFs for converting four-lane to three-lane road with TWLTL on urban and suburban arterials.

28 Application of Crash Modification Factors for Access Management FHWA’s Pedestrian Safety Guide and Countermeasure Selection System (Zegeer et al. 2013) in road design, notes that studies have shown that conversion of two-way streets to one-way operation generally reduces pedestrian crashes. However, one-way streets tend to have higher operating speeds, which can increase the severity of crashes. If a street is converted to one-way, it should be evaluated to see if additional changes are appropriate (e.g., narrowing the cross sec- tion), especially if the street or lanes are overly wide. As a system, one-way streets can increase travel distances for motorists and create some confusion, especially for non-local residents. One-way streets tend to operate better in pairs, separated by a block to no more than ¼ mile, and in a downtown or very heavily congested areas. Conversions can go the other way as well. For example, converting one-way streets to two- way operations may allow better local access and help to slow traffic. Two-way streets tend to pro- duce slower vehicle speeds due to “friction,” especially on residential streets without a marked center line. The Highway Safety Manual (1st Edition) does not provide models to estimate the safety perfor- mance of one-way streets; however, it does provide models for estimating the safety performance of two-way streets, as discussed in Chapter 4 of this guide. Further, NCHRP Project 17-58, “Safety Prediction Models for Six-Lane and One-Way Urban and Suburban Arterials,” provides a framework and models for estimating the safety performance of one-way streets. Similarly, NCHRP Project 17-62, “Improved Prediction Models for Crash Types and Crash Severities,” provides updated models for the upcoming second edition of the Highway Safety Manual. Table 27 presents CMFs inferred from the NCHRP 17-58 and 17-62 research results (publica- tion of these results is pending). These inferred CMFs represent the conversion of two parallel two-way four-lane undivided arterials to two parallel one-way four-lane arterials, assuming dif- ferent traffic volumes and driveway densities. Based on the inferred CMFs, the conversion from two-way to one-way is more effective for larger driveway densities and larger traffic volumes. It may be ineffective and potentially increase crashes at lower traffic volumes. One concern related to this conversion may be higher speeds on one-way streets, particularly at lower traffic volumes. Other considerations related to the potential conversion include access spacing and density, including both signalized and unsignalized intersections and driveways. While a similar approach could be used to develop inferred CMFs for other scenarios, including different lane and median configurations, the preferred method is to estimate and compare the safety performance of alternatives based on the site-specific conditions. Strategy Crash Severity CMF Applicability Convert two parallel 4-lane arterials from two-way to one- way operation All 1.57 • Traffic volume of 10,000 vehicles per day • Driveway density of 20 per mile 1.02 • Traffic volume of 20,000 vehicles per day • Driveway density of 20 per mile 0.81 • Traffic volume of 30,000 vehicles per day • Driveway density of 20 per mile 1.37 • Traffic volume of 10,000 vehicles per day • Driveway density of 40 per mile 0.90 • Traffic volume of 20,000 vehicles per day • Driveway density of 40 per mile 0.71 • Traffic volume of 30,000 vehicles per day • Driveway density of 40 per mile Source: NCHRP Project 17-58 Draft Final Report; NCHRP Project 17-62 Draft Final Report. Table 27. CMFs for converting two, parallel, four-lane, undivided, urban and suburban arterials from two-way to one-way operation (inferred from NCHRP Project 17-58 and Project 17-62 Predictive Methods).

Safety Effects of Access Management 29   A review of the CMF Clearinghouse, in December 2019, indicated that there were eight CMFs rated three stars or above that apply to the conversion of urban and suburban arterials from two-way to one-way operation. Table 28 presents three related CMFs along with the general applicability, CMF ID (linked to the CMF Clearinghouse in the PDF of this publication, which can be found on www.trb.org by searching on “NCHRP Research Report 974”), and base condi- tion. This table only presents CMFs for total crashes and CMFs by crash severity. Refer to the link to the CMF Clearinghouse to check for additional CMFs by crash type. Intersection Treatments This section describes strategies related to intersections, including left-turn lanes, right-turn lanes, and alternative intersection designs. Left-Turn Lanes The treatment of left turns at intersections has an important bearing on safety and opera- tions along arterial roadways and is one of the major access management concerns. Left-turn movements, especially those that are made from lanes shared with through traffic, create con- flict points and delays. Left turns at driveways and street intersections may be accommodated, prohibited, diverted, or separated depending on specific circumstances. A restrictive median is an effective measure to prohibit left turns at locations where it is present; however, this strategy diverts turning movements to downstream locations, which can shift a potential safety issue if not treated properly. See the previous section, Roadway Cross Section, for further discussion on the safety performance of different roadway cross sections and median designs. Left-turn lanes provide a refuge for left-turning vehicles by removing those vehicles from the through-traffic lane(s). As such, left-turn lanes are an effective means of reducing the conflict points and the speed differential that exists between a turning vehicle and the through vehicles that follow when left turns are made from a shared lane. The addition of exclusive left-turn lanes has been shown to provide a variety of traffic safety and operational benefits, including the following: • Reducing the number of conflict points and crashes (particularly rear-end, angle, and side- swipe crashes), • Physically separating left-turning traffic and queues from through traffic, • Decreasing vehicular delay and increasing intersection capacity, • Providing an area for left-turning vehicles to decelerate outside of the through travel lane, and • Providing greater operational flexibility (e.g., additional traffic signal phasing opportunities). Left-turn lanes are normally provided by offsetting the center line or by recessing the physical median. Examples of single and dual left-turn lanes are shown in Figure 7. A typical shared-lane Strategy Crash Severity CMF CMF ID Applicability Convert road from two-way to one-way operations Fatal and injury (KABC on KABCO scale) 0.43 4010 • Frontage road • 2,996 vehicles/day Possible injury (C on KABCO scale) 0.46 4016 Fatal, serious, and minor injury (KAB on KABCO scale) 0.32 4017 Note: The base condition is a two-way frontage road. Source: FHWA n.d. Table 28. CMFs for converting urban and suburban arterials from two-way to one-way operation.

30 Application of Crash Modification Factors for Access Management treatment is shown for comparison purposes. NCHRP Report 745: Left-Turn Accommodations at Unsignalized Intersections (Fitzpatrick et al. 2013), indicates left-turn lanes are likely to be warranted at most unsignalized intersections (except at those with very low volumes). This is based primarily on the cost savings attributable to an expected decrease in the number of crashes as a result of the left-turn-lane installation. Part D of the Highway Safety Manual (1st Edition) presents several CMFs that apply to left- turn lanes at intersections on urban and suburban arterials. These CMFs are also presented in the CMF Clearinghouse. Table 29 presents the CMFs for installing left-turn lanes on one or two approaches of urban and suburban stop-controlled and signalized intersections. According to the Highway Safety Manual (1st Edition), the supporting research did not indicate a change in safety performance for the presence of a left-turn lane on the stop-controlled approaches. For four-legged signalized intersections, the CMF for installing left-turn lanes on three or four approaches is equal to the CMF for installing a left-turn lane on one approach raised to the third or fourth power, respectively. Source: Adapted from Koepke and Levinson 1992, p. 71. 1. Peak left turn volumes over 300 V.P.H. 2. 28’ Min. width on receiving highway 3. Protected signal phasing 4. Divided highway (desirable) 1. Applies to all types of highways 2. May have exclusive, or protected/ permissive, or permissive phasing C) Dual Left-Turn Lane B) Left-Turn Lane A) Shared Through/Left-Turn Lane 1. Limit use to: a. Minor streets b. Major streets where R.O.W. is not available c. Low speed streets d. Low volume streets Figure 7. Examples of left-turn lanes.

Safety Effects of Access Management 31   Beyond the Highway Safety Manual (1st Edition), the CMF Clearinghouse presents several other CMFs rated three stars or higher that apply to left-turn lanes at intersections on urban and suburban arterials. In addition to installing left-turn lanes, there is the option to create a positive offset to improve sight distance and enhance safety. Based on a review of the CMF Clearing- house in December 2019, Tables 30 through 32 present 14 related CMFs along with the general applicability, CMF ID (linked to the CMF Clearinghouse in the PDF of this publication, which can be found on www.trb.org by searching on “NCHRP Research Report 974”), and base condi- tion. These tables only present CMFs for total crashes and CMFs by crash severity. Refer to the link to the CMF Clearinghouse to check for additional CMFs by crash type. Strategy Crash Severity CMF CMF ID1 Applicability Install left-turn lane on one approach All 0.67 254 • Urban 3-legged minor road stop- controlled • 1,500 to 40,600 vehicles/day on major road • 200 to 8,000 vehicles/day on minor road Fatal and injury 0.65 4646 All 0.93 4644 • Urban 3-legged signalized • Note the CMF Clearinghouse indicates these CMFs “cannot be rated” because they were developed through an expert panel; however, both CMFs appear in the Highway Safety Manual (1st Edition). Fatal and injury 0.94 4645 All 0.73 261 • Urban 4-legged minor road stop- controlled • 1,500 to 40,600 vehicles/day on major road • 200 to 8,000 vehicles/day on minor road Fatal and injury 0.71 265 All 0.90 262 • Urban 4-legged signalized • 7,200 to 55,100 vehicles/day on major road • 550 to 2,600 vehicles/day on minor road Fatal and injury 0.91 266 All 0.76 263 • Urban 4-legged (newly) signalized • 4,600 to 40,300 vehicles/day on major road • 100 to 13,700 vehicles/day on minor road Fatal and injury 0.72 267 Install left-turn lane on two approaches All 0.53 269 • Urban 4-legged minor road stop- controlled • 1,500 to 40,600 vehicles/day on major road • 200 to 8,000 vehicles/day on minor road Fatal and injury 0.50 273 All 0.86 -- • Urban 3-legged signalized Fatal and injury 0.88 -- All 0.81 270 • Urban 4-legged signalized • 7,200 to 55,100 vehicles/day on major road • 550 to 2,600 vehicles/day on minor road Fatal and injury 0.83 274 All 0.58 271 • Urban 4-legged (newly) signalized • 4,600 to 40,300 vehicles/day on major road • 100 to 13,700 vehicles/day on minor road Fatal and injury 0.52 275 Note: The base condition is no left-turn lanes present. Minor road stop-controlled approaches are not considered in counting the number of approaches with left-turn lanes. 1CMF IDs are linked to the CMF Clearinghouse in the PDF of this publication, which can be found on www.trb.org by searching on “NCHRP Research Report 974”. Source: AASHTO 2010. Table 29. CMFs for installing left-turn lanes at urban and suburban intersections from the Highway Safety Manual (1st Edition).

32 Application of Crash Modification Factors for Access Management Strategy Crash Severity CMF CMF ID Applicability Install left-turn lane All 0.75 7996 • 3-legged signalized intersections • 2-lane suburban roads • 2,981 to 18,248 vehicles/day on major road • 972 to 13,880 vehicles/day on minor road Fatal and injury 0.57 7999 All 0.92 7997 • 4-legged signalized intersections • 2-lane suburban roads • 1,360 to 17,566 vehicles/day on major road • 746 to 8,884 vehicles/day on minor road Fatal and injury 0.80 8000 All 0.88 7998 • 3- and 4-legged signalized intersections • 2-lane suburban roads • 1,360 to 18,248 vehicles/day on major road • 746 to 13,880 vehicles/day on minor road Fatal and injury 0.74 8001 Note: The base condition is no left-turn lanes present. Source: FHWA n.d. Table 30. CMFs for installing left-turn lanes at urban and suburban signalized intersections. Strategy Crash Severity CMF CMF ID Applicability Install traffic signal and left-turn lane All 0.54 7966 • 3-legged minor road stop-controlled • 2-lane suburban roads • 2,981 to 18,248 vehicles/day on major road • 1,852 to 13,880 vehicles/day on minor road Fatal and injury 0.47 7969 All 0.57 7967 • 4-legged minor road stop-controlled • 2-lane suburban roads • 1,360 to 15,500 vehicles/day on major road • 1,036 to 8,884 vehicles/day on minor road Fatal and injury 0.48 7970 All 0.56 7968 Fatal and injury 0.48 7971 • 3- and 4-legged minor road stop- controlled • 2-lane suburban roads • 1,360 to 18,248 vehicles/day on major road • 1,036 to 13,880 vehicles/day on minor road Note: The base condition is minor road stop-controlled intersections with no left-turn lanes present. Source: FHWA n.d. Table 31. CMFs for installing a traffic signal and left-turn lanes at urban and suburban stop-controlled intersections. Strategy Crash Severity CMF CMF ID Applicability Improve offset for left-turn lanes All 0.66 6095 • 4-legged signalized intersections • 7,150 to 29,200 vehicles/day on major road • 2,200 to 13,350 vehicles/day on minor road Fatal and injury 0.64 6096 Note: The base condition is no offset. Source: FHWA n.d. Table 32. CMFs for improving the offset for left-turn lanes on urban and suburban arterials.

Safety Effects of Access Management 33   Right-Turn Lanes Right-turn movements, especially those that are made from shared lanes, create conflict points and delays. Right-turn lanes provide a refuge for right-turning vehicles by removing those vehicles from the through-traffic lane(s). As such, right-turn lanes are an effective means of reducing the conflict points and the speed differential that exists between a turning vehicle and the through vehicles that follow when right turns are made from a shared lane. The addition of exclusive right-turn lanes has been shown to provide a variety of traffic safety and operational benefits, including the following: • Reducing the frequency and severity of rear-end collisions (Koepke and Levinson 1992), • Physically separating right-turning traffic and queues from through traffic, • Decreasing vehicular delay and increasing intersection capacity, • Providing an area for right-turning vehicles to decelerate outside of the through travel lane, and • Providing greater operational flexibility (e.g., additional traffic signal phasing opportunities). Right-turn lanes may be provided at a single access, or they can be extended to accommodate several nearby driveways. However, to operate as intended, a continuous right-turn lane should not be too long (e.g., not longer than ¼ mile) to avoid additional conflict points that would be introduced between vehicular and bicycle traffic and to discourage the use of the lane for through movements or to avoid traffic congestion. Part D of the Highway Safety Manual (1st Edition) presents several CMFs that apply to right- turn lanes at intersections on urban and suburban arterials. These CMFs are also presented in the CMF Clearinghouse. Table 33 presents the CMFs for installing right-turn lanes on one or two approaches of urban and suburban stop-controlled and signalized intersections. According to the Highway Safety Manual (1st Edition), the supporting research did not indicate a change in safety performance for the presence of a right-turn lane on the stop-controlled approaches. For four-legged signalized intersections, the CMF for installing right-turn lanes on three or four approaches is equal to the CMF for installing a right-turn lane on one approach raised to the third or fourth power, respectively. Beyond the Highway Safety Manual (1st Edition), the CMF Clearinghouse presents several other CMFs rated three stars or higher that apply to right-turn lanes at intersections on urban and Strategy Crash Severity CMF CMF ID1 Applicability Install right-turn lane on one approach All 0.86 285 • 3- and 4-legged minor road stop-controlled • 1,500 to 40,600 vehicles/day on major road • 25 to 26,000 vehicles/day on minor roadFatal and injury 0.77 287 All 0.96 286 • 3- and 4-legged signalized • 7,200 to 55,100 vehicles/day on major road • 550 to 8,400 vehicles/day on minor roadFatal and injury 0.91 288 Install right-turn lane on two approaches All 0.74 289 • 4-legged minor road stop-controlled • 1,500 to 40,600 vehicles/day on major road • 25 to 26,000 vehicles/day on minor roadFatal and injury 0.59 4649 All 0.92 290 • 4-legged signalized • 7,200 to 55,100 vehicles/day on major road • 550 to 8,400 vehicles/day on minor roadFatal and injury 0.83 4650 Note: The base condition is no right-turn lanes present. Minor road stop-controlled approaches are not considered in counting the number of approaches with right-turn lanes. 1CMF IDs are linked to the CMF Clearinghouse in the PDF of this publication that can be found on www.trb.org by searching on “NCHRP Research Report 974”. Source: AASHTO 2010. Table 33. CMFs for installing right-turn lanes at urban and suburban intersections from the Highway Safety Manual (1st Edition).

34 Application of Crash Modification Factors for Access Management suburban arterials. Based on a review of the CMF Clearinghouse in December 2019, Table 34 presents two related CMFs along with the general applicability, CMF ID (linked to the CMF Clearinghouse in the PDF of this publication, which can be found on www.trb.org by searching on “NCHRP Research Report 974”), and base condition. This table only presents CMFs for total crashes and CMFs by crash severity. Refer to the link to the CMF Clearing house to check for addi- tional CMFs by crash type. In addition, the research team for NCHRP Project 17-74 developed CMFs for channelizing right-turn lanes at urban and suburban arterial intersections. Table 35 presents the CMF from this effort. The CMF is applied to the total estimated crashes for locations with a right-turn lane present, excluding vehicle-pedestrian and vehicle-bicycle crashes. The research did not indicate a change in safety performance for the channelization of a right-turn lane on four-legged stop- controlled approaches or three-legged and four-legged signalized approaches. Alternative Intersection Designs There are a number of alternative intersection designs that vary in terms of how they provide for left-turn and U-turn movements. NCHRP Synthesis 404: State of the Practice in Highway Access Management, indicates that “U-turns are being used increasingly as an alternative to direct left turns to reduce conflict points and to improve safety along arterial roads. U-turns make it possible to prohibit left turns from driveway connections onto multilane highways and to eliminate traffic signals that would not fit into time-space (progression) patterns along arterial roads. When incorporated into intersection designs, U-turn provisions enable direct left turns to be rerouted and signal phasing to be simplified” (Gluck and Lorenz 2010). The FHWA Alternative Intersections/Interchanges: Informational Report (AIIR) discusses several alternative treatments that include (Hughes et al. 2010): • Roundabouts, • Median U-turn intersection, • Restricted crossing U-turn intersection (superstreet or J-turn), Table 35. CMF for channelizing right-turn lanes at urban and suburban stop-controlled intersections. Strategy Crash Severity CMF Applicability Channelize right-turn lane All 0.72 • 3-legged minor road stop-controlled • All crashes except vehicle- pedestrian and vehicle-bicycle crashes Note: The base condition is the presence of a right-turn lane with no channelization. Minor road stop-controlled approaches are not considered in counting the number of approaches with right-turn lanes or channelization. Source: NCHRP Project 17-74. Strategy Crash Severity CMF CMF ID Applicability Install right-turn lane on one approach All 0.80 2967 • 3-legged minor road stop-controlled • 7,332 to 66,171 vehicles/day on major road All 0.75 2971 • 4-legged minor road stop-controlled • 8,955 to 66,171 vehicles/day on major road Note: The base condition is no right-turn lanes present. Source: FHWA n.d. Table 34. CMFs for installing right-turn lanes at urban and suburban stop-controlled intersections.

Safety Effects of Access Management 35   • Double crossover diamond interchange (diverging diamond), • Displaced left-turn interchange (continuous flow intersection), and • Quadrant roadway intersection. The safety effects of converting a traditional intersection to a roundabout can be inferred from SPFs. NCHRP Research Report 888: Development of Roundabout Crash Prediction Models and Methods, presents a framework along with SPFs and adjustment factors to estimate the safety performance of roundabouts (Ferguson et al. 2018). The framework provides the flexibility to estimate safety performance at the planning level, intersection level, and leg level for configura- tions (single-lane, multilane) and area types (rural, urban). This framework will be included in the Highway Safety Manual (2nd Edition—forthcoming). Chapter 4 of this guide presents the SPFs and CMFs for estimating the safety performance of traditional stop-controlled and signal- ized intersections. Rather than developing a CMF for converting a traditional intersection to a roundabout, the preferred method is to estimate and compare the safety performances of the alternatives. While the preferred approach is to use the predictive methods provided in Chapter 4 of this guide and NCHRP Research Report 888 to quantify the safety performance of alternative inter- section designs, Part D of the Highway Safety Manual (1st Edition) presents 19 CMFs that apply to roundabouts at intersections on urban and suburban arterials. These CMFs are also presented in the CMF Clearinghouse. For completeness, Table 36 presents the CMFs for converting urban and suburban intersections to roundabouts. Note the variability in the CMFs depending on the characteristics of the intersection before conversion and the roundabout (including number of circulating lanes). Strategy Crash Severity CMF CMF ID1 Applicability Convert signalized intersection to roundabout All 0.52 225 • Urban and suburban intersections • 1- or 2-lane roundabouts Fatal and injury 0.22 226 All 0.99 222 • Urban intersections • 1- or 2-lane roundabouts Fatal and injury 0.40 223 All 0.33 224 • Suburban intersections • 2-lane roundabouts Convert minor road stop- controlled intersection to roundabout All 0.56 227 • Urban and suburban intersections • 1- or 2-lane roundabouts Fatal and injury 0.18 228 All 0.71 231 • Urban intersections • 1- or 2-lane roundabouts Fatal and injury 0.19 232 All 0.61 233 • Urban intersections • 1-lane roundabouts Fatal and injury 0.22 234 All 0.88 235 • Urban intersections • 2-lane roundabouts All 0.68 236 • Suburban intersections • 1- or 2-lane roundabouts Fatal and injury 0.29 237 All 0.22 238 • Suburban intersections • 1-lane roundabouts Fatal and injury 0.22 239 All 0.81 240 • Suburban intersections • 2-lane roundabouts Fatal and injury 0.32 241 Convert all-way stop-controlled intersection to roundabout All 1.03 242 • Urban and suburban intersections • 1- or 2-lane roundabouts Note: The base condition is a conventional intersection. 1CMF IDs are linked to the CMF Clearinghouse in the PDF of this publication, which can be found on www.trb.org by searching on “NCHRP Research Report 974”. Source: FHWA n.d. Table 36. CMFs for converting intersections to roundabouts on urban and suburban arterials.

36 Application of Crash Modification Factors for Access Management Based on a review of the CMF Clearinghouse in December 2019, there were several CMFs rated three stars or higher, including those in Table 36, that apply to alternative intersection designs on urban and suburban arterials. Refer to the previous section, Median Treatment— Non-Traversable Median, for CMFs related to a variation of the median U-turn intersection (i.e., conversion of direct left turns to right turns followed by U-turns). While CMFs are not available for all of the alternative intersection designs listed, Tables 37 through 41 present Strategy Crash Severity CMF CMF ID Applicability Convert intersection to single-lane or multilane high-speed roundabout All 0.66 5229 • 2- and 4-lane roads • 3- and 4-legged intersections • No control, yield-controlled, stop- controlled, and signalized • 4,100 to 48,100 entering vehicles/day Fatal and injury 0.51 5230 Convert intersection to single-lane or multilane low-speed roundabout All 1.10 5227 • 2- and 4-lane roads • 3- and 4-legged intersections • No control, yield-controlled, stop- controlled, and signalized • 4,100 to 48,100 entering vehicles/day Fatal and injury 0.47 5228 Convert intersection to multilane roundabout All 1.06 4926 • 4-lane roads • 3- and 4-legged intersections • No control, yield-controlled, stop- controlled, and signalized • 4,100 to 48,100 entering vehicles/day Fatal and injury 0.37 4927 Convert intersection to single-lane roundabout All 0.64 4924 • 2-lane roads • 3- and 4-legged intersections • No control, yield-controlled, stop- controlled, and signalized • 6,000 to 21,900 entering vehicles/day Fatal and injury 0.82 4925 Note: The base condition is a conventional intersection. Source: FHWA n.d. Table 37. CMFs for converting urban and suburban intersections to roundabouts. Strategy Crash Severity CMF CMF ID Applicability Convert stop-controlled intersection to single- lane roundabout All 0.28 206 • Minor road stop-controlled • 3- and 4-legged intersections • 4,600 to 17,825 vehicles/day Fatal and injury 0.12 210 Convert stop-controlled intersection to multilane roundabout All 0.95 208 • Minor road stop-controlled • 3- and 4-legged intersections • 13,272 to 30,418 vehicles/day Convert stop-controlled intersection to single- lane or multilane roundabout All 0.75 4930 • Minor road stop-controlled • 2- and 4-lane roads • 3- and 4-legged intersections • 4,100 to 48,100 entering vehicles/dayFatal and injury 0.65 4931 Convert no control/yield intersection to single- lane or multilane roundabout All 1.24 4928 • No control and yield-controlled • 2- and 4-lane roads • 3- and 4-legged intersections • 4,100 to 48,100 entering vehicles/dayFatal and injury 0.00 4929 Convert all-way stop- controlled intersection to single-lane or multilane roundabout All 1.11 4932 • All-way stop-controlled • 2- and 4-lane roads • 3- and 4-legged intersections • 4,100 to 48,100 entering vehicles/dayFatal and injury 0.54 4933 Note: The base condition is a conventional unsignalized intersection. Source: FHWA n.d. Table 38. CMFs for converting unsignalized urban and suburban intersections to roundabouts.

Safety Effects of Access Management 37   Strategy Crash Severity CMF CMF ID Applicability Convert signalized intersection to single-lane or multilane roundabout All 0.79 4184 • Urban and suburban • 1 or 2 circulating lanes • 3- and 4-legged intersections • 15–35 mph posted speed • 5,300 to 52,500 vehicles/day on major road Fatal and injury 0.34 4185 All 0.96 4934 • 4-lane roads • 3- and 4-legged intersections • Signalized intersections • 4,100 to 48,100 entering vehicles/day Fatal and injury 0.35 4935 All 1.92 5523 • 1 to 3 circulating lanes • 3- and 4-legged intersections • Signalized intersections • 5,700 to 32,900 vehicles/day on major road • 1,900 to 24,200 vehicles/day on minor road All 0.58 4186 • Suburban • 1 or 2 circulating lanes • 3- and 4-legged intersections • 15–35 mph posted speed • 5,300 to 52,500 vehicles/day on major road Fatal and injury 0.26 4187 All 1.15 4188 • Urban • 1 or 2 circulating lanes • 3- and 4-legged intersections • 15–35 mph posted speed • 5,300 to 52,500 vehicles/day on major road Fatal and injury 0.45 4189 All 1.07 4190 • Urban and suburban • 1 or 2 circulating lanes • 3-legged intersections • 15–35 mph posted speed • 5,300 to 52,500 vehicles/day on major road Fatal and injury 0.37 4191 All 0.76 4192 • Urban and suburban • 1 or 2 circulating lanes • 4-legged intersections • 15–35 mph posted speed • 5,300 to 52,500 vehicles/day on major road Fatal and injury 0.34 4193 Convert signalized intersection to single-lane roundabout All 0.74 4196 • 3- and 4-legged intersections • Signalized intersections • 15–35 mph • 5,300 to 52,500 vehicles/day on major road Fatal and injury 0.45 4197 Convert signalized intersection to multilane roundabout All 0.81 4194 • 3- and 4-legged intersections • Signalized intersections • 15–35 mph • 5,300 to 52,500 vehicles/day on major road Fatal and injury 0.29 4195 Note: The base condition is a conventional signalized intersection. Source: FHWA n.d. Table 39. CMFs for converting signalized urban and suburban intersections to roundabouts. Strategy Crash Severity CMF CMF ID Applicability Convert conventional signalized intersection to signalized RCUT All 0.85 9984 • 3- and 4-legged signalized • 4- to 6-lane roads • Median divided • 40–65 mph posted speed Fatal and injury 0.78 9985 Note: The base condition is a conventional signalized intersection. Source: FHWA n.d. Table 40. CMFs for converting conventional signalized intersection to signalized restricted crossing U-turn (RCUT) on urban and suburban arterials.

38 Application of Crash Modification Factors for Access Management 47 CMFs from the CMF Clearinghouse along with the general applicability, CMF ID (linked to the CMF Clearinghouse in the PDF of this publication, which can be found on www.trb.org by searching on “NCHRP Research Report 974”), and base condition. These tables only present CMFs for total crashes and CMFs by crash severity. Refer to the link to the CMF Clearing- house to check for additional CMFs by crash type. For roundabouts, the CMFs are provided for completeness but note the variability in CMFs depending on the characteristics before and after conversion. Again, the preferred approach is to use the predictive methods provided in Chapter 4 of this guide and NCHRP Research Report 888 (Ferguson et al. 2018) to estimate the safety performance of alternatives. Finally, there is an opportunity to apply a Safe System approach to compare the safety per- formance of alternative intersection designs. A Safe System analysis would complement crash- based predictive methods based on SPFs and CMFs and could serve as an alternative when a crash-based method is not available for the facility type or strategy of interest. The Safe System approach can also strengthen ongoing intersection programs and initiatives, such as Inter- section Control Evaluation (ICE) policies and procedures. Incorporating Safe System principles into ICE would add an additional safety perspective and a new set of performance measures that can be used to inform the selection of an intersection type. The careful selection of an inter- section type and control is critical to future safety performance because it directly influences the number, type, and severity of potential conflict points (as well as capacity and delay). Property Access In addition to the location and design of access for a specific property, there are also strategies to provide reasonable access for a particular property, or properties, such that the resulting access configuration provides for safer and more efficient traffic operations. This section describes strategies related to driveway design elements, sight distance at unsignalized access, frontage/ backage roads, and shared driveways and internal cross connectivity. A related strategy, pro- viding access via secondary roadways (i.e., a roadway that has a lower access classification than Strategy Crash Severity CMF CMF ID Applicability Convert diamond interchange to DDI or DCD All 0.67 8258 • 3- to 6-lane roads • Median divided • 40–45 mph posted speed • 28,168 cross road vehicles/day Fatal and injury 0.59 8278 Fatal and injury 0.37* 9102 • Interchange footprint • 35–40 mph posted speed on cross road • 60–65 mph on freeway • 33,000 to 152,000 vehicles/day on freeway • 16,000 to 29,000 vehicles/day on cross road Possible injury (C on KABCO scale) 0.59* 9104 PDO 0.65* 9103 All 0.63 9107 • 4-legged ramp terminals • 35–45 mph posted speed on cross road • 17,000 to 38,000 vehicles/day on cross road Fatal and injury 0.45 9105 PDO 0.69 9106 All 0.63 10135 Not specified Fatal and injury 0.46 10155 PDO 0.70 10156 Notes: The base condition is a conventional signalized intersection. * indicates the CMF applies to the entire interchange footprint (i.e., ramp terminals, ramp segments, speed-change lanes, crossroad, and freeway segment). Source: FHWA n.d. Table 41. CMFs for converting diamond interchange to diverging diamond interchange (DDI) or double crossover diamond (DCD) on urban and suburban arterials.

Safety Effects of Access Management 39   the intersecting primary roadway), is not included because of the implications it has for traffic circulation patterns on the surrounding roadway network. Driveway Design Elements Driveways are integral to the roadway transportation system. Every driveway connection to a roadway creates an intersection, which, in turn, creates conflict points for the motorist with bicyclists, pedestrians, and other motor vehicles. Other sections of this guide discuss the importance of driveway location and spacing; however, the design of a driveway also merits consideration due to the implications it has on safety. Proper driveway design considers the needs of all users by minimizing conflict points while accommodating mobility and reasonable access. As shown in Figure 8, the following are design elements that influence driveway safety and operations: • Width and number of lanes, • Curb return radius and throat transition geometry, • Throat length and internal site queue storage, • Angle of intersection and horizontal alignment, • Non-traversable medians and islands located on the driveway, • Cross slope, • Driveway edge treatment, • Clearance from fixed objects, and • Vertical alignment and grade. Source: Adapted from Gattis et al. 2010. Figure 8. Key driveway design elements.

40 Application of Crash Modification Factors for Access Management As indicated in FHWA’s Access Management in the Vicinity of Intersections: Technical Sum- mary (FHWA 2010), driveway connections to public roads should be adequately designed to improve the safety and efficiency of vehicle movements to and from the roadway, while bal- ancing safety with mobility interests. There are many elements to consider in proper driveway design, including upstream and downstream sight distance, the angle at which the driveway intersects the major road, the appropriate width of the driveway in tandem with curb radii to accommodate turning movements, the number of lanes (sufficient for the volume at the site), and the vertical grade and length of the driveway throat. In general, driveways should be designed with the appropriate radius, width, and vertical geometry to allow inbound turns to be made without obstructing through vehicles. In addition, the driveway should be of sufficient length to allow motorists to completely pull off the road without interference from on-site parked vehicles, vehicle queues, or pedestrian or vehicle cir- culation once they enter the property adjacent to the roadway. The design of a driveway at any given location is a function of the design vehicle, travel speeds to and from the property, traffic volume, pedestrian and bicycle volume, and the type of traffic control. Based on a review of the CMF Clearinghouse in December 2019, there were three CMFs rated three stars or higher that apply to driveway design elements on urban and suburban arterials. Table 42 presents these CMFs along with the general applicability, CMF ID (linked to the CMF Clearinghouse in the PDF of this publication, which can be found on www.trb.org by searching on “NCHRP Research Report 974”), and base condition. Refer to the CMF Clearinghouse to check for more recent CMFs. Intersection and Driveway Sight Distance The provision of adequate sight distance at all access, including driveways, along roadways is a fundamental aspect of traffic operations and safety. Specified areas along the intersection approaches and corners should be clear of sight obstructions, such as parked vehicles and veg- etation, which might block a driver’s view of potentially conflicting vehicles. The driver of a vehicle approaching an intersection needs to have an unobstructed view of the entire inter- section and any traffic control devices. Drivers also need sufficient time and distance to stop or adjust their speed, as appropriate, to avoid a crash. There are various measures of sight distance, including intersection sight distance, stopping sight distance, and decision sight distance. There are different criteria for each measure as well as variations in the beginning and ending points of the line of sight depending on which motorist is Strategy Crash Severity CMF CMF ID Applicability Convert driveway from right-in-right- out to full access All 2.25 8208 Driveway-related crashes Change driveway width from X to Y feet All exp(0.02656*(Y-X)) 8202 Driveway-related crashes Reduce number of driveway entry lanes from 2 to 1 All 0.72 8203 Driveway-related crashes Note: The base condition for driveway width is X feet, and the condition of interest is Y feet, assuming an ideal driveway width of 12 feet per lane (12 feet for a one-lane driveway and 24 feet for a two-lane driveway). Source: FHWA n.d. Table 42. CMFs for driveway design elements on urban and suburban arterials.

Safety Effects of Access Management 41   in consideration. Based on the application of information in AASHTO’s A Policy on Geometric Design of Highways and Streets, the sight distance needed under various assumptions of physical conditions and driver behavior is directly related to vehicle speeds and to the resultant distances traversed during perception-reaction time and braking (AASHTO 2018). Intersection sight distance (or sight distance from the intersection) is important primarily for drivers on the minor road as it allows them to detect conflicting vehicles and identify appro- priate gaps. Figure 9 shows an example of intersection sight distance considerations at a three- legged intersection with the decision point (DP) on the minor road approach. Stopping sight distance (or sight distance to the intersection) is important for drivers on both the mainline and the minor road. On the mainline, stopping sight distance allows drivers to identify the minor road and potential conflicts with turning vehicles. On the minor road, stop- ping sight distance allows drivers to identify and react to the STOP sign. With limited stopping sight distance, drivers may not have time to identify and react to conflicting movements or the traffic control and, therefore, fail to respond appropriately. Decision sight distance accounts for more nuanced situations that involve greater complexity such as maneuvers ranging from stopping to speed, path, or direction change. With respect to access management, decision sight distance may be a more appropriate design control than stopping sight distance. For example, decision sight distance may be appropriate to allow drivers to perceive and react to vehicle queues or to allow drivers to change speed, path, or direction for a weaving movement to a left- or right-turn lane. NCHRP Project 17-59 developed CMFs related to intersection sight distance (Eccles et al. 2018). Figures 10 to 13 present CMFs from this effort, which apply to three-legged minor road stop-controlled intersections on two- and four-lane roads with a posted speed limit of 35 to 60 mph, major road traffic volume of 272 to 35,671 vehicles per day, and minor road traffic volume of 42 to 15,637 vehicles per day. The CMF is applied to the applicable estimated crashes (total target crashes or fatal and injury target crashes), where the target crashes are those that involve a vehicle on the major road and minor road. Source: Adapted from Eccles et al. 2018. Figure 9. Intersection sight distance for minor road approach at a three-legged intersection.

42 Application of Crash Modification Factors for Access Management Source: Eccles et al. 2018. Figure 10. CMF (target crashes) for changing sight distance. Source: Eccles et al. 2018. Figure 11. CMF (fatal and injury target crashes) for changing sight distance. Source: Eccles et al. 2018. Figure 12. CMF (target crashes) for changing sight distance when AADT and posted speed are unknown. Source: Eccles et al. 2018. Figure 13. CMF (fatal and injury target crashes) for changing sight distance when AADT and posted speed are unknown. Variables for the equations shown in Figures 10 to 13 are defined as follows: • CMFT = Crash modification factor for target crashes. • CMFTFI = Crash modification factor for target fatal and injury crashes. • PSL = Posted speed limit (mph). • LowAADTmaj = 1 if major road annual average daily traffic (AADT) ≤ 5,000; otherwise 0. • MidAADTmaj = 1 if 5,000 < major road AADT ≤ 15,000; otherwise 0. • LowMidAADTmaj = 1 if major road AADT ≤ 15,000; otherwise 0. • ISDnew = New or proposed available intersection sight distance (in feet). • ISDold = Existing or base available intersection sight distance (in feet). Based on a review of the CMF Clearinghouse in December 2019, there were 10 CMFs rated three stars or higher—including two of the equations previously shown—that apply to inter- section sight distance on urban and suburban arterials. Tables 43 and 44 present seven addi- tional CMFs from the CMF Clearinghouse along with the general applicability, CMF ID (linked to the CMF Clearinghouse in the PDF of this publication, which can be found on www.trb. org by searching on “NCHRP Research Report 974”), and base condition. These tables present CMFs for total crashes and CMFs by crash severity, unless otherwise noted in the “applicability” column. Refer to the link to the CMF Clearinghouse to check for additional CMFs by crash type.

Safety Effects of Access Management 43   Frontage/Backage Roads A frontage road is an access roadway that is generally aligned parallel to a main roadway and is located between the right-of-way of the main roadway and the front building setback line. Frontage roads are used as an access management strategy to provide direct access to properties and separate through traffic from local access-related traffic. This reduces the frequency and severity of crashes along the main roadway as well as traffic delays. In addition, the resulting increase in spacing between intersections along the main roadway facilitates the design of auxil- iary lanes for deceleration and acceleration, further improving traffic safety and operations. A “backage” road—also called a “reverse frontage road” or “reverse access”—serves a similar purpose but is located behind the properties that front the main roadway. Frontage and backage roads may be configured for one-way operation or two-way operation. Figure 14 illustrates one potential midblock frontage road configuration. Frontage roads often extend through a number of intersections. The separation of frontage roads at intersecting cross streets should be maximized to ensure sufficient storage for crossroad traffic between the frontage roads and the main roadway. The minimum separation distance should allow adequate left-turn storage and separate operation of the two intersections. Limited CMFs are available for frontage and backage roads. One study reported that frontage roads reduce total crashes by up to 40 percent (Agent et al. 1996). There are no CMFs for total crashes or all crash types by severity rated three stars or higher on the CMF Clearinghouse for frontage or backage roads. Strategy Crash Severity CMF CMF ID Applicability Change right-turn lane geometry to increase line of sight All 0.41 8498 • Crashes related to treated approach • Stop-controlled and signalized intersections • 25–45 mph posted speed • 15,558 to 29,992 total entering vehicles/day All 0.56 8496 • Total intersection crashes • 3- and 4-legged intersections • Stop-controlled and signalized intersections • 25–45 mph posted speed • 15,558 to 29,992 total entering vehicles/dayFatal and injury 0.56 8497 Notes: The base condition is a traditional right-turn lane design, and changes made to study approaches include sharpening the flat approach angle typical in the traditional design, reducing the radius, adjusting the stop/yield bar position, and/or modifying the corner island to improve safety by increasing the line of sight of approaching through traffic. Source: FHWA n.d. Table 44. CMFs for changing right-turn lane geometry to improve sight distance on urban and suburban arterials. Strategy Crash Severity CMF CMF ID Applicability Increase triangle sight distance Fatal (K on KABCO scale) 0.44 1637 Not specified Serious, minor, and possible injury (ABC on KABCO scale) 0.63 1638 Serious, minor, and possible injury (ABC on KABCO scale) 0.53 307 4-legged PDO 0.89 308 Notes: The base condition is not specified but assumed to be less than the proposed sight distance. Source: FHWA n.d. Table 43. CMFs for increasing triangle sight distance on urban and suburban arterials.

44 Application of Crash Modification Factors for Access Management Shared Driveways and Internal Cross Connectivity Access management promotes the implementation of shared-access driveways and cross-access easements between adjacent properties, where possible; these allow pedestrians and vehicles to circulate between properties without reentering the abutting roadway (see Figure 15). The sharing of access driveways improves roadway safety and operations by reducing the number of conflict points and separating conflict points along these roadways. The longer spacing between access driveways also facilitates the provision of left-turn and right-turn lanes, reducing potential conflicts between through and turning movements. In addition, smoother traffic flow on the abutting street helps to reduce the propensity for vehicular crashes and to increase egress capacity. While there are no CMFs available on the CMF Clearinghouse for cross connectivity, Table 6 provides CMFs for driveway density, and Table 7 provides CMFs for driveway spacing. Source: FHWA. Figure 15. Improved access configuration with cross connectivity. Source: FHWA. Figure 14. Potential frontage road configuration.

Safety Effects of Access Management 45   Counterintuitive Results (Do Not Use) While the prior sections present numerous CMFs for estimating the safety effects of access management strategies, there are some results in the literature that are counter- intuitive and require additional research before the results could be used with confi- dence. This section presents results that are counterintuitive and SHOULD NOT BE USED until further research can verify the findings. The following sections indicate counterintuitive results for the strategies described in prior sections. Access Spacing This section describes counterintuitive findings for strategies related to access spacing, including unsignalized access density and spacing and traffic signal spacing. There were no counterintuitive findings associated with functional area, corner clear- ance, and interchange crossroad spacing. Unsignalized Access Density and Spacing (Intersections and Driveways) The Highway Safety Manual (1st Edition) provides models to estimate the safety per- formance of segments and intersections as discussed in Chapter 4; however, the results should not be used to estimate the safety effect of variables related to access spacing and density. The Highway Safety Manual (1st Edition) Part C Predictive Method and Chapter 4 of this guide are only applicable to estimating the safety performance of individual segments and intersections, assuming independence among each unit of analysis. While the results can be aggregated from multiple segments and intersections to estimate the safety performance of a corridor, as suggested in the Highway Safety Manual (1st Edition), this method does not consider the potential interactions among adjacent or nearby sites (e.g., access spacing and density). The existing Part C Predic- tive Method may even produce counterintuitive results (e.g., fewer estimated segment crashes with an increase in the number of intersections along a corridor). As such, the corridor-level predictive method presented in Chapter 5 of this guide is more appropriate for considering interactions among access management features and estimating the safety effect of variables related to access spacing and density. Refer to Chapter 5 for further guidance on when to combine the results from segment- and intersection-level analysis (from Chapter 4) and when to use a corridor-level prediction model for corridor-level analysis. Traffic Signal Density and Spacing The Highway Safety Manual (1st Edition) provides models to estimate the safety per- formance of segments and intersections as discussed in Chapter 4 of this guide; how- ever, the results should not be used to estimate the safety effect of variables related to traffic signal spacing and density. See the previous section, Unsignalized Access Density and Spacing (Intersections and Driveways), for further discussion of potential counterintuitive results. Roadway Cross Section This section describes counterintuitive findings for strategies related to roadway cross section, including non-traversable median treatments and TWLTL median treat- ments. There were no counterintuitive findings associated with median opening spacing and design or converting two-way streets to one-way operation.

46 Application of Crash Modication Factors for Access Management Median Treatment—Non-Traversable Median e Highway Safety Manual (1st Edition) provides models to estimate the safety performance of arterials with and without medians as discussed in Chapter 4 of this guide; however, the results should not be used to infer CMFs. Table 45 presents CMFs inferred from the Highway Safety Manual (1st Edition) for installing a non-traversable median on a four-lane undivided arterial. Similarly, Table 46 presents CMFs inferred from NCHRP Project 17-62, “Improved Prediction Models for Crash Types and Crash Severities.” Based on the inferred CMFs from the Highway Safety Manual (1st Edition), the addition of a non-traversable median to a four-lane undivided arterial appears to reduce crashes and be slightly more eective as trac volume decreases and driveway density increases. Based on the inferred CMFs from NCHRP Project 17-62, the addi- tion of a non-traversable median to a four-lane undivided arterial appears to increase crashes under certain conditions and be more eective as trac volume increases and driveway density decreases. e inconsistency between the two sets of CMFs suggests that the inferred CMFs should not be used to quantify the safety eect of installing non- traversable medians on undivided roads. is appears to be a case where cross-sectional comparisons will not give reasonable and reliable CMFs because the SPF comparisons do not account for all dierences in safety between the two site types even aer the SPFs are calibrated to the same spatial and temporal conditions. e CMFs from the CMF Clearinghouse are more reliable for estimating the safety impacts of this strategy. e Highway Safety Manual (1st Edition) and NCHRP Project 17-62, “Improved Pre- diction Models for Crash Types and Crash Severities,” provide models to estimate the safety performance of arterials with non-traversable medians and with TWLTLs, as dis- cussed in Chapter 4; however, the results should not be used to infer CMFs. Table 47 Strategy Crash Severity CMF Applicability Install non- traversable median on 4- lane undivided road All 0.85 • Traffic volume of 5,000 vehicles per day • Driveway density of 20 residential driveways per mile 0.88 • Traffic volume of 15,000 vehicles per day • Driveway density of 20 residential driveways per mile 0.90 • Traffic volume of 25,000 vehicles per day • Driveway density of 20 residential driveways per mile 0.62 • Traffic volume of 5,000 vehicles per day • Driveway density of 40 residential driveways per mile 0.66 • Traffic volume of 15,000 vehicles per day • Driveway density of 40 residential driveways per mile 0.68 • Traffic volume of 25,000 vehicles per day • Driveway density of 40 residential driveways per mile Note: These inferred CMFs SHOULD NOT BE USED to quantify the safety effect of installing non-traversable medians on undivided roads. Source: AASHTO 2010. Table 45. CMFs for installing a non-traversable median on a four-lane undivided urban or suburban arterial (inferred from Highway Safety Manual, 1st Edition).

Safety Effects of Access Management 47   Strategy Crash Severity CMF Applicability Install non- traversable median on 4- lane undivided road All 1.18 • Traffic volume of 5,000 vehicles per day • Driveway density of 20 residential driveways per mile 0.81 • Traffic volume of 15,000 vehicles per day • Driveway density of 20 residential driveways per mile 0.68 • Traffic volume of 25,000 vehicles per day • Driveway density of 20 residential driveways per mile 1.38 • Traffic volume of 5,000 vehicles per day • Driveway density of 40 residential driveways per mile 0.95 • Traffic volume of 15,000 vehicles per day • Driveway density of 40 residential driveways per mile 0.80 • Traffic volume of 25,000 vehicles per day • Driveway density of 40 residential driveways per mile Note: These inferred CMFs SHOULD NOT BE USED to quantify the safety effect of installing non-traversable medians on undivided roads. Source: NCHRP Project 17-62. Table 46. CMFs for installing a non-traversable median on a four-lane undivided urban or suburban arterial (inferred from NCHRP Project 17-62). Note: These inferred CMFs SHOULD NOT BE USED to quantify the safety effect of installing non-traversable medians on five-lane roads with a TWLTL. Source: NCHRP Project 17-62. Strategy Crash Severity CMF Applicability Install non- traversable median on 5- lane undivided road (i.e., replace TWLTL) All 1.31 • Traffic volume of 5,000 vehicles per day • Driveway density of 20 residential driveways per mile 1.26 • Traffic volume of 15,000 vehicles per day • Driveway density of 20 residential driveways per mile 1.24 • Traffic volume of 25,000 vehicles per day • Driveway density of 20 residential driveways per mile 1.83 • Traffic volume of 5,000 vehicles per day • Driveway density of 40 residential driveways per mile 1.76 • Traffic volume of 15,000 vehicles per day • Driveway density of 40 residential driveways per mile 1.73 • Traffic volume of 25,000 vehicles per day • Driveway density of 40 residential driveways per mile Table 47. CMFs for installing a non-traversable median on a ve-lane, urban or suburban arterial with a TWLTL (inferred from NCHRP Project 17-62). presents CMFs inferred from NCHRP Project 17-62 for installing a non-traversable median on a ve-lane arterial (i.e., replacing a TWLTL with a non-traversable median). Based on the inferred CMFs from NCHRP Project 17-62, the installation of a non- traversable median on a road that has a TWLTL is expected to increase crashes, which contradicts conventional wisdom. Further, the results are exacerbated at higher driveway

48 Application of Crash Modication Factors for Access Management densities; this also contradicts conventional wisdom. is appears to be another case where cross-sectional comparison will not give reasonable and reliable CMFs because the SPF comparisons do not account for all differences in safety between the two site types even aer the SPFs are calibrated to the same spatial and temporal condi- tions. e CMFs from the CMF Clearinghouse are more reliable for estimating the safety impacts of this strategy. Median Treatment—Two-Way Left-Turn Lane e Highway Safety Manual (1st Edition) provides models to estimate the safety performance of arterials with and without TWLTLs as discussed in Chapter 4 of this guide; however, the results should not be used to infer CMFs. Table 48 presents CMFs inferred from the Highway Safety Manual (1st Edition) for installing a TWLTL on two-lane undivided arterials without taking away existing lanes. Similarly, Table 49 presents CMFs inferred from NCHRP Project 17-62, “Improved Prediction Models for Crash Types and Crash Severities.” Based on the inferred CMFs from the Highway Safety Manual (1st Edition), the addition of a TWLTL to a two-lane undivided arterial appears to be more eective as trac volume increases but the safety impacts are insen- sitive to driveway density. e insensitivity suggests that these inferred CMFs should not be used as previous research has shown the eects to vary with driveway density. While the inferred CMFs from NCHRP Project 17-62 are mildly sensitive to driveway density, the trend with respect to trac volume is the opposite (i.e., CMFs increase as trac volume increases). e inconsistency further suggests that the inferred CMFs should not be used to quantify the safety eect of TWLTLs. is appears to be a case where cross-sectional comparisons will not give reasonable and reliable CMFs because the SPF comparisons do not account for all dierences in safety between the two site types even aer the SPFs are calibrated to the same spatial and temporal conditions. Strategy Crash Severity CMF Applicability Install TWLTL All 1.04 • Traffic volume of 5,000 vehicles per day • Driveway density of 20 residential driveways per mile 1.01 • Traffic volume of 15,000 vehicles per day • Driveway density of 20 residential driveways per mile 0.99 • Traffic volume of 25,000 vehicles per day • Driveway density of 20 residential driveways per mile 1.04 • Traffic volume of 5,000 vehicles per day • Driveway density of 40 residential driveways per mile 1.01 • Traffic volume of 15,000 vehicles per day • Driveway density of 40 residential driveways per mile 0.99 • Traffic volume of 25,000 vehicles per day • Driveway density of 40 residential driveways per mile Note: These inferred CMFs SHOULD NOT BE USED to quantify the safety effect of installing a TWLTL on undivided roads. Source: AASHTO 2010. Table 48. CMFs for installing a TWLTL on a two-lane undivided urban or suburban arterial [inferred from Highway Safety Manual (1st Edition)].

Safety Effects of Access Management 49   e CMFs from the CMF Clearinghouse are more reliable for estimating the safety impacts of this strategy. Intersection Treatments ere were no counterintuitive ndings associated with intersection treatments, including le-turn lanes, right-turn lanes, and alternative intersection designs. Property Access ere were no counterintuitive ndings associated with property access, including driveway design, sight distance at unsignalized access, frontage/backage roads, shared driveways, and internal cross connectivity. Note: These inferred CMFs SHOULD NOT BE USED to quantify the safety effect of installing a TWLTL on undivided roads. Source: NCHRP Project 17-62. Strategy Crash Severity CMF Applicability Install TWLTL All 0.63 • Traffic volume of 5,000 vehicles per day • Driveway density of 20 residential driveways per mile 1.18 • Traffic volume of 15,000 vehicles per day • Driveway density of 20 residential driveways per mile 1.58 • Traffic volume of 25,000 vehicles per day • Driveway density of 20 residential driveways per mile 0.58 • Traffic volume of 5,000 vehicles per day • Driveway density of 40 residential driveways per mile 1.09 • Traffic volume of 15,000 vehicles per day • Driveway density of 40 residential driveways per mile 1.47 • Traffic volume of 25,000 vehicles per day • Driveway density of 40 residential driveways per mile Table 49. CMFs for installing a TWLTL on a two-lane undivided urban or suburban arterial (inferred from NCHRP Project 17-62).