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10 Literature Review Introduction The objectives of Task 2 were to (1) review the literature related to the safety effects of access management, (2) identify the quality of existing CMFs for access management techniques, and (3) identify access management techniques that are typically applied concurrently and for which cumulative and/or interactive effects must be known. The project team conducted a thorough and critical review of existing published literature from U.S. and international sources, focusing on definitive accessÂmanagementÂrelated strategies and quantitative, crashÂbased research. This chapter presents the results of the literature review, structured by strategy. Table 2 provides a list of strategies, substrategies, and general principles related to access management. Following Table 2 is a description of each strategy with a summary of related CMFs and SPFs, focusing on the availability and quality of quantitative safety information. Alternative Intersection and Interchange Design Alternative intersection designs are viewed as access management strategies that may be used to reduce and separate conflict points. While there are a number of atÂgrade intersection designs that are noticeably different from each other, there is a common aspect among them: the treatment of leftÂturn movements. As indicated in the FHWAâs Alternative Intersections/ Interchanges: Informational Report (AIIR) (Hughes et al. 2010), alternative designs attempt to remove one or more of the conventional leftÂturn movements from the major intersection. There are safety and operational benefits of these designs, including fewer conflict points and improved signal phasing. An example is a Restricted Crossing UÂturn Intersection (RCUT), which is also referred to as a superstreet or as a JÂturn. As indicated in NCHRP Synthesis 404 (Gluck and Lorenz 2010), superstreets present an alternative leftÂturn treatment. Instead of allowing leftÂturn and through movements from side streets to be made directly through a twoÂway median opening, a superstreet redirects these movements downstream on the major street to a oneÂway median opening. For example, converting a conventional unsignalized inter section to an unsignalized superstreet is shown to reduce total crashes by 46 percent (Hummer et al. 2010). Installing JÂturns is shown to reduce total crashes by 35 percent (Edara et al. 2013). There are additional alternative intersection strategies that include roundabouts and jug handles. The common objective of these strategies is to reduce leftÂturn conflicts. Roundabouts, for example, involve rightÂturn movements to enter and exit the intersection. As indicated in the AIIR, although similar in concept to rotaries and traffic circles, roundabouts differ in geometry and operation (Hughes et al. 2010). The slower speeds and other differences related to round abouts result in generally safer operations than rotaries and traffic circles. Converting a minor road stopÂcontrolled intersection to a roundabout is shown to reduce total crashes by 70 percent C H A P T E R  2
Literature Review 11  (continued on next page) Table 2. Access management strategies, substrategies, and principles. Strategy Substrategy Applicable Access Management Principles Alternative intersection and interchange design Convert from 4-legged intersection to two 3- legged intersections ⢠Limit the number of conflict points ⢠Separate conflict areas ⢠Manage left-turn movements Install roundabout at roadway intersection Provide median acceleration lane Install continuous green T at signalized 3-legged intersection Replace direct left turn with grade-separated interchange Superstreet [restricted crossing U-turn (RCUT), J-Turn] Convert to jug handle intersections Control driveway design elements Change class/type of driveway ⢠Separate conflict areas ⢠Manage left-turn movements Change movement restriction (e.g., right-in-right- out) Require design of driveways with the appropriate return radii, throat width, channelization, number of lanes, and throat length for the type of traffic to be served Convert two-way streets to one-way operation Convert two-way operation to one-way operation ⢠Limit the number of conflict points ⢠Manage left-turn movements Establish corner clearance criteria Driveways at signalized intersections ⢠Preserve the functional area of intersections ⢠Separate conflict areas Improve cross connectivity Allow vehicles to access adjacent properties without returning to the mainline ⢠Limit the number of conflict points ⢠Remove turning vehicles from through-traffic lanes Install non-traversable medians and accommodate left turns and U-turns Convert traversable median [non-two-way left- turn lane (non-TWLTL)] to non-traversable median ⢠Limit the number of conflict points ⢠Separate conflict areas ⢠Manage left-turn movements Install isolated median barriers Install non-traversable median on undivided highway Replace TWLTL with non-traversable median Install TWLTL on undivided highway Non-road diet scenarios ⢠Remove turning vehicles from through-traffic lanes Road diet scenarios Install isolated median barriers Install non-traversable median on undivided highway Replace TWLTL with non-traversable median Install service or frontage roads Change proportion of primary roadway with frontage road ⢠Limit the number of conflict points ⢠Remove turning vehicles from through-traffic lanes Install frontage road to provide access to individual parcels Install traversable medians Convert undivided to divided by traversable median ⢠Separate conflict areas ⢠Manage left-turn movements
12 Application of Crash Modification Factors for Access Management or more (Persaud et al. 2001; Rodegerdts et al. 2007). Converting a signalized intersection to a roundabout is shown to reduce total crashes by up to 67 percent (Rodegerdts et al. 2007; Srinivasan et al. 2011; Uddin et al. 2012). A jug handle intersection is defined by the New Jersey Department of Transportation Roadway Design Manual (New Jersey Department of Trans portation 2015) as an atÂgrade ramp provided at or between intersections to permit motorists to make indirect left turns and/or UÂturns. There are several variants of the jug handle, including the forward jug handle, reverse jug handle, and the UÂturn ramp jug handle. All of these have the same objective of reducing leftÂturn movements and the associated conflicts. Table 2. (Continued). Strategy Substrategy Applicable Access Management Principles Left-turn treatment Change storage capacity of existing left-turn deceleration lane ⢠Remove turning vehicles from through-traffic lanes Channelize left-turn lane Control/improve design elements of left-turn lanes Install left-turn deceleration lanes at roadway intersections Provide turning bypass lanes Prohibit left turn Manage location and spacing of unsignalized access Establish density for unsignalized access (e.g., maximum driveway density) ⢠Limit the number of conflict points ⢠Separate conflict areasEstablish spacing for unsignalized access (e.g., minimum driveway spacing) Manage spacing of traffic signals Establish traffic signal density criteria ⢠Locate signals to favor through movements ⢠Limit the number of conflict points ⢠Separate conflict areas Establish traffic signal spacing criteria Manage the location, spacing, and design of median openings and crossovers Create directional median opening ⢠Limit the number of conflict points ⢠Separate conflict areas Install U-turns as an alternative to direct left turns Regulate median opening density Regulate median opening spacing Replace full median opening with median designed for left turns from the major roadway Manage the spacing of signalized and unsignalized access on crossroads in the vicinity of freeway interchanges Establish spacing criteria for interchange ramp terminals ⢠Limit the number of conflict points ⢠Separate conflict areas Provide adequate sight distance at access points Manage design elements to improve sight distance ⢠Preserve the functional area of intersections Manage the location and placement of parking (e.g., replace curb parking with off-street parking or restrict on-street parking near driveways or intersections to improve sight distance) Manage vegetation to improve sight distance (e.g., in landscaped medians or sight triangles) Right-turn treatment Channelize right-turn lane ⢠Remove turning vehicles from through-traffic lanes Control/improve design elements of right-turn lanes Install right-turn deceleration lane at roadway intersections
Literature Review 13  CMFs for Alternative Intersection and Interchange Design Table 3 provides a summary of CMFs related to the location and spacing of unsignalized access points at the intersection level. Based on a review of the CMF Clearinghouse (FHWA n.d.) in June 2017, there were 237 related CMFs, 126 of which were rated three stars or higher. All CMFs available in this category are relevant at the intersection level, with no CMFs for the site or corridor level. SPFs for Alternative Intersection and Interchange Design Table 4 provides a summary of SPFs related to alternative intersection designs. These SPFs are relevant at the intersection level, with no SPFs for the site or corridor level. Control Driveway Design Elements Driveways are integral to the roadway transportation system. Every driveway connection to a roadway creates an intersection, which in turn creates conflicts for the motorist with bicyclists, pedestrians, and other motor vehicles. Proper driveway design balances the needs of all users by minimizing conflicts while accommodating demands for mobility and access. As indicated in FHWAâs Technical Summary: Access Management in the Vicinity of Inter- sections (FHWA 2010), driveway connections to public roads must be adequately designed to ensure safe and efficient movement of vehicles to and from the roadway while balancing safety with mobility interests. There are many elements to consider in proper driveway design, includ ing 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 following through vehicles. In addition, the driveway should be of sufficient length to allow motorists to completely pull off the road without interference from onÂsite parked vehicles, vehicle queues, or pedestrian or vehicle circulation 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 Substrategy Number of CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Convert from 4-legged intersection to two 3-legged intersections 19 7 4 1 4 2 0 1 Install roundabout at roadway intersection 127 0 5 14 43 59 4 2 Provide median acceleration lane 22 17 5 0 0 0 0 0 Install continuous green T at signalized 3-legged intersection 3 0 0 0 3 0 0 0 Replace direct left turn with grade-separated interchange 3 0 0 0 3 0 0 0 Superstreet (RCUT, J-Turn) 63 0 27 28 7 1 0 0 Jug handle intersections 0 0 0 0 0 0 0 0 Total 234 24 41 43 57 62 4 3 Table 3. Summary of intersection-level CMFs for alternative intersection and interchange design.
14 Application of Crash Modification Factors for Access Management property, traffic volume, pedestrian and bicycle volume, and the type of traffic control. CMFs to estimate the safety effects of changing these driveway characteristics are limited. For motorists leaving a property, the vertical alignment of the driveway should be as close to level and perpendicular as possible where the driveway intersects with the roadway. The driveway should be level for a sufficient distance to allow the motorist to easily stop, with an unobstructed view upstream and downstream prior to entering the major roadway. An FHWA study indicates a potential reduction in crashes at urban, threeÂlegged, stop controlled intersections with physical turning movement restrictions (compared to similar intersections with no turning restrictions). The CMFs for total, intersectionÂrelated, and fatal and injury crashes are 0.55, 0.32, and 0.20 for urban, threeÂlegged, stopÂcontrolled intersections with physical turning movement restrictions compared to similar intersections with no turning restrictions (Le et al. 2018a). While there is a potential to reduce crashes at stopÂcontrolled intersections by restricting movements, there is a need to consider the potential for crash migra tion in determining the net benefits. The same report also indicates a potential increase in total, intersectionÂrelated, and fatal and injury crashes at signalized and stopÂcontrolled intersections that are downstream from urban, threeÂlegged, stopÂcontrolled intersections with physical turning movement restrictions (compared to similar intersections with no turning restrictions). Table 4. Summary of SPFs related to alternative intersection designs. Substrategy Intersection-Level SPFs Convert from 4-legged intersection to two 3- legged intersections SPFs are available for intersections by number of legs and traffic control from many sources, including the Highway Safety Manual (1st Edition). It would be challenging to make credible inferences from the SPFs since factors such as the distance between legs would need to be considered and, in general, many factors related to differences in location of 3-legged and 4-legged intersections tend not to be considered in the SPFs. (AASHTO 2010) Install roundabout at roadway intersection SPFs for roundabouts are available in NCHRP Report 572 and are updated in NCHRP Research Report 888. Comparing these SPF predictions to those available for controlled intersections is unlikely to yield credible CMFs since the two sets of SPFs pertain in general to different jurisdictions and to locations that may have safety-related factors that are different for the different site types. (Rodegerdts et al. 2007, Ferguson et al. 2018) The developed SPF was based on a relatively large sample size and included signalized intersections that were not converted to roundabouts and several newly constructed roundabouts in the same jurisdictions that otherwise would have been constructed as signalized intersections. Interaction terms, which were significant for total crashes, allowed investigation of the relationship between traffic volume and the effect of installing roundabouts at signalized intersections. The implied CMF for total crashes indicates that the CMF exceeds 1.0 for annual average daily traffic (AADT) greater than approximately 14,400 vehicles per day. (Gross et al. 2013) Replace direct left turn with grade- separated interchange SPFs for interchanges are available from the final report for NCHRP Project 17-45. These would be applicable not for directly estimating a CMF but for estimating expected changes in safety for a contemplated change from direct left-turn design if the safety of the latter can be reliably estimated from crash history. Install continuous green T at signalized 3- legged intersection A propensity scores-potential outcomes framework was used to compare the safety performance of the continuous green T with conventional signalized T-intersections. The results showed that expected total, fatal and injury, and target (rear-end, angle, and sideswipe) crash frequencies were lower at the continuous green T intersection relative to the conventional signalized T intersection. CMFs of 0.958, 0.846, and 0.920 were estimated for total, fatal and injury, and target (rear-end, angle, and sideswipe) crashes, respectively. (Donnell et al. 2016) Median acceleration lanes, jug handle, or superstreet designs The literature review found no SPFs specifically related to the provision of median acceleration lanes, superstreet, or jug handle designs.
Literature Review 15  The report concludes that this strategy can be costÂeffective in reducing crashes, but results may vary by location and there is a need to consider the specific costs and estimated benefits on a caseÂbyÂcase basis. CMFs for Controlling Driveway Design Elements Tables 5 and 6 provide summaries of CMFs related to controlling driveway design elements. Based on a review of the CMF Clearinghouse and related literature, there were 11 related CMFs, all of which were rated three stars or higher (or not yet rated due to recent publication). Among these, six CMFs are relevant at the intersection level, and five CMFs are relevant at the site level. There were no CMFs for the corridor level. SPFs for Controlling Driveway Design Elements Table 7 provides a summary of SPFs related to driveway design elements. These SPFs are relevant at the site level, with no CMFs for the intersection or corridor level. Convert Two-Way Streets to One-Way Operation As indicated in FHWAâs Technical Summary: Access Management in the Vicinity of Inter- sections (FHWA 2010), oneÂway couplets are often found in urban areas and provide access management benefits. OneÂway streets limit the number of conflicting movements at each intersection. They also limit driveways to rightÂinÂrightÂoutÂonly or leftÂinÂleftÂoutÂonly turning maneuvers. Thus, oneÂway streets reduce the number and types of conflict points that occur at each driveway. OneÂway 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 Table 5. Summary of intersection-level CMFs for controlling driveway design elements. Substrategy Numberof CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Change class/type of driveway 3 0 0 0 3 0 0 0 Change movement restriction (e.g., right-in-right-out) 3 0 0 0 0 0 0 3 Require design of driveways with the appropriate return radii, throat width, channelization, number of lanes and throat length for the type of traffic to be served 0 0 0 0 0 0 0 0 Total 6 0 0 0 3 0 0 3 Table 6. Summary of site-level CMFs for controlling driveway design elements. Substrategy Number of CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Change class/type of driveway 2 0 0 0 2 0 0 0 Change movement restriction (e.g., right-in-right-out) 1 0 0 0 1 0 0 0 Require design of driveways with appropriate return radii, throat width, channelization, number of lanes and throat length for the type of traffic to be served 2 0 0 0 2 0 0 0 Total 5 0 0 0 5 0 0 0
16 Application of Crash Modification Factors for Access Management opportunities to use available roadway width to provide auxiliary lanes for rightÂturn and/or leftÂturn movements, reducing conflicts between through and turning vehicles. OneÂway oper ation has additional benefits related to traffic signal progression. Converting twoÂway frontage roads to oneÂway operation is shown to reduce fatal and injury crashes by up to 68 percent (Eisele et al. 2011). CMFs for Converting Two-Way Streets to One-Way Operation Table 8 provides a summary of CMFs related to twoÂway to oneÂway conversion. Based on a review of the CMF Clearinghouse in June 2017, there were 12 related CMFs, eight of which were rated three stars or higher. All CMFs available in this category are relevant at the site level, with no CMFs for the intersection or corridor level. SPFs for Converting Two-Way Streets to One-Way Operation Table 9 provides a summary of SPFs related to oneÂway roads. There are many sources of SPFs for twoÂway arterial roads, including the Highway Safety Manual (1st Edition) (AASHTO 2010). SPFs available in this category are relevant at the intersection and site level, with no SPFs for the corridor level. Substrategy Site -Level SPFs Require design of driveways with the appropriate return radii, throat width, channelization, number of lanes , and throat length for the type of traffic to be served SPF exists for access-related crashes including mainline annual average daily traffic (AADT), driveway width, and 95th percentile queue at downstream intersection in peak hour. SPF indicates that wider driveways are associated with more access-related crashes. SPF does not account for driveway volumes or the median design, which are likely related to the driveway width and impact crash risk. (Jafari and Hummer 2013) Changing the movement restriction of driveways or specific driveway design parameters (type of driveway, radii, throat width, channelization, number of lanes, throat length) The literature review found no further SPFs specifically related to changing the movement restriction of driveways or specific driveway design parameters (type of driveway, radii, throat width, channelization, number of lanes, or throat length). Table 7. Summary of SPFs related to driveway design elements. Table 8. Summary of site-level CMFs for two-way to one-way conversion. Substrategy Numberof CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Convert two-way streets to one-way operation 12 0 0 4 8 0 0 0 Total 12 0 0 4 8 0 0 0 Table 9. Summary of SPFs related to two-way to one-way conversion. Substrategy Site-Level and Intersection-Level SPFs Convert two-way streets to one- way operation SPFs for one-way arterials will be available when the final report for NCHRP Project 17-58 is released. Comparing these SPF predictions to those available for two-way operation is unlikely to yield credible CMFs since the two sets of SPFs pertain in general to different jurisdictions and to locations that may have safety- related factors that are different for the different site types.
Literature Review 17  Establish Corner Clearance Criteria Protecting the functional integrity of intersections is extremely important from safety and operations perspectives. One strategy to help accomplish this is to locate driveways outside of the functional area of an intersection. Figure 2 illustrates the physical area of the intersection. Figure 3 illustrates the functional area of the intersection. As shown in Figure 3, the inter- section functional area extends beyond the physical intersection limits to include the upstream approaches where deceleration, maneuvering, and queuing take place, as well as the down- stream departure area beyond the intersection where driveways could introduce con icts and generate queues backing up through the intersection. As noted in AASHTOâs A Policy on Geometric Design of Highways and Streets (AASHTO 2011), driveways should not be located within the functional area of an intersection or in the in uence area of an adjacent driveway. An FHWA study indicates a potential increase in total crashes for downstream corners with driveways within 50 compared to downstream corners with no driveways within 50 . e CMFs for total crashes are 1.33 and 1.76 for corner clearance of 50 or less on one and two downstream corners, respectively, compared to no driveways within 50 of both downstream corners. e report also indicates a potential reduction in total crashes for upstream corners with driveways within 50 compared to upstream corners with no driveways within 50 (Le et al. 2018b). e CMFs for total crashes are 0.82 and 0.67 for corner clearance of 50 or less on one and two upstream corners, respectively, compared to no driveways within 50 of both upstream corners. e CMFs for limited corner clearance on the downstream corners were consistent with expectation, indicating statistically signicant increases in total, fatal and injury, rear-end, sideswipe, right-angle, and nighttime crashes. For limited corner clearance on the upstream corners, the CMFs were counterintuitive, indicating statistically signicant decreases in total, fatal and injury, and rear-end crashes. CMFs for Establishing Corner Clearance Criteria Table 10 provides a summary of CMFs related to establishing corner clearance criteria. Based on a review of the CMF Clearinghouse in June 2017, there were 19 related CMFs, 15 of which were rated three stars or higher. All CMFs available in this category are relevant at the inter- section level, with no CMFs for the site or corridor level. Source: Adapted from Transportation Research Circular 456: Driveway and Street Intersection Spacing, Figure 4, p.16. Figure 2. Intersection physical area.
18 Application of Crash Modi cation Factors for Access Management SPFs for Establishing Corner Clearance Criteria e literature review found no SPFs speci cally related to establishing corner clearance criteria. Improve Cross Connectivity Access management promotes the implementation of shared-access driveways and cross-access easements between (compatible) adjacent properties, where possible, which allow pedestrians and vehicles to circulate between properties without reentering the abutting roadway (see Fig- ure 4). e sharing of access driveways improves roadway safety and operations by reducing the number of con ict points and separating con ict points along these roadways. e longer spacing between access driveways also facilitates the provision of le-turn and right-turn lanes, eliminating con icts between through and turning movements. In addition, smoother trac ow on the abutting street helps to reduce the propensity for vehicular crashes and to increase egress capacity. CMFs for Improving Cross Connectivity e literature review found no CMFs speci cally related to cross-connectivity improvements. Source: Adapted from Transportation Research Circular 456: Driveway and Street Intersection Spacing, Figure 4, p.16. Figure 3. Intersection functional area. Substrategy Number of CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Driveways at signalized intersections 19 0 4 0 15 0 0 0 Total 19 0 4 0 15 0 0 0 Table 10. Summary of intersection-level CMFs for establishing corner clearance criteria.
Literature Review 19  SPFs for Improving Cross Connectivity The literature review found no SPFs specifically related to crossÂconnectivity improvements. Install Non-Traversable Medians and Accommodate Left Turns and U-Turns Allowing unrestricted leftÂturn movements to and from all access driveways increases the number of vehicular conflict points with other vehicles, pedestrians, and bicyclists. LeftÂturning vehicles have been shown to account for nearly threeÂquarters (74 percent) of all accessÂrelated crashes. Installations of nonÂtraversable (i.e., raised) medians with provisions for median openings to accommodate left turns and UÂturns have proven to be among the most effective techniques for reducing conflicts and improving traffic operations along roadways. Research shows this strategy can reduce total crashes by 14 to 71 percent (Schultz et al. 2008; Yanmaz Tuzel and Ozbay 2010; Alluri et al. 2012; AbdelÂAty et al. 2014). The installation of a nonÂtraversable median reduces the number of conflicts along a high way corridor by restricting driveways (not located at median openings) to rightÂinÂrightÂout movements and directing leftÂturn and UÂturn movements to designated median openings, as shown in Figure 5. NonÂtraversable medians with designated median openings to allow for leftÂturn and UÂturn movements offer the following advantages over the other types of roadway cross sections: ⢠Vehicles traveling in opposite directions are physically separated, eliminating the propensity for headÂon crashes. Converting a twoÂway leftÂturn lane (TWLTL) to a raised median is shown to reduce headÂon crashes by up to 73 percent (AbdelÂAty et al. 2014). ⢠When properly designed, the physical space provided for the deceleration and storage of leftÂturning and UÂturning vehicles occurs outside of the throughÂtraffic lanes. The resulting reduction in speed differential between the turning and through vehicles improves traffic operations and reduces the potential for crashes. Source: FHWA. Figure 4. Improved access configuration with cross connectivity.
20 Application of Crash Modi cation Factors for Access Management ⢠e number of le -turn con icts with vehicles, pedestrians, and bicyclists is reduced. Install- ing a raised median is shown to reduce pedestrian and bicyclist crashes by 29 and 3 percent, respectively (Miranda-Moreno et al. 2011; Alluri et al. 2012). ⢠e non-traversable median provides a refuge area for pedestrians crossing the roadway at intersections. In addition, midblock pedestrian crossings can be provided and signalized without interfering with trac progression (i.e., by stopping trac approaching from the le rst and then stopping trac from the right). ⢠Locations for making le turns and U-turns are identiable to the driver, thus reducing driver workload. ⢠Non-traversable medians reduce the frequency and severity of crashes as compared to both undivided roadways and roadways with a TWLTL. Converting a TWLTL to a raised median is shown to reduce total and fatal and injury crashes by up to 47 and 42 percent, respectively (Alluri et al. 2012; Abdel-Aty et al. 2014). CMFs for Installing Non-Traversable Medians and Accommodating Left Turns and U-Turns Tables 11 to 13 provide summaries of CMFs related to the installation of non-traversable medians at the intersection, site, and corridor levels, respectively. Based on a review of the CMF Clearinghouse in June 2017, there were 410 related CMFs, 174 of which were rated three stars or higher. SPFs for Installing Non-Traversable Medians and Accommodating Left Turns and U-Turns Table 14 provides a summary of site- and corridor-level SPFs related to non-traversable medians with le - and U-turn provisions. ere were no SPFs for the intersection level. Source: Adapted from Gluck et al. 1999, Figure 30, p. 72. Figure 5. Allowable traf c movements before and after raised median installation.
Literature Review 21  Install a Two-Way Left-Turn Lane on an Undivided Highway Like installations of nonÂtraversable medians on formerly undivided highways, installations of TWLTLs offer safety and operational benefits. Compared to undivided highways, TWLTLs allow the deceleration and storage of leftÂturning vehicles outside of the throughÂtraffic 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 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 headÂon crashes; and ⢠The TWLTL provides a refuge area for passenger cars making a twoÂstage 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). Substrategy Number of CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Convert traversable median (non-TWLTL) to non-traversable median 2 0 0 0 2 0 0 0 Install isolated median barriers 0 0 0 0 0 0 0 0 Install non-traversable median on undivided highway 3 0 0 0 1 0 0 2 Replace TWLTL with non-traversable median 0 0 0 0 0 0 0 0 Total 5 0 0 0 3 0 0 2 Table 11. Summary of intersection-level CMFs for installing non-traversable medians and accommodating left turns and U-turns. Substrategy Numberof CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Convert traversable median (non-TWLTL) to non-traversable median 8 0 0 6 0 2 0 0 Install isolated median barriers 38 0 0 3 3 4 1 27 Install non-traversable median on undivided highway 33 0 4 1 13 1 4 10 Replace TWLTL with non-traversable median 129 0 24 47 58 0 0 0 Total 208 0 28 57 74 7 5 37 Table 12. Summary of site-level CMFs for installing non-traversable medians and accommodating left turns and U-turns. Substrategy Number of CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Convert traversable median (non-TWLTL) to non-traversable median 8 0 0 6 0 2 0 0 Install isolated median barriers 29 0 0 3 3 4 1 18 Install non-traversable median on undivided highway 31 0 4 1 12 1 4 9 Replace TWLTL with non-traversable median 129 0 24 47 58 0 0 0 Total 197 0 28 57 73 7 5 27 Table 13. Summary of corridor-level CMFs for installing non-traversable medians and accommodating left turns and U-turns.
22 Application of Crash Modification Factors for Access Management Table 14. Summary of SPFs related to installing non-traversable medians and accommodating left turns and U-turns. Substrategy Site-Level SPFs Corridor-Level SPFs Install non-traversable median on undivided highway Not applicable Corridor-level SPFs are available for total, property-damage-only, and fatal plus injuries crashes and address the presence of TWLTL or non-traversable median. The information is dated, so its relevance may be in question (Brown et al. 1998). NCHRP Report 395 has SPFs for corridor crashes excluding signalized intersections for TWLTL, undivided, and raised curb medians. The information is dated, so its relevance may be in question (Bonneson and McCoy 1997). SPFs are available from which CMFs may be inferred for median opening density (right-angle crashes increase for mixed-use corridors with increase in density), proportion of segment with TWLTL (rear-end crashes decrease with increase in TWLTL proportion for residential corridors), and proportion with divided median (decrease in right- angle crashes for mixed-use corridors with increasing proportion) (Gross et al. 2015). Replace TWLTL with non-traversable median Information is available on distance between driveway exit and downstream U-turn opening (Liu et al. 2008). Corridor-level SPFs are available for total, property-damage-only, and fatal plus injuries crashes and address the presence of TWLTL or non-traversable median. The information is dated, so its relevance may be in question (Brown et al. 1998). NCHRP Report 395 has SPFs for corridor crashes excluding signalized intersections for TWLTL, undivided, and raised curb medians. The information is dated, so its relevance may be in question (Bonneson and McCoy 1997). SPFs are available from which CMFs may be inferred for median opening density (right-angle crashes increase for mixed-use corridors with increase in density), proportion of segment with TWLTL (rear-end crashes decrease with increase in TWLTL proportion for residential corridors), and proportion with divided median (decrease in right- angle crashes for mixed-use corridors with increasing proportion) (Gross et al. 2015). Persaud and co-authors inferred CMFs from the Part C Predictive Method for providing a median, depending on annual average daily traffic (AADT) and driveway spacing and type. The inferred CMFs show that CMFs are smaller for lower AADT, for higher driveway densities, and for major commercial (compared to major residential) (AASHTO 2010; Persaud et al. 2011). Provide isolated median barriers or converting non- TWLTL traversable medians to non- traversable medians The literature review found no SPFs specifically related to isolated median barriers or converting non-TWLTL traversable medians to non- traversable medians. The literature review found no SPFs specifically related to isolated median barriers or converting non-TWLTL traversable medians to non- traversable medians.
Literature Review 23  As reported in the second edition of the Access Management Manual (Williams et al. 2014), crash models developed for and discussed in NCHRP Report 395: Capacity and Operational Effects of Midblock Left-Turn Lanes (Bonneson and McCoy 1997), indicated that roadways 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) similar to that of an undivided roadway. It also indicated that TWLTLs do not provide the same safety benefits as nonÂtraversable medians, which help to physically separate opposing traffic (Williams et al. 2014). NCHRP Report 420, after presenting the results of safety analyses comparing nonÂtraversable medians and TWLTLs, indicates that literature compiled since the 1980s reflects the safety benefits of nonÂtraversable medians over TWLTLs (Gluck et al. 1999). Specifically, fourÂlane and sixÂlane divided roadways with nonÂtraversable medians (and protected leftÂturn lanes) have shown better safety perfor mance (lower average crash rates) than fiveÂlane and sevenÂlane roadways with a TWLTL. A few studies have shown benefits based on beforeÂandÂafter studies of the same roadway; however, most use a crossÂsectional comparison of crash rates for the two basic types of roads. While NCHRP Report 420 (p. 76) concluded that roadways with nonÂtraversable medians appear safer than similar roadways with TWLTLs, care should be exercised in selecting the appropriate design. Specifically, for roadways with nonÂtraversable 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. CMFs for Installing a Two-Way Left-Turn Lane on an Undivided Highway Tables 15 to 17 provide summaries of CMFs related to the installation of TWLTLs on undivided highways at the intersection, site, and corridor levels, respectively. Based on a review of the CMF Clearinghouse in June 2017, there were 203 related CMFs, 99 of which were rated three stars or higher. SPFs for Installing a Two-Way Left-Turn Lane on an Undivided Highway Table 18 provides a summary of SPFs related to TWLTLs. These SPFs are relevant at the corridor level, with no SPFs for the intersection or site levels. Substrategy Number of CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Non-road diet scenarios 3 0 0 0 3 0 0 0 Road diet scenarios 0 0 0 0 0 0 0 0 Total 3 0 0 0 3 0 0 0 Table 15. Summary of intersection-level CMFs for installing a TWLTL on an undivided highway. Substrategy Number of CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Non-road diet scenarios 67 1 2 7 9 14 5 29 Road diet scenarios 32 0 0 12 16 3 1 0 Total 99 1 2 19 25 17 6 29 Table 16. Summary of site-level CMFs for installing a TWLTL on an undivided highway.
24 Application of Crash Modification Factors for Access Management Install Service or Frontage Roads A frontage road is an access roadway that is generally aligned parallel to a main roadway and is located between the rightÂofÂway of the main roadway and the front building setback line. Frontage roads are used as an access management technique to provide direct access to properties and separate through traffic from local accessÂrelated traffic. This reduces the frequency and severity of conflicts along the main roadway as well as traffic delays. Installing frontage roads is shown to reduce total crashes by up to 40 percent (Agent et al. 1996). In addition, the result ing increase in spacing between intersections along the main roadway facilitates the design of auxiliary lanes for deceleration and acceleration, further improving traffic safety and operations. A âbackageâ roadâalso called a âreverse frontage roadâ or âreverse accessââserves a similar purpose but is located behind the properties that front the main roadway. Frontage and backage roads may be configured for oneÂway operation or twoÂway operation. Figure 6 illustrates one potential frontage road configuration. CMFs for Installing Service or Frontage Roads Tables 19 and 20 provide summaries of CMFs related to installation of service or frontage roads for the site and corridor levels, respectively. Based on a review of the CMF Clearing house in June 2017, there were eight related CMFs, all of which were rated three stars or higher. There were no CMFs for the intersection level, as this strategy does not apply to individual intersections. SPFs for Installing Service or Frontage Roads The literature review found no SPFs specifically related to installation of service or front age roads. Substrategy Number of CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Non-road diet scenarios 67 1 2 7 9 14 5 29 Road diet scenarios 32 0 0 12 16 3 1 0 Total 99 1 2 19 25 17 6 29 Table 17. Summary of corridor-level CMFs for installing a TWLTL on an undivided highway. Substrategy Corridor-Level SPFs Non-road diet scenarios Corridor SPFs are available for total, property-damage-only, and fatal plus injury crashes and address presence of TWLTL or non-traversable median. The information is dated, so its relevance may be in question (Brown et al. 1998). NCHRP Report 395 has SPFs for corridor crashes excluding signalized intersections for TWLTL, undivided, and raised curb medians. The information is dated, so its relevance may be in question (Bonneson and McCoy 1997). SPFs are available with proportion of TWLTL (rear-end crashes decrease with increase in TWLTL proportion for residential corridors) (Gross et al. 2015). Provides SPFs for 5T (4-lane roadway segment with a TWLTL) and for 4U (4-lane undivided roadway segment) and 4D (4-lane divided roadway segment) from which CMFs may be inferred depending on annual average daily traffic (AADT) and driveway type and frequency. Indications are that 5T has substantially more crashes, all else being equal, but perhaps all else is not equal in terms of factors not accounted for in the SPFs, including driveway volumes (AASHTO 2010). Road diet scenario The literature review found no SPFs specifically related to providing TWLTL in a road diet scenario. Table 18. Summary of SPFs related to TWLTLs.
Literature Review 25  Install Traversable Medians Similar to installations of non-traversable medians and TWLTLs, installations of travers- able medians reduce the frequency and severity of crashes as compared to undivided roadways. Installing ush (traversable) medians on undivided roads is shown to reduce total crashes by up to 78 percent (Agent et al. 1996; Gan et al. 2005). While not physically divided, vehicles traveling in opposite directions are further separated, reducing the potential for head-on crashes. e added separation o en provides space for le -turn lanes at intersections, allowing the deceleration and storage of le -turning vehicles outside of the through-trac lanes. e resulting reduction in speed dierential between the turning and through vehicles improves trac operations and reduces the potential for crashes. Depending on the type and width of traversable median, it may serve as a refuge area for passenger cars making a two-stage le turn from a sidestreet or driveway (i.e., crossing trac approaching from the le , waiting in the traversable median, and then merging with trac approaching from the right). Source: FHWA. Figure 6. Potential frontage road con guration. Substrategy Numberof CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Change proportion of primary roadway with frontage road 2 0 0 0 2 0 0 0 Install frontage road to provide access to individual parcels 2 0 0 0 0 0 0 2 Total 4 0 0 0 2 0 0 2 Table 19. Summary of site-level CMFs for installation of service or frontage roads. Substrategy Numberof CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Change proportion of primary roadway with frontage road 2 0 0 0 2 0 0 0 Install frontage road to provide access to individual parcels 2 0 0 0 0 0 0 2 Total 4 0 0 0 2 0 0 2 Table 20. Summary of corridor-level CMFs for installation of service or frontage roads.
26 Application of Crash Modification Factors for Access Management CMFs for Installing Traversable Medians Tables 21 to 23 provide summaries of CMFs related to the installation of traversable medians at the intersection, site, and corridor levels, respectively. Based on a review of the CMF Clearinghouse in June 2017, there were 19 related CMFs, three of which were rated three stars or higher. SPFs for Installing Traversable Medians The literature review found no SPFs specifically related to the installation of traversable medians. Left-Turn Treatment LeftÂturn movements, especially those that are made from lanes shared with through traffic, cause conflicts and delays. LeftÂturn lanes provide a refuge for leftÂturning vehicles by removing those vehicles from the throughÂtraffic lane(s). As such, they are an effective means of reducing the conflicts 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 leftÂturn lanes has been shown to provide a variety of traffic safety and operational benefits, including the following: ⢠Reducing the number of conflicts and crashes (particularly rearÂend, angle, and sideswipe crashes), ⢠Physically separating leftÂturning traffic and queues from through traffic, ⢠Decreasing vehicular delay and increasing intersection capacity, Substrategy Numberof CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Convert undivided to divided by traversable median 0 0 0 0 0 0 0 0 Remove traversable median to convert divided to undivided 1 0 0 0 1 0 0 0 Total 1 0 0 0 1 0 0 0 Table 21. Summary of intersection-level CMFs for installing traversable medians. Substrategy Numberof CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Convert undivided to divided by traversable median 9 0 0 0 1 0 0 8 Remove traversable median to convert divided to undivided 0 0 0 0 0 0 0 0 Total 9 0 0 0 1 0 0 8 Table 22. Summary of site-level CMFs for installing traversable medians. Substrategy Number of CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Convert undivided to divided by traversable median 9 0 0 0 1 0 0 8 Remove traversable median to convert divided to undivided 0 0 0 0 0 0 0 0 Total 9 0 0 0 1 0 0 8 Table 23. Summary of corridor-level CMFs for installing traversable medians.
Literature Review 27  ⢠Providing an area for leftÂturning vehicles to decelerate outside of the through travel lane, and ⢠Providing greater operational flexibility (e.g., additional traffic signal phasing opportunities). Installing exclusive leftÂturn lanes is shown to reduce total, fatal and injury, rearÂend, and angle crashes by up to 48, 58, 59, and 68 percent, respectively (Harwood et al. 2003; ITE 2004; Srinivasan et al. 2014). In addition, NCHRP Report 745: Left-Turn Accommodations at Unsignalized Intersections (Fitzpatrick et al. 2013) indicated that leftÂturn 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 leftÂturn lane installation. CMFs for Left-Turn Treatment Tables 24 and 25 provide summaries of CMFs related to leftÂturn treatment at the inter section and site level, respectively. Based on a review of the CMF Clearinghouse in June 2017, there were 208 related CMFs, 81 of which were rated three stars or higher. There were no CMFs identified for the corridor level. SPFs for Left-Turn Treatment The literature review found no SPFs specifically related to intersections with any of the following leftÂturn treatments: ⢠Length of leftÂturn deceleration lanes, ⢠Channelized leftÂturn lanes, ⢠Design elements of leftÂturn lanes, ⢠Presence of leftÂturn deceleration lanes, ⢠Provision of turning bypass lanes, or ⢠Prohibition of left turns. Substrategy Number of CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Change storage capacity of existing left-turn deceleration lane 4 0 0 2 2 0 0 0 Channelize left-turn lane 51 0 0 1 6 1 0 43 Control/improve design elements of left-turn lanes 2 0 0 0 2 0 0 0 Install left-turn lanes at roadway intersections 119 0 3 3 16 37 11 49 Provide turning bypass lanes 12 0 0 1 2 0 0 9 Prohibit left turn 11 0 0 0 0 4 0 7 Total 199 0 3 7 28 42 11 108 Table 24. Summary of intersection-level CMFs for left-turn treatment. Substrategy Number of CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Change storage capacity of existing left-turn deceleration lane 1 0 0 1 0 0 0 0 Channelize left-turn lane 0 0 0 0 0 0 0 0 Control/improve design elements of left-turn lanes 2 0 0 0 2 0 0 0 Install left-turn lanes at roadway intersections 0 0 0 0 0 0 0 0 Provide turning bypass lanes 0 0 0 0 0 0 0 0 Prohibit left turn 0 0 0 0 0 0 0 0 Total 3 0 0 1 2 0 0 0 Table 25. Summary of site-level CMFs for left-turn treatment.
28 Application of Crash Modification Factors for Access Management Manage Location and Spacing of Unsignalized Access Access points, commonly referred to as driveways or intersections, introduce conflicts and friction into the flow of traffic along a roadway. Vehicles entering and leaving the roadway often slow the movement of through traffic, and the difference in speed between through traffic and turning traffic increases the potential for crashes. AASHTOâs A Policy on Geometric Design of Highways and Streets (i.e., the âGreen Bookâ) indicates that the number of crashes is disproportionately higher at driveways than at other intersections. Therefore, driveway design and location merit special consideration (AASHTO 2011). Where an access point is needed, its location should be selected to minimize its adverse effects on roadway safety and traffic flow. Increasing the spacing between access points, through proper planning of future access and closing or consolidating existing access, improves traffic flow and safety along the roadway by: ⢠Reducing the number of conflicts per mile; ⢠Providing a greater distance for motorists to anticipate and recover from turning maneuvers; and ⢠Providing opportunities for the construction of acceleration lanes, deceleration lanes, or exclusive leftÂturn or rightÂturn lanes. Reducing driveway density from 48 per mile to 26 to 48 per mile is shown to reduce injury crashes by up to 29 percent. Reducing driveway density from 26 to 48 per mile to 10 to 24 per mile is shown to reduce injury crashes by up to 31 percent. Reducing driveway density from 10 to 24 per mile to less than 10 per mile is shown to reduce injury crashes by up to 25 percent (Elvik and Vaa 2004). Figure 7 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. CMFs for Managing Location and Spacing of Unsignalized Access Tables 26 to 28 provide summaries of CMFs related to managing location and spacing of unsignalized access at the intersection, site, and corridor levels, respectively. Based on a review Source: FHWA. Figure 7. Illustration of unsignalized driveway spacing.
Literature Review 29  of the CMF Clearinghouse in June 2017, there were 83 related CMFs, 82 of which were rated three stars or higher. SPFs for Managing Location and Spacing of Unsignalized Access Table 29 provides a summary of corridorÂlevel SPFs related to the location and spacing of unsignalized access points. There were no SPFs identified for the intersection or site level. Manage Spacing of Traffic Signals Establishing traffic signal spacing criteria for arterial roadways is one of the most important and basic access management techniques. These criteria apply to both signalized driveways and signalized roadway intersections. The proper spacing of traffic signals in terms of frequency and uniformity is important because of the effects traffic signals have on arterial safety and traffic flow. Frequency, in this context, refers to the number of traffic signals for a given length of roadway and is sometimes referred to as âsignal density.â It is typically expressed as the number of signals per mile. Uniformity, in this context, refers to the variation in the distances between individual traffic signals along a given length of roadway. It is desirable to minimize this varia tion and to space traffic signals at uniform distances, as shown in Figure 8. Closely spaced or improperly spaced traffic signals can result in increased crashes, frequent stops, unnecessary delays for motorists and pedestrians, increased fuel consumption, and excessive vehicular emissions. For example, if a 2Âmile segment of roadway would require Substrategy Number of CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Establish density for unsignalized access (e.g., maximum driveway density) 1 0 0 1 0 0 0 0 Establish spacing for unsignalized access (e.g., minimum driveway spacing) 0 0 0 0 0 0 0 0 Total 1 0 0 1 0 0 0 0 Table 26. Summary of intersection-level CMFs for managing location and spacing of unsignalized access. Substrategy Number of CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Establish density for unsignalized access (e.g., maximum driveway density) 27 0 0 0 24 0 3 0 Establish spacing for unsignalized access (e.g., minimum driveway spacing) 13 0 0 0 13 0 0 0 Total 40 0 0 0 37 0 3 0 Table 27. Summary of site-level CMFs for managing location and spacing of unsignalized access. Substrategy Numberof CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Establish density for unsignalized access (e.g., maximum driveway density) 27 0 0 0 24 0 3 0 Establish spacing for unsignalized access (e.g., minimum driveway spacing) 13 0 0 0 13 0 0 0 Total 40 0 0 0 37 0 3 0 Table 28. Summary of corridor-level CMFs for managing location and spacing of unsignalized access.
30 Application of Crash Modification Factors for Access Management four traffic signals (i.e., a signal density of two signals per mile), it is generally more desir able 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 signalÂtiming plans to reduce traffic conflicts. For example, Figure 9 presents a CMF for estimating the change in total crashes by changing traffic signal density from X to Y signals per mile, where X is the number of signals per mile before and Y is the number of signals per mile after (Schultz et al. 2008). Substrategy Corridor-Level SPFs Establish density for unsignalized access (e.g., maximum driveway density) NCHRP Report 395 has SPFs for corridor crashes excluding signalized intersections for TWLTL, undivided, and raised curb medians. The information is dated, so its relevance may be in question (Bonneson and McCoy 1997). SPFs are available from which CMFs can be inferred for unsignalized intersection density (e.g., total and turning crashes increase with increased density for mixed-use corridors) and access density (crashes increase with increased density) (Gross et al. 2015). Provides corridor SPFs for total, property-damage-only, and fatal plus injury crashes and addresses access density and proportion of access points that are signalized. The information is dated, so its relevance may be in question (Brown et al. 1998). Persaud and colleagues inferred CMFs from the Part C Predictive Method. That methodology provides separate models for multivehicle driveway and non-driveway crashes per mile considering the annual average daily traffic (AADT), the number and type of driveways, and whether the arterial is divided or undivided. CMFs for reducing driveway density are lower for undivided arterials and lower AADT (AASHTO 2010; Persaud et al. 2011). Establish spacing for unsignalized access (e.g., minimum driveway spacing) The literature review found no SPFs specifically related to minimum spacing of unsignalized access points. Table 29. Summary of SPFs related to the location and spacing of unsignalized access points. Source: Adapted from Gluck et al. 1999, Figure 5, p. 23. Figure 8. Comparison of uniform and non-uniform signal spacing. Figure 9. Example CMF for total crashes for changing traffic signal density.
Literature Review 31  As another example, Figure 10 presents a CMF for estimating the change in fatal and serious injury pedestrian crashes by changing traffic signal density from X to Y signals per mile, where X is the number of signals per mile before and Y is the number of signals per mile after (Ukkusuri et al. 2011). Figure 10. Example CMF for fatal and serious injury crashes involving pedestrians for changing traffic signal density. CMFs for Managing Spacing of Traffic Signals Tables 30 and 31 provide summaries of CMFs related to managing signal spacing at the site and corridor levels, respectively. Based on a review of the CMF Clearinghouse in June 2017, there were 20 related CMFs, all of which were rated three stars or higher. This strategy does not apply to the intersection level. SPFs for Managing Spacing of Traffic Signals Table 32 provides a summary of corridorÂlevel SPFs related to the spacing of signalized inter sections. There were no SPFs identified for the intersection and site level. Substrategy Numberof CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Establish traffic signal density criteria 3 0 0 0 3 0 0 0 Establish traffic signal spacing criteria 7 0 0 0 7 0 0 0 Total 10 0 0 0 10 0 0 0 Table 30. Summary of site-level CMFs for managing spacing of traffic signals. Substrategy Numberof CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Establish traffic signal density criteria 3 0 0 0 3 0 0 0 Establish traffic signal spacing criteria 7 0 0 0 7 0 0 0 Total 10 0 0 0 10 0 0 0 Table 31. Summary of corridor-level CMFs for managing spacing of traffic signals. Substrategy Corridor-Level SPFs Establish traffic signal density criteria SPFs are available from which CMFs for various crash types and severities can be inferred for signalized intersection density (crashes increase with increased density) (Gross et al. 2015). Corridor-level SPFs are available for total, property-damage-only, and fatal plus injury crashes and address access density and proportion of access points that are signalized. The information is dated, so its relevance may be in question (Brown et al. 1998). Did not provide a CMF or model for signal spacing directly but found that coordinated intersections are more unsafe than the isolated ones and raised the possibility that the relatively short distance between coordinated intersections could lead to more traffic interactions among those intersections, and thus more crashes (Guo et al. 2010). Establish traffic signal spacing criteria The literature review found no SPFs specifically related to the spacing or minimum spacing of signalized access points. Table 32. Summary of SPFs related to the spacing of signalized intersections.
32 Application of Crash Modification Factors for Access Management Figure 11. Example CMF for total crashes for changing median opening density. Manage the Location, Spacing, and Design of Median Openings and Crossovers A median opening is an opening in a nonÂtraversable median that provides for crossing and turning traffic. A âfullâ median opening allows all turning movements, whereas a âpartialâ 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. Figure 3 illustrated the functional area of an intersection. As an example, Figure 11 presents a CMF for estimating the change in total crashes by changing median opening density from X to Y median openings per mile, where X is the number of median openings per mile before and Y is the number of median openings per mile after (Mauga and Kaseko 2010). Substrategy Numberof CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Create directional median opening 1 0 0 1 0 0 0 0 Install U-turns as an alternative to direct left turns 18 0 5 0 12 1 0 0 Regulate median opening density 0 0 0 0 0 0 0 0 Regulate median opening spacing 0 0 0 0 0 0 0 0 Replace full median opening with median designed for left turns from the major roadway 1 0 0 0 1 0 0 0 Total 20 0 5 1 13 1 0 0 Table 33. Summary of intersection-level CMFs for managing the location, spacing, and design of median openings and crossovers. Converting an open median to a directional median is shown to reduce total crashes and fatal and injury crashes by up to 7 and 23 percent, respectively (Zhou et al. 2013). Converting an open median to a leftÂinÂonly median is shown to reduce total crashes and fatal and injury crashes by up to 5 and 7 percent, respectively (Zhou et al. 2013). CMFs for Managing the Location, Spacing, and Design of Median Openings and Crossovers Tables 33 to 35 provide summaries of CMFs related to managing the location, spacing, and design of median openings and crossovers at the intersection, site, and corridor levels, respec tively. Based on a review of the CMF Clearinghouse in June 2017, there were 94 related CMFs, 72 of which were rated three stars or higher. SPFs for Managing the Location, Spacing, and Design of Median Openings and Crossovers Table 36 provides a summary of corridorÂlevel SPFs related to the location, spacing, and design of median openings and crossovers. There were no SPFs identified for the intersection or site level.
Literature Review 33  Substrategy Numberof CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Create directional median opening 1 0 0 1 0 0 0 0 Install U-turns as an alternative to direct left turns 18 0 5 0 12 1 0 0 Regulate median opening density 0 0 0 0 0 0 0 0 Regulate median opening spacing 2 0 0 0 2 0 0 0 Replace full median opening with median designed for left turns from the major roadway 15 0 0 2 9 4 0 0 Total 36 0 5 3 23 5 0 0 Table 34. Summary of site-level CMFs for managing the location, spacing, and design of median openings and crossovers. Substrategy Numberof CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Create directional median opening 1 0 0 1 0 0 0 0 Install U-turns as an alternative to direct left turns 18 0 5 0 12 1 0 0 Regulate median opening density 5 0 0 0 5 0 0 0 Regulate median opening spacing 0 0 0 0 0 0 0 0 Replace full median opening with median designed for left turns from the major roadway 14 0 0 2 8 4 0 0 Total 38 0 5 3 25 5 0 0 Table 35. Summary of corridor-level CMFs for managing the location, spacing, and design of median openings and crossovers. Substrategy Corridor-Level SPFs Regulate median opening density SPFs are available from which CMFs may be inferred for median opening density (right-angle crashes increase for mixed-use corridors with increase in density) (Gross et al. 2015). Directional median openings, U-turns as an alternative to direct left turns, or median opening spacing, or median openings designed for left turns from the major roadway The literature review found no SPFs specifically related to directional median openings, U-turns as an alternative to direct left turns, median opening spacing, or median openings designed for left turns from the major roadway. Table 36. Summary of SPFs related to the location, spacing, and design of median openings and crossovers. Manage the Spacing of Signalized and Unsignalized Access on Crossroads in the Vicinity of Freeway Interchanges Freeway interchanges provide the means of moving traffic between freeways and inter secting crossroads. Although direct property access is prohibited on the freeway itself, safety and operational problems can arise when driveways and intersections along the crossroad are spaced too close to the interchange ramp termini. Heavy weaving volumes, complex traffic signal operations, frequent crashes, and recurrent congestion could result. In addition, drive ways and median breaks that are provided for direct access to properties along the crossroad compound these problems. Managing access on crossroads in the vicinity of interchanges protects the longevity of both the interchange and the intersecting crossroad by reducing crashes, minimizing conges tion, and simplifying driving tasks. Improperly managing access on the crossroad near the interchange 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
34 Application of Crash Modification Factors for Access Management mainline safety and operations. For these reasons, access management should be applied to interchange crossroads such that access points, including both driveways and intersections, are sufficiently separated from freeway interchange ramp termini. While research in this area is limited, Figure 12 presents three CMFs for estimating the change in total, rearÂend, and angle crashes, respectively, by changing the spacing distance between two ramp terminals at a diamond interchange from X to Y ft, where X is the spacing distance before and Y is the spacing distance after (Wang et al. 2011). Figure 12. Example CMFs for ramp terminal spacing. CMFs for Managing the Spacing of Signalized and Unsignalized Access on Crossroads in the Vicinity of Freeway Interchanges Table 37 provides a summary of siteÂlevel CMFs related to managing the spacing of signal ized and unsignalized access on crossroads in the vicinity of freeway interchanges. Based on a review of the CMF Clearinghouse in June 2017, there were three related CMFs, all of which were rated three stars or higher. There were no CMFs identified for the intersection or corridor level. SPFs for Managing the Spacing of Signalized and Unsignalized Access on Crossroads in the Vicinity of Freeway Interchanges Table 38 provides a summary of intersectionÂlevel SPFs related to the spacing of signalized and unsignalized access on crossroads in the vicinity of freeway interchanges. There were no SPFs identified for the site or corridor level. Provide Adequate Sight Distance at Access Points The provision of adequate sight distance at all intersectionsâincluding drivewaysâalong roadways is a fundamental aspect of traffic operations and safety. Sufficient sight distance is needed to allow drivers to perceive the presence of potentially conflicting vehicles, whether Substrategy Numberof CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Establish spacing criteria for interchange ramp terminals 3 0 0 0 3 0 0 0 Total 3 0 0 0 3 0 0 0 Table 37. Summary of site-level CMFs for managing the spacing of signalized and unsignalized access on crossroads in the vicinity of freeway interchanges.
Literature Review 35  they are relying on a traffic control device to determine rightÂofÂway or, in the absence of such a device, relying on the rules of the road. This perception should occur in sufficient time for drivers to stop or adjust their speed, as appropriate, to avoid a crash. The driver of a vehicle approaching an intersection needs to have not only an unobstructed view of the entire intersection and any traffic control devices but also sufficient time and distance along the intersecting roadway to anticipate and avoid potential collisions. 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 perceptionÂreaction time and braking. Specified areas along the intersectionâs approach legs and across their corners should be clear of sight obstructions, such as parked vehicles and vegetation, which might block a driverâs view of potentially conflicting vehicles. CMFs for Providing Adequate Sight Distance at Access Points Table 39 provides a summary of intersectionÂlevel CMFs related to providing adequate sight distance at access points. Based on a review of the CMF Clearinghouse in June 2017, there were 15 related CMFs, four of which were rated three stars or higher. There were no CMFs identified for the site or corridor level. Substrategy Intersection-Level SPFs Establish spacing criteria for interchange ramp terminals The final report for NCHRP Project 17-45 has SPFs for ramp terminals that consider distance to nearest ramp or intersection. Results show logically that an increase in both fatal plus injury and property-damage-only crashes is associated with presence and frequency of driveways or unsignalized public street approaches within 250 ft and decreasing distance to the adjacent ramp terminal and intersection. Provides SPFs that relate crashes to access spacing. These were used to develop lookup tables that quantify the impact of access road spacing on the expected number of crashes per unit distance. The tables demonstrate a decrease in the crash rate as the access road spacing increases. The models were said to satisfy statistical requirements (Rakha et al. 2008). Table 38. Summary of SPFs related to the spacing of signalized and unsignalized access on crossroads in the vicinity of freeway interchanges. Substrategy Numberof CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Manage design elements to improve sight distance 3 0 0 3 0 0 0 0 Manage the location and placement of parking (e.g., replace curb parking with off-street parking or restrict on-street parking near driveways or intersections to improve sight distance) 3 0 0 0 0 0 0 3 Manage vegetation to improve sight distance (e.g., in landscaped medians or sight triangles) 9 0 2 3 4 0 0 0 Total 15 0 2 6 4 0 0 3 Table 39. Summary of intersection-level CMFs for providing adequate sight distance at access points.
36 Application of Crash Modification Factors for Access Management SPFs for Providing Adequate Sight Distance at Access Points The literature review found no SPFs related to providing adequate sight distance at access points. Right-Turn Treatment RightÂturn movements, especially those that are made from shared lanes, cause conflicts and delays. RightÂturn lanes provide a refuge for rightÂturning vehicles by removing those vehicles from the throughÂtraffic lane(s). As such, they are an effective means of reducing the conflicts 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 rightÂturn lanes has been shown to provide a variety of traffic safety and operational benefits, including the following: ⢠Reducing the number of conflicts and crashes (particularly rearÂend, angle, and sideswipe crashes), ⢠Physically separating rightÂturning traffic and queues from through traffic, ⢠Decreasing vehicular delay and increasing intersection capacity, ⢠Providing an area for rightÂturning vehicles to decelerate outside of the through travel lane, and ⢠Providing greater operational flexibility (e.g., additional traffic signal phasing opportunities). Installing exclusive rightÂturn lanes is shown to reduce total, fatal and injury, rearÂend, and angle crashes by up to 26, 23, 65, and 56 percent, respectively (Harwood et al. 2003; Gan et al. 2005). CMFs for Right-Turn Treatment Tables 40 and 41 provide summaries of CMFs related to rightÂturn treatments at the inter section and site level, respectively. Based on a review of the CMF Clearinghouse in June 2017, there were 266 related CMFs, 207 of which were rated three stars or higher. There were no CMFs identified for the corridor level. SPFs for Right-Turn Treatment The literature review found no SPFs specifically related to rightÂturn treatments. Substrategy Numberof CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Channelize right-turn lane 4 0 3 0 0 0 0 1 Control/improve design elements of right-turn lanes 20 0 0 16 1 3 0 0 Install right-turn lane at roadway intersections 225 0 0 1 198 3 2 21 Total 249 0 3 17 199 6 2 22 Table 40. Summary of intersection-level CMFs for right-turn treatment. Substrategy Numberof CMFs CMF Clearinghouse Star Quality 0 1 2 3 4 5 Not Rated Channelize right-turn lane 0 0 0 0 0 0 0 0 Control/improve design elements of right-turn lanes 16 0 0 16 0 0 0 0 Install right-turn lane at roadway intersections 0 0 0 0 0 0 0 0 Total 16 0 0 16 0 0 0 0 Table 41. Summary of site-level CMFs for right-turn treatment.
Literature Review 37  Strategy Substrategy CMF Available CMF rated 3 stars or higher Alternative intersection and interchange design Convert from 4-legged intersection to two 3- legged intersections 19 6 Install roundabout at roadway intersection 127 106 Provide median acceleration lane 22 0 Install continuous green T at signalized 3-legged intersection 3 3 Replace direct left-turn with grade-separated interchange 3 3 Superstreet (RCUT, J-Turn) 63 8 Convert to jug handle intersections 0 0 Control driveway design elements Change class/type of driveway 5 5 Change movement restriction (e.g., right-in-right- out) 4 1 Require design of driveways with the appropriate return radii, throat width, channelization, number of lanes, and throat length for the type of traffic to be served 2 2 Convert two-way streets to one-way operation Convert two-way operation to one-way operation 12 8 Establish corner clearance criteria Driveways at signalized intersections 19 15 Improve cross connectivity Allow vehicles to access adjacent properties without returning to the mainline 0 0 Install non-traversable medians, and accommodate left turns and U-turns Convert traversable median (non-TWLTL) to non-traversable median 18 6 Install isolated median barriers 67 17 Install non-traversable median on undivided highway 67 36 Replace TWLTL with non-traversable median 258 116 Install TWLTL on undivided highway Non-road diet scenarios 139 59 Road diet scenarios 64 40 Install service or frontage roads Change proportion of primary roadway with frontage road 4 4 Install frontage road to provide access to individual parcels 4 0 Table 42. Summary of available and high-quality CMFs for each category and subcategory. (continued on next page) Summary The literature review focused on the safety effects of access management strategies, including CMFs and SPFs. The CMF Clearinghouse was the primary source for identifying CMFs, provid ing more than 1,000 CMFs for the access management strategies of interest. Table 42 provides a summary of all available CMFs and highÂquality CMFs (rated three stars or higher in the CMF Clearinghouse) for each category and subcategory of access management strategy. The research team also conducted a thorough and critical review of ongoing and completed research to identify potentially relevant SPFs that include access management strategies of interest. Table 43 indicates the availability of relatively highÂquality SPFs for each category and subcategory. These summaries provided a basis for the gap analysis and helped to inform the prioritization of research conducted in Phase 2.
38 Application of Crash Modification Factors for Access Management Strategy Substrategy CMF Available CMF rated 3 stars or higher Manage spacing of traffic signals Establish traffic signal density criteria 6 6 Establish traffic signal spacing criteria 14 14 Manage the location, spacing, and design of median openings and crossovers Create directional median opening 3 0 Install U-turns as an alternative to direct left turns 54 39 Regulate median opening density 5 5 Regulate median opening spacing 2 2 Replace full median opening with median designed for left turns from the major roadway 30 26 Manage the spacing of signalized and unsignalized access on crossroads in the vicinity of freeway interchanges Establish spacing criteria for interchange ramp terminals 3 3 Provide adequate sight distance at access points Manage design elements to improve sight distance 3 0 Manage the location and placement of parking (e.g., replace curb parking with off-street parking or restrict on -street parking near driveways or intersections to improve sight distance) 3 0 Manage vegetation to improve sight distance (e.g., in landscaped medians or sight triangles) 9 4 Right-turn treatment Channelize right-turn lane 4 0 Control/improve design elements of right-turn lanes 36 4 Install right-turn deceleration lane at roadway intersections 225 203 Install traversable medians Convert undivided to divided by traversable median 18 2 Left-turn treatment Change storage capacity of existing left-turn deceleration lane 5 2 Channelize left-turn lane 51 7 Control/improve design elements of left-turn lanes 4 4 Install left-turn lanes at roadway intersections 119 64 Provide turning bypass lanes 12 2 Prohibit left turn 11 4 Manage location and spacing of unsignalized access Establish density for unsignalized access (e.g., maximum driveway density) 55 54 Establish spacing for unsignalized access (e.g., minimum driveway spacing) 26 26 Table 42. (Continued). Strategy Substrategy SPF Available Alternative intersection and interchange design Convert from 4-legged intersection to two 3-legged intersections Yes Install roundabout at roadway intersection Yes Provide median acceleration lane No Replace direct left-turn with grade-separated interchange Yes Superstreet (RCUT, J-Turn) No Convert to jug handle intersections No Table 43. Summary of available SPFs for each category and subcategory.
Literature Review 39Â Â Strategy Substrategy SPF Available Manage the spacing of signalized and unsignalized access on crossroads in the vicinity of freeway interchanges Establish spacing criteria for interchange ramp terminals Yes Provide adequate sight distance at access points Manage design elements to improve sight distance No Manage the location and placement of parking (e.g., replace curb parking with off-street parking or restrict on- street parking near driveways or intersections to improve sight distance) No Manage vegetation to improve sight distance (e.g., in landscaped medians or sight triangles) No Right-turn treatment Channelize right-turn lane No Control/improve design elements of right-turn lanes No Install right-turn deceleration lane at roadway intersections No Convert two-way streets to one-way operation Convert two-way operation to one-way operation Yes Establish corner clearance criteria Driveways at signalized intersections No Improve cross connectivity Allow vehicles to access adjacent properties without returning to the mainline No Install non-traversable medians, and accommodate left turns and U-turns Convert traversable median (non-TWLTL) to non- traversable median No Install isolated median barriers No Install non-traversable median on undivided highway Yes Replace TWLTL with non-traversable median Yes Install TWLTL on undivided highway Non-road diet scenarios Yes Road diet scenarios No Install service or frontage roads Change proportion of primary roadway with frontage road No Install frontage road to provide access to individual parcels No Install traversable medians Convert undivided to divided by traversable median No Left-turn treatment Change storage capacity of existing left-turn deceleration lane No Channelize left-turn lane No Control/improve design elements of left-turn lanes No Install left-turn deceleration lanes at roadway intersections No Provide turning bypass lanes No Prohibit left turn No Manage location and spacing of unsignalized access Establish density for unsignalized access (e.g., maximum driveway density) Yes Establish spacing for unsignalized access (e.g., minimum driveway spacing) No Manage spacing of traffic signals Establish traffic signal density criteria Yes Establish traffic signal spacing criteria No Manage the location, spacing, and design of median openings and crossovers Create directional median opening No Install U-turns as an alternative to direct left turns No Regulate median opening density Yes Regulate median opening spacing No Replace full median opening with median designed for left turns from the major roadway No Control driveway design elements Change class/type of driveway No Change movement restriction (e.g., right-in-right-out) No Require design of driveways with the appropriate return radii, throat width, channelization, number of lanes, and throat length for the type of traffic to be served Yes Table 43. (Continued).
