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10 Description Techniques in this group have the common element of installing a non-traversable median to manage access by restricting left-turn access to a limited number of locations. A non-traversable median is installed along an extended section of undivided highway, or a TWLTL lane is replaced with a non-traversable median. Non-traversable medians include raised curbs, slightly depressed medians (e.g., flush grass), and median barriers (1). See Chapters 1 and 11 for information about controlling left-turn access, egress, or both at specific driveways, which may include installing short sections of non-traversable median. Tables 4 through 7 follow. Quantitative Analysis Methods Motor Vehicle Operations Exhibit 18-11 in the HCM6 gives the change in roadway free-flow speed resulting from a conversion from an undivided roadway or a nonrestrictive median to a restrictive (i.e., non- traversable) median (3). The change in free-flow speed (in mph) equals 1.5 prm â 3.7 prm, pcurb, where prm is the proportion of the link length (decimal) with a restrictive median and pcurb is the proportion of the link length (decimal) with a curb on the right side of the roadway. C H A P T E R 2 Non-Traversable Medians Source: Photograph provided by Google Earth.
Non-Traversable Medians 11 Access Management Technique Performance Trends and Documented Performance Relationships Operations Safety Install non-traversable median along undivided highway. â â â â â â â â Â Â Â Â Â Â Â Â Â Convert TWLTL to non-traversable median. â â â â â â â â Â Â Â Â Â Â Â Â Â Install median barrier with no direct left-turn access or egress. â â â â â â â â Â Â Â Â Â Â Â Â Â Table 4. Multimodal operations and safety performance summary. Mode Operations Safety Motor vehicle free-flow and travel speeds increase by up to 1.5 miles per hour (mph) (with no curb on the right side of the roadway) or decrease by up to 2.2 mph (with a curb on the right side), depending on the proportion of the roadway with a raised median (2, 3). See also Chapters 5 and 9. The motorized vehicle crash rate decreases (4â6). There is greatly reduced potential for head-on collisions (7). The number of vehicleâ vehicle conflict points decreases. Provides opportunities to develop two-stage pedestrian crossings that reduce pedestrian delay when crossing the street (8), except for barrier designs, which block pedestrian crossings (9). Pedestrian LOS goes down with increased motor vehicle speeds and up with decreased motor vehicle speeds (3, 10). Decreases the number of vehicleâpedestrian conflict points and can provide a refuge in the middle of the roadway at pedestrian crossings, both of which improve pedestrian safety (1, 7, 11, 12). Increases in vehicle speeds may negatively affect pedestrian safety and vice versa (13, 14). May reduce legal bicycle left-turn opportunities, although bicyclists may be able to dismount and cross as a pedestrian (7). Bicycle LOS goes down with increased motor vehicle speeds and up with decreased motor vehicle speeds (3, 10). Bicycle speeds similar to slightly higher than before (15). Reduces vehicleâbicycle crash frequency at signalized intersections (16). Decreases the number of potential vehicleâbicycle conflict points (7). Increases in vehicle speeds may negatively affect bicycle safety and vice versa (13, 14). Similar effects as for motor vehicles. If necessary, bus left turns can be served with bus-only left- turn lanes (7). Can facilitate access to midblock bus stops if pedestrian crossing opportunities are provided, as bus passengers generally need to cross the roadway at some point during a round trip (3, 17). No documented effect beyond what is generally observed for motor vehicle traffic (for buses) and pedestrians (for boarding and alighting passengers). Truck speeds increase, as long as traffic volumes do not increase by more than 285 vehicles per hour per lane as a result of changes in traffic patterns (15). Truck LOS goes up with increased speeds and down with decreased speeds (18). No documented effect beyond that generally observed for motor vehicle traffic. Table 5. General trends associated with installing non-traversable medians.
12 Guide for the Analysis of Multimodal Corridor Access Management The magnitude of the corresponding change in average running speed will be slightly lower, as discussed in the appendix. When a non-traversable median is installed on an undivided roadway or a roadway with a TWLTL, through traffic will experience less delay than before, due to the reduction in the number of locations where left turns can be made (3). See Chapter 1 for details. When a non-traversable median is installed on an undivided roadway and left-turn lanes are provided at median openings, through traffic will experience less delay than before, because vehicles stopped to make a left turn will no longer block through vehicles (3). See Chapter 9 for more information. If no median openings are provided between signalized intersections or if U-turns are prohibited at median openings, delay may increase at the signalized intersections as a result of the increased U-turning volume (5). Chapter 19 in the HCM6 (3) can be used to estimate the change in delay resulting from the additional U-turns. Alternative intersection designs such as the Michigan U-turn or restricted crossing U-Turn address this issue by relocating U-turns from the main signalized intersection to adjacent secondary intersections. These intersection forms can be analyzed by using the methods in Chapter 23 in the HCM6 (3). See Chapter 12 for more information. Motor Vehicle Safety NCHRP Report 420 (5) summarized the results of a number of studies between 1983 and 1995 that investigated the change in crash rates following the installation of non-traversable medians (6), as shown in Table 6. A 2012 study of 18 Florida locations where TWLTLs had been converted to non-traversable medians found an average 30% reduction in the crash rate following the installation of the non-traversable median (6). The crash-rate reduction was much greater on 6-lane arterials (â37%) than on 4-lane arterials (â5%). Tables 13-10 and 13-11 in the HSM provide the following crash modification factors related to installing a median on urban and rural roadways (4): â¢ Urban 2-lane roadways: 0.61 â¢ Urban multi-lane arterials: 0.78 (injury crashes), 1.09 (non-injury crashes) â¢ Rural multi-lane arterials: 0.88 (injury crashes), 0.82 (non-injury crashes) Bowman et al. (19) developed a set of predictive models that collectively addressed vehicleâ vehicle crashes for three cross-section types (i.e., raised-curb median, TWLTL, and undivided). The appendix provides additional detail about these models. Pedestrian Operations Equations 18-32 and 18-35 in the HCM6 (3) are used to determine the effect of motorized vehicle speeds on pedestrian LOS. An increase in average traffic speed of 2 mph worsens the Percent Change in Crash Rate Before Condition Number of Studies Range of Results Average (Median) Result Undivided 10 â2 to â67 â35 TWLTL 16 +15 to â57 â27 Table 6. Crash rates following the installation of non-traversable medians.
Non-Traversable Medians 13 pedestrian LOS score by 0.03 points (at 20 mph) to 0.08 points (at 50 mph), with 0.75 points representing the range covered by one LOS letter. Installing a non-traversable median may improve pedestrian LOS in one of two ways: (a) by potentially providing an opportunity to develop legal midblock pedestrian crossings where none existed before or (b) by reducing pedestrian delay at an existing legal crossing by facilitating two-stage pedestrian crossings (3). The impact on the pedestrian LOS score depends on how much time the pedestrian saves and the quality of the pedestrian environment along the roadway (âlinkâ) and at signalized intersections. Greater time-savings and poorer link and intersection pedestrian environments result in greater improvements in the pedestrian LOS score. Providing a midblock pedestrian crossing can potentially improve the pedestrian LOS score by 1 to 2 points, while converting a one-stage crossing to a two-stage crossing typically improves the pedestrian LOS score by 0.1 to 0.8 points. Pedestrian Safety Table 7 summarizes the results of studies that have evaluated the effect of non-traversable medians on vehicleâpedestrian crash rates. Bowman et al. (19) developed a set of predictive models that collectively addressed three cross- section types (i.e., raised-curb median, TWLTL, and undivided). The dependent variable (i.e., crash rate) was expressed in terms of vehicleâpedestrian crashes per 100 million vehicle miles. The model indicates that vehicleâpedestrian crash rate is lowest for the raised-curb median, regardless of area type or land use. The appendix provides additional detail about these models. Zegeer et al. (20) developed a crash prediction model that predicts the frequency of vehicleâ pedestrian crashes at unsignalized crossing locations. The model includes an input variable that is used to indicate whether a raised-curb median is present (as a refuge) for part of the cross- ing. The model indicates that vehicleâpedestrian crash frequency decreases when a raised-curb median is present. Bicycle Operations Equations 18-41 and 18-44 in the HCM6 (3) are used to determine the effect of motorized vehicle speeds on bicycle LOS. An increase in average traffic speed of 2 mph worsens the bicycle LOS score by 0.57 points (at 21 mph or less), 0.17 points (at 25 mph), 0.05 points (at 40 mph), and 0.03 points (at 50 mph), with 0.75 points representing the range covered by one LOS letter. Average Percent Change in Crash Rate by Previous Median Type Source Crash Type Undivided TWLTL Bowman and Vecellio (12) Midblock â42 â42 intersection â58 â61 Central business location â â54 Suburban location â â51 Parsonson et al. (11) Fatal â â78 Alluri et al. (6) All â â29a Note: A dash indicates that the combination of median type and crash type was not studied. aNot statistically significant. Table 7. Studies that have evaluated the effect of non-traversable medians on vehicleâpedestrian crash rates.
14 Guide for the Analysis of Multimodal Corridor Access Management A simulation study (15) found that converting a TWLTL to a raised median reduced bicycle speeds by 0.04 mph if no change in traffic volume resulted from the conversion. However, this speed reduction would be offset if traffic volumes increased as a result of the conversion. With a raised median, bicycle speeds increased by 0.23 mph for each increase of 100 vehicles per hour per lane. Higher through traffic volumes seemed to reduce opportunities for driveway traffic to turn onto the roadway, which decreased the chance that driveway traffic would interfere with bicycle traffic. The break-even point for bicycle speed occurred at an increase of about 20 vehicles per hour per lane. Bicycle Safety Miranda-Moreno et al. (16) developed two models indicating that the vehicleâbicycle crash rate at signalized intersections decreases when a raised median is present; however, this relation- ship was not statistically significant in either model. Alluri et al. (6) studied the effect of converting TWLTLs to raised medians in Florida. With respect to vehicleâbicycle crashes, the study found a 4.5% reduction in the crash rate that was not statistically significant. Bus Operations The information in HCM6 Chapter 18 (3) can be used to estimate the change in average bus speeds resulting from improvements in midblock running speed. This estimation in turn can be used to estimate the change in bus LOS for the segment. At 30-minute bus headways and with seated loads and reliable service, a 1-mph increase in average bus speed produces a 0.08â0.12 improvement in the bus LOS score, with 0.75 points representing the range covered by one LOS letter (3, 17). Truck Operations Section P in Exhibit 3 of NCHRP Report 825: Planning and Preliminary Engineering Applica- tions Guide to the Highway Capacity Manual (21), which is derived from NCFRP Report 31: Incorporating Truck Analysis into the Highway Capacity Manual (18), can be used to estimate the effect of improved truck free-flow and travel speeds on overall truck LOS. On a level street with a 35-mph free-flow speed, increasing average truck speeds from 25 to 26 mph results in a 1.3 percentage point increase in the truck LOS index, while increasing average truck speeds from 17.5 to 18.5 mph results in a 7.0 percentage point increase, with 10 percentage points representing the range covered by one LOS letter. A simulation study (15) found that converting a TWLTL to a raised median improved truck speeds by 2.1 mph if no change in traffic volume occurred as a result of the conversion. However, this speed increase would be partially offset if traffic volumes increased as a result of the conversion. With a raised median, truck speeds decreased by 0.72 mph for each increase of 100 vehicles per hour per lane. The break-even point for truck speed occurred at an increase of about 280 vehicles per hour per lane. Additional Information â¢ Chapters 1, 3, 5, 7, 11, and 12 in this guide. â¢ Access Management Manual, Second ed.: Chapter 17, Medians and Two-Way Left-Turn Lanes, Section 20.2.7 â¢ Access Management Application Guidelines: Chapter 15, Median Applications and Design. â¢ NCHRP Report 420: Chapter 6, Median Alternatives.
Non-Traversable Medians 15 References 1. Williams, K. M., V. G. Stover, K. K. Dixon, and P. Demosthenes. Access Management Manual, Second ed. Transportation Research Board of the National Academies, Washington, D.C., 2014. 2. Texas Transportation Institute; Kittelson & Associates, Inc.; and Purdue University. Predicting the Perfor- mance of Automobile Traffic on Urban Streets. Final report, NCHRP Project 03-79. Transportation Research Board of the National Academies, Washington, D.C., Jan. 2008. 3. Highway Capacity Manual: A Guide for Multimodal Mobility Analysis, 6th ed. Transportation Research Board, Washington, D.C., 2016. 4. Highway Safety Manual, 1st ed. American Association of State Highway and Transportation Officials, Washington, D.C., 2010. 5. Gluck, J., H. S. Levinson, and V. Stover. NCHRP Report 420: Impacts of Access Management Techniques. Transportation Research Board, Washington, D.C., 1999. 6. Alluri, P., A. Gan, K. Haleem, S. Miranda, E. Echezabal, A. Diaz, and S. Ding. Before and After Safety Study of Roadways Where New Medians Have Been Added. Lehman Center for Transportation Research, Florida International University, Miami, Dec. 2012. 7. Dixon, K. K., R. D. Layton, M. Butorac, P. Ryus, J. L. Gattis, L. Brown, and D. Huntington. Access Management Application Guidelines. Transportation Research Board, Washington, D.C., 2016. 8. Redmon, T. Safety Benefits of Raised Medians and Pedestrian Refuge Areas. Publication FHWA-SA-10-020. Federal Highway Administration, Washington, D.C., 2010. 9. New Jersey Department of Transportation and Pennsylvania Department of Transportation. Smart Transportation Guidebook. Trenton, N.J., and Harrisburg, Pa., March 2008. 10. Dowling, R., D. Reinke, A. Flannery, P. Ryus, M. Vandehey, T. Petritsch, B. Landis, N. Rouphail, and J. Bonneson. NCHRP Report 616: Multimodal Level of Service Analysis for Urban Streets. Transportation Research Board of the National Academies, Washington, D.C., 2008. 11. Parsonson, P. S., M. G. Waters III, and J. S. Fincher. Georgia Study Confirms the Continuing Safety of Raised Medians Over Two-Way Left-Turn Lanes. Fourth National Conference on Access Management, Portland, Ore., Aug. 2000. 12. Bowman, B. L., and R. L. Vecellio. Effect of Urban and Suburban Median Types on Both Vehicular and Pedestrian Safety. Transportation Research Record: Journal of the Transportation Research Board, No. 1445, 1994, pp. 169â179. 13. Chandler, B. E., M. C. Myers, J. E. Atkinson, T. E. Bryer, R. Retting, J. Smithline, J. Trim, P. Wojtkiewicz, G. B. Thomas, S. P. Venglar, S. Sunkari, B. J. Malone, and P. Izadpanah. Signalized Intersections Informational Guide, 2nd ed. Publication No. FHWA-SA-13-027. Federal Highway Administration, Washington, D.C., July 2013. 14. National Association of City Transportation Officials. Urban Street Design Guide. National Association of City Transportation Officials, Washington, D.C., Oct. 2013. 15. Butorac, M., J. Bonneson, K. Connolly, P. Ryus, B. Schroeder, K. Williams, Z. Wang, S. Ozkul, and J. Gluck. Web-Only Document 256: Assessing Interactions Between Access Management Treatments and Multimodal Users. Transportation Research Board, Washington, D.C., 2018. 16. Miranda-Moreno, L. F., J. Strauss, and P. Morency. Disaggregate Exposure Measures and Injury Frequency Models of Cyclist Safety at Signalized Intersections. Transportation Research Record: Journal of the Trans- portation Research Board, No. 2236, 2011, pp. 74â82. 17. Kittelson & Associates, Inc.; Parsons Brinckerhoff; KFH Group, Inc.; Texas A&M Transportation Institute; and Arup. TCRP Report 165: Transit Capacity and Quality of Service Manual, 3rd ed. Transportation Research Board of the National Academies, Washington, D.C., 2013. 18. Dowling, R., G. List, B. Yang, E. Witzke, and A. Flannery. NCFRP Report 31: Incorporating Truck Analysis into the Highway Capacity Manual. Transportation Research Board of the National Academies, Washington, D.C., 2014. 19. Bowman, B., R. Vecellio, and J. Miao. Vehicle and Pedestrian Accident Models for Median Locations. Journal of Transportation Engineering, Vol. 121, No. 6, American Society of Civil Engineers, Washington, D.C., Nov.âDec. 1994, pp. 531â537. 20. Zegeer, C. V., J. R. Stewart, H. H. Huang, P. A. Lagerwey, J. Feaganes, and B. J. Campbell. Safety Effects of Marked Versus Unmarked Crosswalks at Uncontrolled Locations: Final Report and Recommended Guidelines. Report FHWA-HRT-04-100. Federal Highway Administration, Washington, D.C., Aug. 2005. 21. Dowling, R., P. Ryus, B. Schroeder, M. Kyte, T. Creasey, N. Rouphail, A. Hajbabaie, and D. Rhoades. NCHRP Report 825: Planning and Preliminary Engineering Applications Guide to the Highway Capacity Manual. Transportation Research Board, Washington, D.C., 2016.