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16 Description A TWLTL provides a location in the center of the roadway for storing vehicles from either direction that are waiting to make a left turn. TWLTLs can be developed along undivided road- ways by widening the roadway or by reducing the number of through lanes (i.e., a road diet). Tables 8 through 10 follow. Quantitative Analysis Methods Motor Vehicle Operations Exhibit 18-13 in the HCM6 gives average through vehicle delay in terms of seconds per vehicle per full, unsignalized access point (2). Delay values in this table assume 10% of the traffic on the street turns right at the access point and 10% turns left. Adjust the delay values proportionately for other turning percent- ages. Reduce the delay values by 50% if one turning movement is provided with an appro- priately dimensioned turn lane or the turning movement does not exist. There is no delay if both turning movements are provided with turn lanes (or if one movement has a turn lane and the other movement does not exist). See Chapter 9 for additional information specific to left-turn lanes. C H A P T E R 3 Continuous Two-Way Left-Turn Lanes Source: Photograph provided by the authors.
Continuous Two-Way Left-Turn Lanes 17 Access Management Technique Performance Trends and Documented Performance Relationships Operations Safety Install continuous TWLTL. â â â â â â â â Â ÂÂ Â Â Â Â Â Â Â Note: Trends are relative to a before condition of an undivided roadway. Table 8. Multimodal operations and safety performance summary. Mode Operations Safety Motor vehicle free-flow and travel speeds increase by up to a few mph, depending on traffic volumes, the number of access points, and the proportion of the roadway with a TWLTL (1, 2). The motor vehicle crash rate decreases, but by a smaller amount compared with installing a non- traversable median (3). Creates the potential for overlapping left-turn movements (4). Increased pedestrian delay and decreased pedestrian LOS at midblock pedestrian crossings where the crossing distance is increased as a result of the TWLTL. Small negative effect on pedestrian LOS due to increased motor vehicle speeds (2, 5). Increased pedestrian exposure when the crossing distance is increased as a result of the TWLTL; the TWLTL does not provide a pedestrian refuge (4). Similar vehicleâ pedestrian crash rates as undivided highways (6). Increases in vehicle speeds may negatively affect pedestrian safety (7, 8). Negative effect on bicycle LOS due to increased motor vehicle speeds (2, 5). Increases in vehicle speeds may negatively affect bicycle safety (7, 8). Similar effects as for motor vehicles. May make access to midblock bus stops more difficult, as bus passengers generally need to cross the roadway at some point during a round trip (9, 10). No documented effect beyond that generally observed for motor vehicle traffic (for buses) and pedestrians (for boarding and alighting passengers). Improved truck LOS due to improved speeds (11). No documented effect beyond that generally observed for motor vehicle traffic. Note: Trends are relative to a before condition of an undivided roadway. Table 9. General trends associated with installing continuous TWLTLs. Midsegment Volume (vehicles per hour per lane) Through Vehicle Delay (seconds per vehicle per access point) by Number of Through Lanes 1 Lane 2 Lanes 3 Lanes 200 0.04 0.04 0.05 300 0.08 0.08 0.09 400 0.12 0.15 0.15 500 0.18 0.25 0.15 600 0.27 0.41 0.15 700 0.39 0.72 0.15 Table 10. Average through vehicle delay per full, unsignalized access point.
18 Guide for the Analysis of Multimodal Corridor Access Management Motor Vehicle Safety NCHRP Report 420 (3) summarized the results of 12 studies between 1974 and 1994 that investigated the change in crash rates following the installation of TWLTLs. The average (median) change in crash rate was â38%, with greater improvements seen in rural areas (â53%) than in urban and suburban areas (â23%). Table 13-6 in the HSM provides a crash modification factor of 0.71 for the situation where a 4-lane undivided urban arterial is converted into two through lanes plus a TWLTL (12): 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. (13) 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 (2) are used to determine the effect of motorized vehicle speeds on pedestrian LOS. An increase in average traffic speed of 2 mph worsens the 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 TWLTL may decrease pedestrian LOS when (a) the crossing distance is widened and (b) it is nevertheless faster to cross the street at legal midblock locations than to detour to the nearest signalized intersection to cross. The impact on the pedestrian LOS score depends on the extra delay experienced by pedestrians and the quality of the pedestrian environment along the roadway (âlinkâ) and at signalized intersections. Higher delays and better link and intersection pedestrian environments result in greater decreases in the pedestrian LOS score. With a midrange pedestrian environment, the extra delay typically results in a reduction in the pedestrian LOS score of 0.2 to 0.9 points. Pedestrian Safety A study by Bowman and Vecellio (6) found that undivided roadways and roadways with TWLTLs had similar vehicleâpedestrian crash rates (midblock: 6.69 crashes per 100 million vehicle milesâundivided, 6.66âTWLTL and intersection: 2.32 crashes per 100 million enter- ing vehiclesâundivided, 2.49âTWLTL). The vehicleâpedestrian crash models developed by Bowman et al. (13) can be used to estimate crash rates on roadways with TWLTLs, as well as on undivided roadways and roadways with raised medians. The appendix provides additional detail about these models. Bicycle Operations Equations 18-41 and 18-44 in the HCM6 (2) are used to determine the effect of motor- ized 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.
Continuous Two-Way Left-Turn Lanes 19 Bus Operations Chapter 18 in the HCM6 (2) 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 (2, 10). Truck Operations Section P in Exhibit 3 of NCHRP Report 825 (14), which is derived from NCFRP Report 31 (11), 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. Additional Information â¢ Chapters 2 and 9 in this guide. â¢ Access Management Manual, Second ed.: Chapter 17, Medians and Two-Way Left-Turn Lanes, Section 20.3.3. â¢ Access Management Application Guidelines: Chapter 15, Median Applications and Design. â¢ NCHRP Report 420: Chapter 6, Median Alternatives. References 1. 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. 2. Highway Capacity Manual: A Guide for Multimodal Mobility Analysis, 6th ed. Transportation Research Board, Washington, D.C., 2016. 3. Gluck, J., H. S. Levinson, and V. Stover. NCHRP Report 420: Impacts of Access Management Techniques. Transportation Research Board, Washington, D.C., 1999. 4. 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. 5. 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. 6. 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. 7. 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. 8. National Association of City Transportation Officials. Urban Street Design Guide. National Association of City Transportation Officials, Washington, D.C., Oct. 2013. 9. Redmon, T. Safety Benefits of Raised Medians and Pedestrian Refuge Areas. Publication FHWA-SA-10-020. Federal Highway Administration, Washington, D.C., 2010. 10. 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.
20 Guide for the Analysis of Multimodal Corridor Access Management 11. 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. 12. Highway Safety Manual, 1st ed. American Association of State Highway and Transportation Officials, Washington, D.C., 2010. 13. 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. 14. 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.