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44 Description A left-turn lane is typically an auxiliary lane in the middle of a two-way roadway. Left-turn deceleration lanes allow left-turning vehicles from the roadway to conduct most of their deceleration and, if necessary, queue and wait for a safe gap in opposing traffic before turning. Tables 30 through 35 follow. Quantitative Analysis Methods Motor Vehicle Operations The table from NCHRP Report 745 (3) gives intersection-wide delay reductions resulting from adding a left-turn lane at an unsignalized intersection, developed from simulation (see Table 32). Chapters 19 and 20 in the HCM6 can be used to compare the change in delay as a result of installing left-turn lanes (2), in cases in which the left-turn lane is adequately sized to prevent queue spillback into the through lanes. Chapter 31, Section 4, in the HCM6 can be used to calculate a desired percentile back-of- queue for left-turn lanes at signalized intersections, while Equation 20-68 in Chapter 20 in the HCM6 can be used to determine the 95th percentile queue length for the left-turn lanes on major-street approaches to two-way stop-controlled intersections (2). A taper distance, allowing vehicles to maneuver from the through lane into the left-turn lane, and a deceleration distance C H A P T E R 9 Left-Turn Lanes Source: Photograph provided by the authors.
Left-Turn Lanes 45 Access Management Technique Performance Trends and Documented Performance Relationships Operations Safety Install left-turn deceleration lanes where none exists. â â â â â â â â â Â Â Â Â Â Â Install alternating left-turn lane. â â â â â â â â â Â Â Â Â Â Â Install isolated median and left-turn lane to shadow and store left-turn vehicles. â â â â â â â â â Â Â Â Â Â Â Install left-turn deceleration lane in lieu of right-angle crossover. Â Â Â Â Â Â Â Â Â Â Increase storage capacity of existing left-turn lane. â Â Â Â Â â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Table 30. Multimodal operations and safety performance summary. Mode Operations Safety Motor vehicle free-flow and travel speeds increase by up to a few miles per hour. Speed increases are greater when the number of through lanes is less, when through traffic volumes are higher, and when turning traffic volumes are higher (1â3). The motorized vehicle crash rate decreases (4). Small negative effect on pedestrian LOS due to increased motor vehicle speeds (2, 5). At unsignalized intersections, crossing distance and pedestrian delay increase and pedestrian LOS may decrease, if the roadway is widened to install the left-turn lane and no pedestrian refuge is provided (2, 6). The traffic signal cycle length may need to increase to accommodate longer pedestrian crossing distances, increasing pedestrian delay (6). Increases pedestrian exposure to traffic if the roadway is widened to install the left-turn lane (7). If traffic conditions permit bicyclists to access the turn lane, provides easier left turns for bicyclists at unsignalized intersections (6). Increases cross-street bicycle exposure to traffic due to widened cross-section (6). Improves transit speeds at locations where left- turning traffic blocks through traffic (7). Buses need longer gaps in traffic to make left turns; therefore, a left-turn lane reduces exposure by providing a refuge for buses waiting for a suitable gap (6). Similar to motor vehicles, but with greater benefit for trucks, as they accelerate more slowly to their running speed after stopping or slowing (8). Improved truck LOS due to improved speeds (9). No documented effect beyond that generally observed for motor vehicle traffic. Table 31. General trends associated with providing left-turn lanes.
46 Guide for the Analysis of Multimodal Corridor Access Management Total Major Street Through Lanes Speed Limit (mph) Major Street Volume (vehicles per hour per lane) Intersection Delay Reduction (seconds per vehicle) by Left-Turning Volume (vehicles per hour) 20 60 100 140 2 30 400 0.0 0.4 1.0 1.5 600 0.7 1.2 1.8 2.4 800 1.5 2.0 2.6 3.2 40 400 0.0 0.0 0.6 1.1 600 0.3 0.8 1.4 1.9 800 1.1 1.6 2.1 2.7 50 400 0.0 0.0 0.5 1.0 600 0.2 0.8 1.3 1.9 800 1.0 1.6 2.1 2.7 4 30 400 0.0 0.0 0.3 0.6 600 0.2 0.5 0.8 1.1 800 0.7 1.0 1.3 1.6 40 400 0.0 0.0 0.1 0.5 600 0.0 0.4 0.8 1.2 800 0.7 1.1 1.5 1.9 50 400 0.0 0.0 0.1 0.7 600 0.2 0.7 1.3 a 800 1.3 1.9 2.4 a Note: Define delay reduction based on all vehicles entering the intersection in the hour and assume adequate left-turn storage and deceleration distance. aBeyond the limit of regression, use value scaled from Figures 23 or 24 in NCHRP Report 745 (3). Source: NCHRP Web-Only Document 193, Table 46 (10). Table 32. Intersection-wide delay reductions. Intersection Type Traffic Volume: AADT (vehicles per day) Crash Severity Crash Modification Factor by Number of Approaches with Left-Turn Lanes One Approach Two Approaches Rural, 4-leg, TWSC Major: 1,600â32,400 Minor: 50â11,800 All 0.72 0.52 Injury 0.65 0.42 Urban, 4-leg, TWSC Major: 1,500â40,600 Minor: 200â8,000 All 0.73 0.53 Injury 0.71 0.50 Rural, 4-leg, signalized Unspecified All 0.82 0.67 Urban, 4-leg, signalized Major: 1,500â32,400 Minor: 50â11,800 All 0.90 0.81 Injury 0.91 0.83 Urban, 4-leg, newly signalized Major: 4,600â40,300 Minor: 100â13,700 All 0.76 0.58 Injury 0.72 0.52 Note: Values apply to major street approaches at unsignalized intersections and any approach at signalized intersections. For signalized intersections with three or four approaches with left-turn lanes, the crash modification factor is the value for one approach raised to the third or fourth power, respectively. TWSC = two-way stop controlled (minor street stop controlled); AADT = annual average daily traffic. Source: Highway Safety Manual, 1st ed., Tables 14-11 and 14-12 (4). Table 33. Crash modification factors for approaches to four-leg, two-way stop-controlled intersections.
Left-Turn Lanes 47 need to be added to this storage length when determining the total left-turn lane length. NCHRP Report 745 (3) and the Access Management Manual (11), among others, provide guidance on designing left-turn lanes. Workbook 11 that accompanies the linked version of the Access Management: Manual and Application Guidelines (12) is a spreadsheet tool for calculating the total left-turn lane length at signalized intersections. Motor Vehicle Safety The HSM provides crash modification factors related to installing left-turn lanes on the major street approaches to four-leg, two-way stop-controlled intersections and on any approach to a four-leg signalized intersection (4) (see Table 33). The HSM also provides crash modification factors for installing a left-turn lane on one major street approach to a three-leg minor street stop-controlled intersection and on any approach to a three-leg signalized intersection (4) (see Table 34 above). An FHWA study (13) found an average 43% reduction in crash rates at four rural four-leg unsignalized intersections where left-turn lanes had been lengthened. No information was available about left-turning volumes or overflow from the existing turn lane at the intersections. Sample sizes were too small to permit drawing conclusions about lengthening turn lanes at other types of intersections. Left-turn lanes that are too short to accommodate demand or that otherwise provide insuf- ficient deceleration distance result in a greater speed differential between left-turning vehicles and through vehicles relative to the typical design differential of 10 mph (1). Table 35 (14, 15) gives crash rate ratios for various speed differentials on rural highways relative to no speed dif- ferential. For example, the crash rate with a 10-mph speed differential is twice the crash rate with no speed differential. See Chapter 5 for information related to channelized left-turn lanes. Pedestrian Operations Equations 18-32 and 18-35 in the HCM6 (2) can determine the effect of motorized vehi- cle speeds on pedestrian link LOS. An increase in average traffic speed of 2 mph worsens the Intersection Type Traffic Volume: AADT (vehicles per day) Crash Severity Crash Modification Factor Rural, three-leg, TWSC Major: 1,600â32,400 Minor: 50â11,800 All 0.56 Injury 0.45 Urban, three-leg, TWSC Major: 1,500â40,600 Minor: 200â8,000 All 0.67 Urban, three-leg, TWSC Unspecified Injury 0.65 Rural, three-leg, signalized Unspecified All 0.85 Urban, three-leg, signalized Unspecified All 0.93 Injury 0.94 Note: Values apply to the major street approach at unsignalized intersections and any approach at signalized intersections. Source: Highway Safety Manual, 1st ed., Table 14-10 (4). Table 34. Crash modification factors for approaches to three-leg, two-way stop-controlled intersections.
48 Guide for the Analysis of Multimodal Corridor Access Management pedestrian link 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. Equation 19-72 in the HCM6 (2) can determine the effect of intersection width on pedestrian intersection LOS. An increase of one lane of width worsens the pedestrian intersection LOS score by 0.13 points (widening from 7 to 8 lanes, including turn lanes) to 0.23 points (2 to 3 lanes), with 1.00 point representing the range covered by one LOS letter. Bicycle Operations Equations 18-41 and 18-44 in the HCM6 (2) can determine the effect of motorized vehicle speeds on bicycle link LOS. An increase in average traffic speed of 2 mph worsens the bicycle link 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. Equation 19-80 in the HCM6 (2) can determine the effect of intersection width on bicycle intersection LOS. An increase in intersection width of 12 feet worsens the bicycle intersection LOS score by 0.18 points, with 1.00 point representing the range covered by one LOS letter. Truck Operations Section P in Exhibit 3 of NCHRP Report 825 (16), derived from NCFRP Report 31 (9), 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 increas- ing 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¢ Chapter 5 in this guide. â¢ Access Management Manual, Second ed.: Chapter 16, Auxiliary Lanes, Section 20.3.4. â¢ Access Management Application Guidelines: Chapter 21, Left-Turn Lanes. â¢ NCHRP Report 420: Chapter 7, Left-Turn Lanes. 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. Speed Differential (mph) Crash Rate Ratio Relative to no Speed Differential 0 1.0 â10 2.0 â20 6.5 â30 45 â35 180 Source: Solomon (14), Stover and Koepke (15). Table 35. Crash rate ratios for various speed differentials on rural highways relative to no speed differential.
Left-Turn Lanes 49 2. Highway Capacity Manual: A Guide for Multimodal Mobility Analysis, 6th ed. Transportation Research Board, Washington, D.C., 2016. 3. Fitzpatrick, K., M. A. Brewer, W. L. Eisele, H. S. Levinson, J. S. Gluck, and M. R. Lorenz. NCHRP Report 745: Left-Turn Accommodations at Unsignalized Intersections. Transportation Research Board of the National Academies, Washington, D.C., 2013. 4. Highway Safety Manual, 1st ed. American Association of State Highway and Transportation Officials, Washington, D.C., 2010. 5. Dowling, R., R. David, 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. 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. 7. Ryus, P., K. Laustsen, K. Blume, S. Beaird, and S. Langdon. TCRP Report 183: A Guidebook on Transit- Supportive Roadway Strategies. Transportation Research Board, Washington, D.C., 2016. 8. Harwood, D. W., D. J. Torbic, K. R. Richard, W. D. Glauz, and L. Elefteriadou. NCHRP Report 505: Review of Truck Characteristics as Factors in Roadway Design. Transportation Research Board of the National Academies, Washington, D.C., 2003. 9. 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. 10. 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. 11. 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. 12. Transportation Research Board. Access Management: Manual and Application GuidelinesâLinked. Transportation Research Board, Washington, D.C., 2017. 13. Harwood, D. W., K. M. Bauer, I. B. Potts, D. J. Torbic, K. R. Richard, E. R. Kohlman Rabbani, E. Hauer, and L. Elefteriadou. Safety Effectiveness of Intersection Left- and Right-Turn Lanes. Report FHWA-RD-02-089. Federal Highway Administration, Washington, D.C., July 2002. 14. Solomon, D. H. Accidents on Main Rural Highways Related to Speed, Driver, and Vehicle. Bureau of Public Roads, Washington, D.C., 1964. 15. Stover, V. G., and F. J. Koepke, Transportation and Land Development, 2nd ed., Institute of Transportation Engineers, Washington, D.C., 2002. 16. 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.