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16 side of the turn lanes may be extended through the intersec- of a dotted line instead of a solid line at the upstream end of tion using dotted lines if engineering judgment indicates that the full-width turn lane.) this would be helpful to drivers. The design standards for three agencies (OH, OR, and The design standards for Indiana require that if dual left- WY) note that dotted lines are optional in turn lane tapers. turn lanes are provided in opposing directions, the dotted line extensions of the lane lines between the two turn lanes should The design standards for Florida note that dotted lines are be separated from each other in the center of the intersection not typically used in turn lane tapers. However, in curves or by at least 30 ft. in other areas where drivers in the through lane might need extra guidance to avoid unintentionally drifting into a turn The design standards for the city of Tucson/Pima County lane, a broken lane line (6-ft segments with 10-ft gaps) may require that if dual left-turn lanes are provided in opposing be used. directions, the dotted line extensions of the lane lines between the two turn lanes should typically be separated from each The design standards for Virginia note that dotted lines other in the center of the intersection by at least 26 ft. are used if the distance between the upstream end of the turn lane taper and the upstream end of the lane line separating the turn lane from the adjacent through lane is longer than 100 ft. USE AND TYPE OF DOTTED LINES IN TURN LANE TAPERS If the distance between the upstream end of the turn lane taper and the upstream end of the lane line separating the turn lane Figure 2 shows an example of a dotted line in a turn lane taper. from the adjacent through lane is 100 ft or less, a dotted line is not used. 2003 MUTCD Provisions The design standards for the District of Columbia state that dotted lines are used where a turn lane occurs on a hori- The MUTCD does not require or recommend that dotted zontal curve. lines be used in turn lane tapers to discourage through traffic from unintentionally entering the turn lane. None of the fig- The design standards for seven agencies (AL, DE, GA, ures in Part 3 that show the formation of a turn lane at an NC, RI, VT, and CLT) specify that dotted lines are to be used intersection (see Figures 3B-2, 3B-7, 3B-11, and 3B-22) in all turn lane tapers. show dotted lines in the turn lane tapers. However, the MUTCD does not prohibit dotted lines from being used for this purpose (see Appendix A for figures). Pattern of Dotted Lines (12 agencies) Item E in Paragraph 1 of Section 3A.05 contains the fol- The following patterns are used for dotted lines in turn lane lowing standards: "A dotted line shall consist of noticeably tapers: shorter line segments separated by shorter gaps than used for a broken line. The width of a dotted line shall be at least the AL, RI, VT, VA, DC--2-ft segments with 4-ft gaps same as the width of the line it extends." Even though these DE, GA, OR, WY--2-ft segments with 6-ft gaps provisions are classified as requirements they do not specify CLT--2-ft segments with 10-ft gaps the width or the pattern of dotted lines, which varies signifi- NC--2-ft segments with 13-ft gaps cantly among the policies of the various agencies. FL--6-ft segments with 10-ft gaps. Paragraph 3 of Section 3A.05 contains the following Width of Dotted Lines (6 agencies) option: "A dotted line for line extensions may consist of 2 ft line segments and 2 ft to 6 ft gaps." The following widths are used for dotted lines in turn lane tapers: Use Versus Non-Use of Dotted Lines (43 agencies) AL, VT, WY--4 in. The design standards for 30 agencies (AK, AZ, AR, CA, CO, AL--4 to 6 in. ID, KS, KY, MD, MI, MN, MS, MT, NE, NV, NH, NY, OK, GA--5 in. PA, SC, SD, TN, TX, UT, WA, WV, WI, PR, LAN, and DC--8 in. TUC) do not show dotted lines in turn lane tapers. The design standards for Colorado specify that dotted lines are not to be LEFT-TURN LANE ADDED BETWEEN THROUGH used in turn lane tapers. The design standards for Idaho show LANES OF TWO-LANE HIGHWAYS a dotted line only in the taper of a left-turn lane formed by widening between the through lanes on a two-way highway. Figure 5 shows examples of adding a left-turn lane between the (The design standards for Maryland show the upstream end through lanes of two-lane highways.
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17 2003 MUTCD Provisions width of the turn lane becomes available. These are known as fully shadowed left-turn lanes. The MUTCD does not include an illustration of adding a left- turn lane between the through lanes on a two-lane highway, The design standards for four agencies use the shifting nor does it address the length of left-turn lane tapers. How- taper to widen the roadway until less than the full-width ever, the following formulas are recommended in several of the turn lane becomes available. These are known as places in Part 3 (Paragraph 10 of Section 3B.03, Paragraph 2 partially shadowed left-turn lanes. In all four cases, the of Section 3B.09, Paragraph 2 of Section 3B.10, and Figures upstream end of the left-turn lane taper begins at the point 3B-12 and 3B-13) for the length (L) in feet of a shifting taper where the partial width of the turn lane first becomes avail- for through traffic: L = WS for speeds of 45 mph or more and able. The following are the amounts of partial widening L = WS2/60 for speeds of less than 45 mph, where W is the that occur: width of the offset distance in feet and S is the speed of traf- fic in mph (see Appendix A for figures). ID--half of the width of the left-turn lane AR, SD--two-thirds of the width of the left-turn lane TN--three-fourths of the width of the left-turn lane. Shifting Taper (21 agencies) The design standards for 15 agencies (AZ, AR, CA, ID, KS, Distance from Full Shadowing to Upstream End MI, MO, NH, OH, PA, SD, TN, TX, WY, and TUC) use the of Left-Turn Taper (17 agencies) recommended MUTCD formulas for the length of the shifting taper for the through lane as it approaches the left-turn lane The design standards for 13 of the 17 agencies that use location. However, one of these agencies (CA) allows the speed fully shadowed left-turn lanes (AZ, CA, IA, KS, LA, MI, (S) in urban areas to be reduced by 10 to 20 mph. MO, MT, NH, OR, WA, WY, and TUC) show the upstream end of the left-turn lane taper beginning at the In the design standards for Oregon the recommended point where the full width of the turn lane first becomes MUTCD formulas are used but the threshold between the use available. of the high-speed formula versus the low-speed formula is 35 mph instead of 45 mph. The design standards for four agencies do not show the upstream end of the left-turn lane taper beginning at the In the design standards for Washington State the recom- point where the full width of the turn lane first becomes mended MUTCD high-speed formula (L = WS) is used for all available. The following are the distances between the point speeds, even low speeds. where the full width of the turn lane first becomes available and the upstream end of the left-turn lane taper for these four In the design standards for Utah the recommended agencies: MUTCD formulas are used to compute L, but only half of L is used for the shifting taper. UT--20 ft OH--100 ft In the design standards for Montana the length of the shift- PA--half of the shifting taper ing taper is calculated from a taper rate based on design speed TX--unspecified distance. and is shown in a chart. The shifting taper rate varies from 10-to-1 for 20 mph (shown as 30 km/h) to 75-to-1 for 70 mph Length of Left-Turn Taper (21 agencies) (shown as 120 km/h). The design standards for 14 agencies show a straight-line The design standards for Iowa and Louisiana show reverse left-turn taper. The following lengths are shown for straight- curves being used to transition from a two-lane section to a line left-turn tapers: three-lane section of roadway. The Iowa design standards specify 12,000 ft for the radii of the reverse curves. The OH--50 ft Louisiana design standards do not specify the radii of the NH--75 ft if the posted speed limit is 40 mph or less and reverse curves. (The Louisiana design standards also show a 100 ft if the posted speed limit is 45 mph or more reverse curve being used to transition from a three-lane sec- MO--100 ft tion back to a two-lane section on the departure side of the CA--120 ft in high-speed rural areas and 60 or 90 ft in intersection.) business, residential, or urban areas WY--150 ft Amount of Widening (21 agencies) PA--one-sixth of the shifting taper AR--one-third of the shifting taper The design standards for 17 agencies (AZ, CA, IA, KS, LA, IA--10-to-1 taper rate MI, MO, MT, NH, OH, OR, PA, TX, UT, WA, WY, and MT--the taper rate, which varies from 8-to-1 for 20 mph TUC) use the shifting taper to widen the roadway until the full- (shown as 30 km/h) to 18-to-1 for 70 mph (shown as
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18 120 km/h), is based on design speed and is shown in cent to the departure lane in the opposite direction. The fol- a table lowing lengths are shown for the gaps in the lane line: WA--the length, which varies from 75 ft for a 25 mph speed limit to 180 ft for a 60 mph speed limit, is based AZ--60 ft if the posted or design speed is less than on the posted speed limit and is shown in a table 40 mph, 90 ft if the posted or design speed is 40 to SD, TN, TX, UT--unspecified length. 50 mph, and 140 ft if the posted or design speed is more than 50 mph The design standards for three agencies show a left-turn TUC--60 ft if the posted speed limit is 35 mph or less, taper comprised of reverse curves. The following lengths are 90 ft if the posted speed limit is 40 or 45 mph, and shown for reverse-curve left-turn tapers: 120 ft if the posted speed limit is 50 mph or more MI--at least 75 ft. ID--length in feet is five times the speed limit in mph KS--unspecified length (reverse curves with a radius of The design standards for Oregon show two options for 150 ft for speeds of 40 mph or less and 300 ft for speeds providing a left-turn taper. One option is to use a set of of 45 mph or more are used to form the left-turn lane reverse curves with the length of the reverse curves, which taper) varies from 98 ft for 35 mph to 180 ft for 65 mph, based on LA--unspecified length. design speed and shown in a table. A second option, which uses the same length as the reverse curves, is to extend the The design standards for three agencies show a gap in the double yellow centerline on the left-hand side of the lane line on the left-hand side of the approach through lane approach through lane for one-eighth of the length and then instead of a marked taper (see Figure 6). The only longitudi- leave a gap in the lane line on the left-hand side of the nal pavement marking in the gap area is the centerline adja- approach through lane instead of a marked taper. FIGURE 6 Gap in lane line instead of marked taper (Source: Michigan DOT pavement markings typical plans).