Design Guidance for Intersection Auxiliary Lanes (2014)

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44 Typical Designs Introduction As part of the questionnaire sent to key state transporta- tion agencies, respondents were asked to identify locations with installations that would be considered best-practice sites. These best-practice sites were to demonstrate preferred design treatments for five design categories: island design, deceleration lane design, double left-turn lane design, triple left-turn lane design, and double right-turn lane design. Colorado, Florida, Maine, Minnesota, North Carolina, and Washington all identified locations for consideration. An underlying question associated with the identification of these locations centered on whether the Green Book pro- vides sufficient guidance for implementing the treatment. Each of the identified sites was examined using aerial imag- ery to gain a better understanding of the key features of the design, as implemented. A single site considered representa- tive of the design treatment was identified for each of the five design categories. This representative site was examined in detail through a case study approach. The case studies focused on the key design features associ- ated with the treatment under consideration. To the extent possible, these design features were quantified and referenced to the appropriate standards. A review of each of the com- pleted case studies was then done to determine if the guid- ance provided by the Green Book would have been sufficient or if supplemental guidance would have been necessary to complete the design. In those cases where supplemental guid- ance was necessary, this supplementary guidance has been summarized. Case Studies Case studies were undertaken to evaluate a typical, best- practice design for five design categories: island design, deceleration lane design, double left-turn lane design, triple left-turn lane design, and double right-turn lane design. Typical Design: Island Context The intersection reviewed for island design is in a suburban area of Lakewood, Colorado, a western suburb of Denver (see Figure 4-1). The primary land use in the area of the inter- section is light-density residential, with commercial uses fronting the primary arterials. State Highway 391, also known as S. Kipling Street, is a four-lane divided arterial with a posted speed limit of 45 mph. West Alameda Avenue is a four-lane arterial with a posted speed limit of 45 mph. All right-turning movements at the intersection are provided a deceleration lane, channelization island, and acceleration lane. The chan- nelization islands are raised, curbed islands. The central area of the islands is paved with a colored, textured pavement and contains sidewalks and curb ramps that are depressed and of different texture than the rest of the island. For this case study, the feature being considered was the island in the northeast quadrant of the intersection (see Figure 4-2). Design Considerations According to AASHTO’s A Policy on Geometric Design of Highways and Streets (4), an island is a defined area between traffic lanes used to control vehicle movements and to pro- vide an area for pedestrian refuge and placement of traffic control devices. The Colorado Roadway Design Guide (68) defines a channelized intersection as an at-grade intersection in which traffic is directed into definite paths by islands. The Colorado Roadway Design Guide further identifies the pur- poses for which islands are generally included in intersection design: • Separation of conflicts. • Control of angle of conflict. • Reduction in excessive pavement areas. C H A P T E R 4

45 • Regulation of traffic and indication of proper use of intersection. • Arrangements to favor a predominant turning movement. • Protection of pedestrians (ADA requirements should be considered). • Protection and storage of turning and crossing vehicles. • Location of traffic control devices. • Access control. The turning roadway is made up of five distinct compo- nents: an approach taper, a deceleration lane, the turning roadway curve, an acceleration lane, and a merging taper. Approach Taper. An approach taper is used to develop the necessary width for the auxiliary lane. This taper allows a driver to recognize that an exclusive lane is being developed and provides a location where some deceleration may occur prior to entering the auxiliary lane. The Colorado Roadway Design Guide recommends a taper ratio of 15:1 for a design speed of 50 mph. At this location, an auxiliary lane width of 20 ft was selected to provide for truck movements. Based on the 15:1 taper requirements, 300 ft of approach taper should be provided. At this location, 350 ft of taper was provided. Deceleration Lane. The Colorado Roadway Design Guide points out that it may not be feasible to provide full decelera- tion lengths on many arterial facilities. As such, it assumes that braking begins “where two-thirds of the lane width is devel- oped” and, thus, the deceleration length begins at that point and extends to the turning roadway curve. Further, a “10 mph differential is commonly considered acceptable on arterial roadways” representing a deceleration of 10 mph within the through lanes and the approach taper. As a continuous- flow turn, it is assumed that the operating speed on the turn- ing roadway curve is 15 mph and that a vehicle entering the deceleration lane will only need to decelerate to 15 mph before the curve, not to a complete stop. When determining deceleration lengths that do not end in a stop condition, the Colorado Roadway Design Guide refers the designer to Exhibit 10-73 of the 2004 AASHTO A Policy on the Geometric Design of Highways and Streets (4) to calculate the required decelera- tion distance. At this location, the design speed is 50 mph. Assuming that 10 mph of deceleration occurs on the through lane and within the initial taper area, the design vehicle speed at the beginning of the deceleration lane is 40 mph. Based on Exhibit 10-73, a deceleration distance of 350 ft must be provided to deceler- ate from 40 mph to 15 mph. This location provides 350 ft of deceleration distance. Turning Roadway Curve. Based on AASHTO’s A Pol- icy on the Geometric Design of Highways and Streets (4), the “principal controls for the design of turning roadways are the alignment of the traveled way edge and the turning road- way width.” The Colorado Roadway Design Guide specifies curve design for the inner edge of pavement, turning road- way pavement width, and approximate island size for three design classifications. The design data for an angle of turn of 90 degrees, which matches this location, is shown in Table 4-1. To accommodate a WB-50, Design Classification C is used at this location, resulting in a minimum curve of 65 ft radius, an offset of 6 ft, and terminal curves of 180 ft radius. This design is shown in Figure 4-3. This design approach permits the right and left wheel tracks of the design vehicle to traverse the traveled way within 2 ft of the edge of the traveled way. While this guidance is generally applicable, designers should always check the turn- ing paths of potential design vehicles to ensure that the final design meets their needs. Acceleration Lane. The Colorado Roadway Design Guide points out that provision for acceleration clear of the through- traffic lanes is a desirable objective on arterial roads and streets. Source: Google Earth™ Mapping Service Figure 4-1. State Highway 391 (S. Kipling Street) and West Alameda Avenue in Lakewood, Colorado. Source: Google Earth™ Mapping Service Figure 4-2. Close-up of northeast quadrant.

46 The distance to achieve a safe and comfortable speed at the point of convergence is based on the difference in speed between the operating speed on the turning roadway curve and the operating speed on the arterial. It is assumed that the operating speed on the turning roadway curve is 15 mph. At this location, a parallel-type acceleration lane is provided, which provides operational and safety benefits through addi- tional time for merging vehicles to find an acceptable open- ing. When determining acceleration lengths that do not begin in a stop condition, the Colorado Roadway Design Guide refers the designer to Exhibit 10-70 of the 2004 Green Book to cal- culate the required acceleration distance. At this location, the design speed is 50 mph. Based on Exhibit 10-70, an acceleration distance of 660 ft must be pro- vided to accelerate from 15 mph to 39 mph. However, at this location, there is a cross street to the north of the intersection. This physically limits the acceleration distance that can be provided. To accommodate this, the acceleration lane begins as full width and is then tapered to a width of 12 ft, which is then present to the cross-street intersection as an auxiliary lane. The configuration used is 130 ft of full 20-ft width, fol- lowed by 170 ft of taper from 20 ft to 12 ft, followed by 200 ft of 12 ft width auxiliary lane. This provides 500 ft of accelera- tion distance. Merging Taper. As with the approach taper design, the Colorado Roadway Design Guide recommends a taper ratio of 15:1 for a design speed of 50 mph. Due to the use of the drop lane described above, the acceleration lane only needs to taper from 20 ft to 12 ft. The configuration used is 130 ft of Three-Centered Curve Angle of Turn (degrees) Design Classification Radii (ft) Offset (ft) Width of Lane (ft) Approximate Island Size (sq ft) 90 A 150-50-150 3.0 14 50 B 150-50-150 5.0 18 80 C 180-65-180 6.0 20 125 Notes: A—Primarily passenger vehicles: permits occasional design single-unit truck to turn with restricted clearances. B—Provides adequately for SU: permits occasional WB-50 to turn with slight encroachment on adjacent traffic lanes. C—Provides fully for WB-50. Asymmetric three-centered compound curves and straight tapers with a simple curve can also be used without significantly altering the width of roadway or corner island size. Table 4-1. Colorado tabular values for treatment of turn lanes (68). Source: A Policy on Geometric Design of Highways and Streets (2004) by the American Association of State Highway and Transportation Officials, Washington, D.C. Used by permission. Figure 4-3. Minimum turning roadway designs with corner islands at urban locations (WB-50 design vehicle) (4).

47 full 20 ft width, followed by 170 ft of taper from 20 ft to 12 ft, followed by 200 ft of 12 ft width auxiliary lane. Corner Island Delineation. The Green Book states that “the island in all instances should be about 2 ft outside the traveled way edges.” It further provides guidance for the delineation of islands to ensure that the approach nose of a curbed island is conspicuous to approaching drivers and clear of vehicle paths so that drivers will not shy away from the island. Details on this guidance are shown in Figure 4-4. The guidance calls for the offset of the approach nose (4 to 6 ft) to be greater than the offset to the face of the curbed island (2 to 3 ft). At this location, the offset of the approach nose is 4 ft, while the offset to the face of the curbed island is 2 ft. This allows the island to be gradually widened to its full width past the approach nose. In addition, the approach nose should be provided with devices to give advance warning to drivers of the island, especially for nighttime operation. In a northern climate, it is also important to clearly delineate the island location because snowfall may obscure the curbed edge. To account for both of these concerns at this location, post-mounted reflectors are positioned on the island near the approach nose, merging nose, and approach corner to help delineate the island location. It is further recommended that all curbed island corners should be rounded with appropriate curve radii for vis- ibility and construction simplicity. It recommends that the approach nose and merging nose use a 2- to 3-ft radius and the approach corner use a radius of 2 to 5 ft. At this location, the approach nose uses a 2.5-ft radius, the merging nose a 2-ft radius, and the approach corner a 4-ft radius. Additional Considerations Pedestrians. It is preferable that passages for pedestrians and wheelchairs on raised islands be cut-through or at-grade installations. This better facilitates guidance for visually impaired pedestrians and eliminates the need to traverse additional grades for wheelchairs. At this location, the pedes- trian crossings within the channelized islands, while not at grade, are depressed as compared to the surrounding island. In addition, they are of different texture then other parts of the island. Reduced Visibility. The Colorado Roadway Design Guide points out that curbed islands can be difficult to see at night. As such, intersections using curbed islands should provide fixed-source lighting or appropriate delineation. At this loca- tion, both are provided. Snow. An additional consideration for islands in northern climates is their potential impact on snow removal activities. Colorado Roadway Design Guide recommends that designers consider the implementation of plowable end treatments. While this was not done at this location, post-mounted reflec- tors are installed at each of the three island corners (approach nose, merging nose, and approach corner) to provide clear delineation of the island dimensions even during inclement weather. Functional Intersection Area. The Colorado Roadway Design Guide lists functional intersection area as one of the five basic elements that enter into design considerations of intersections. The functional intersection area includes areas both upstream and downstream of the intersection. Source: A Policy on Geometric Design of Highways and Streets (2004) by the American Association of State Highway and Transportation Officials, Washington, D.C. Used by permission. Figure 4-4. Details of corner island design for turning roadways (urban location, large island) (4).

48 While this area is variable, based on local conditions, it is accepted that vehicles leaving a major roadway disrupt traffic flow, which affects overall operation and safety. Thus, good access management is critical in planning and designing a roadway so that it performs according to its functional classification. At this location, there is no access within the intersection functional area in three of the four quadrants. Typical Design: Deceleration Lane Context The intersection reviewed for deceleration is in a rural area to the west of Fuquay-Varina, North Carolina (see Fig- ure 4-5). The primary land use in the area of the inter section is low-density residential. State Route 42, also known as W. Academy Street, is a two-lane undivided highway. This intersection along State Route 42 is where the cross-section transitions from a curb-and-gutter section east of the inter- section to unpaved shoulders west of the intersection. West of the intersection, State Route 42 consists of two 11-ft lanes with a posted speed of 45 mph. East of the intersection, it consists of two 15-ft lanes, also with a posted speed of 45 mph. Coley Farm Road is a two-lane arterial with a posted speed of 35 mph north of the intersection and an assumed speed of 35 mph (based on North Carolina’s general statutes) south of the intersection as no posted speed was found there. This case study examined the east and west leg of the intersection and focused on the design of the deceleration lanes provided for turning traffic. The design speed for State Route 42 is 50 mph. Design Considerations According to the 2004 AASHTO A Policy on Geometric Design of Highways and Streets (4), “Provision for decelera- tion clear of the through-traffic lanes is a desirable objective on arterial roads and streets and should be incorporated into design, whenever practical.” The North Carolina Roadway Design Manual (69) considers some deceleration within the taper area of the developing auxiliary lane and, thus, reduces the required deceleration distances appropri- ately. The design of a deceleration lane based on the North Carolina Roadway Design Manual is presented in Table 4-2 and Figure 4-6. Source: Google Earth™ Mapping Service Figure 4-5. State Route 42 (W. Academy Street) and Coley Farm Road in Fuquay-Varina, North Carolina. Design Speed (mph) Posted Speed (mph) Minimum Deceleration Length (D) Desirable Deceleration Length (D) Bay Taper Length (T) Approach/Departure Taper (A) 30 ≤ 25 100' 150' 75' A = WS2/60 (If S ≤ 40 mph) 35 30 100' 150' 75' A = WS (If S > 40 mph) 40 35 150' 200' 100' S = Design Speed 45 40 150' 250' 100' W = Width of Lateral Shift 50 45 150' 300' 100' Storage length for waiting vehicles should be calculated based on the latest version of the Highway Capacity Manual or Policy on Street and Driveway Access to North Carolina Highways. 55 50 200' 500' 150’ 60 55 200' 575' 200' Table 4-2. North Carolina tabular values for treatment of turn lanes (69). Figure 4-6. Recommended treatment for turn lanes (69).

49 Intersection East Leg The development of the left-turn deceleration lane for traffic traveling westbound on State Route 42 was done sym- metrically about the centerline at this location. The roadway cross-section approaching the intersection area consists of two 15-ft lanes (excluding curb and gutter). This is then devel- oped into three 11-ft lanes within the intersection (excluding curb and gutter)—a through lane in each direction and a westbound left-turn lane. To accommodate the intersection lanes, the pavement must be widened by 1.5 ft on each side of the centerline (W=1.5 ft). Table 4-2 provides formulas for calculating the approach taper lengths based on the design speed (S) and width of lateral shift (W). Because the design speed is greater than 40 mph, the formula A=WS with values of S=50 and W=1.5 was used to calculate a value for the approach taper (A) of 75 ft. Further, based on Figure 4-6, the dimensions for the channelization of the deceleration lane can be identified using a value of 2⁄3 A (50 ft) and T (100 ft). However, at this location, a value of T=75 ft was used. Intersection West Leg The development of both the left-turn and right-turn decel- eration lanes for traffic traveling eastbound on State Route 42 was done symmetrically about the centerline at this location. The roadway cross-section approaching the intersection area consists of two 11-ft lanes. This is then developed into four 10-ft lanes within the intersection—a through lane in each direction, an eastbound right-turn lane, and an eastbound left-turn lane. To accommodate the intersection lanes, the pavement must be widened by 9 ft on each side of the center line (W=9 ft). Table 4-2 provides formulas for calculating the approach taper lengths based on the design speed (S) and width of lateral shift (W). Because the design speed is greater than 40 mph, the formula A=WS with values of S=50 and W=9 was used to calculate a value for the approach taper (A) of 450 ft. Further, based on Figure 4-6, the dimensions for the chan- nelization of the deceleration lane can be identified using a value of 2⁄3 A (300 ft) and T (100 ft). However, at this loca- tion, a value of T=75 ft was used. Additional Considerations The North Carolina DOT also maintains a series of Road- way Standard Drawings. When providing for deceleration lanes, the standard drawing for Pavement Markings: Turn Lanes specifies their treatment within the intersection area (see Figure 4-7). Under this guidance, skip striping is used to outline the entry taper area. Past this point, solid striping is used to delineate the auxiliary lane. At this location, this results in the skip striping being used on both the east and west leg of the intersection for the left-turn auxiliary lanes from the beginning of the entering taper to 75 ft past the start of the taper. At this point, all striping reverts to solid. In addi- tion, on the west leg of the intersection, skip striping is used to delineate the entry taper for the right-turn lane from the beginning of the entering taper to 300 ft past the entering taper. At this point, all striping reverts to solid. Typical Design: Double Right-Turn Lanes Context The intersection reviewed for double right-turn lanes is in an urbanized area of Wellington, Florida (see Figure 4-8). The primary land use in the area of the intersection is residen- tial. The southwest quadrant of this intersection, however, is a large regional mall—the Mall at Worthington Green—and there is commercial development facing the major arterials. State Road 7, also known as Range Line Road, runs approxi- mately north/south at this location. The cross-section of State Road 7 is an eight-lane divided arterial with a bike lane in each direction. It has a posted speed limit of 50 mph. State Road 882, also known as Forest Hill Boulevard, runs approxi- mately east/west at this location. It is a six-lane divided arte- rial with a bike lane in each direction. It has a posted speed limit of 50 mph to the east of the intersection and 45 mph to the west of the intersection. This case study examined the west leg of the intersection and focused on the design of the double right-turn lanes pro- vided for traffic turning from eastbound State Road 882 to southbound State Road 7 (see Figure 4-9). The intersection was recently upgraded with a design speed of 60 mph. Source: North Carolina Roadway Standard Drawings, North Carolina Department of Transportation, 2006. Figure 4-7. Pavement markings for turn bays 125 to 200 ft in length.

50 Design Considerations According to the Florida Manual of Uniform Minimum Stan- dards for Design, Construction and Maintenance for Streets and Highways (referred to as the Florida Greenbook) (70), “Storage (or deceleration lanes) to protect turning vehicles should be provided, particularly where turning volumes are significant” to accommodate speed change and maneuvering of turning traffic. Based on the projected demand for turns on this leg of the intersection, double right-turn lanes were provided. Auxiliary Lane Design The Florida Intersection Design Guide (71) details the require- ments for the design of the auxiliary lanes to accommodate turn lanes. Based on this guidance, the auxiliary lane is defined by three key components: entering taper, deceleration length, and storage length. The minimum lengths associated with these key components are outlined in Table 4-3 and Fig- ure 4-10. The guidance focuses on left-turn examples, but the approach is the same for right-turn auxiliary lanes. The entering taper requirement is 100 ft for double left- turn lanes (see Figure 4-10). The short taper length is meant to “provide approaching road users with positive identification” of the upcoming auxiliary lane and to provide for the greatest length of full-width auxiliary lanes to the extent possible. At this location, a taper of 100 ft is provided. To account for the actual path that road users will use to access the auxiliary lanes based on speed conditions, two clear- ance distances (L1 and L3) are identified (see Figure 4-10). These distances also help identify the locations at which to begin the striping of the lane lines for the auxiliary lanes. Based on a design speed of 60 mph, L1 has a value of 145 ft and L3 has a value of 230 ft. At this location, the striping for both turn lanes begins at 230 ft from the beginning of the entering taper. The outside turn lane striping did not begin at 145 ft due to the requirements associated with providing for bike lanes discussed below. The total deceleration length (shown as L in Figure 4-10) is “that needed for a safe and comfortable stop from the design speed of the highway.” This distance is made up of the clear- ance distance, L1, and the brake-to-stop distance, L2. The clearance distance identifies the point at which vehicles have entered the auxiliary lane. In urbanized areas, it is assumed a turning vehicle decelerates by 10 mph while traversing the through lane and this is clearance distance. However, for design Source: Google Earth™ Mapping Service Figure 4-8. State Road 7 (Range Line Road) and State Road 882 (Forest Hill Boulevard) in Wellington, Florida. Source: Google Earth™ Mapping Service Figure 4-9. Close-up of double right-turn lanes. Turn Lanes—Curbed and Uncurbed Medians Design Speed (mph) Entry Speed (mph) Clearance Distance L1 (ft) Urban Conditions Rural Conditions Brake- To-Stop Distance L2 (ft) Total Decel. Distance L (ft) Clearance Distance L3 (ft) Brake- To-Stop Distance L2 (ft) Total Decel. Distance L (ft) Clearance Distance L3 (ft) 35 25 70 75 145 110 ---- ---- ---- 40 30 80 75 155 120 ---- ---- ---- 45 35 85 100 185 135 ---- ---- ---- 50 40/44 105 135 240 160 185 290 160 55 48 125 ---- ---- ---- 225 350 195 60 52 145 ---- ---- ---- 260 405 230 65 55 170 ---- ---- ---- 290 460 270 Table 4-3. Minimum deceleration lengths based on design speed (71).

51 speeds over 50 mph, the average running speed is used. Thus, for a design speed of 60 mph, the assumed entry speed is 52 mph. The brake-to-stop distance then represents the dis- tance to bring a vehicle to a stop at the assumed entry speed. At this location, L1 has a value of 145 ft and L2 has a value of 260 ft, resulting in a total deceleration length, L, of 405 ft. Finally, the storage length accounts for the distance neces- sary to accommodate the number of vehicles likely to accu- mulate during a critical period. It is assumed that when two turning lanes are used, the storage length required is approxi- mately half of what would be necessary for a single lane (note: not all states assume an even distribution of queue between the two provided turn lanes). At this location, 220 ft of queue storage is provided in each turn lane, which represents a total storage length of 440 ft. Bike Lane Design When providing for the through movement of a bicycle lane at a major intersection in an urbanized area with curb and gutter, the Florida Greenbook specifies the location and treatment of that bicycle lane within the intersection area (see Figure 4-11). The bicycle lane is maintained between the through and turning vehicle lanes. Under this guidance, skip striping is used to outline the area where the bicycle and right-turning traffic weaving area is located. When this is Figure 4-10. Double left-turn with raised separation (71). Figure 4-11. Major intersection with separate right-turn lane urban typical section (curb and gutter) (70).

52 applied to double turn lanes, this skip striping is extended to the beginning of the outermost turn lane. At this location, this results in the skip striping being used from the beginning of the entering taper to 230 ft past the entering taper. At this point, all striping reverts to solid. Turning Radii Design The Florida Greenbook specifies that the design of corner radii should be based on selected design vehicles. According to the guidance, in an urban area the designer must balance several key issues: the needs of the road users using them, the amount of right-of-way available, the angle of turn between intersection legs, the number of pedestrians using the cross- walk, the width and number of lanes on the intersecting street, and the speeds on each street. The minimum-edge-of-traveled- way design is based on the angle of turn and design vehicle (see Figure 4-12). At this location, the angle of turn is 90 degrees and the design vehicle is a WB-62. Based on Table 4-4, the sym- metric curve radii for a three-centered compound curve are 400-70-400 ft with an offset of 10 ft. Additional Considerations When double right-turn lanes are provided, the design must provide intersection turning radii to accommodate vehicles turning two abreast. In Florida, most intersections on the state highway system must be able to accommodate, at a minimum, an SU truck and passenger car turning simulta- neously. Depending on the design vehicles selected, the width of the receiving throat may need to be expanded to accommo- date the necessary turning radii. At this location, no additional widening was necessary. Based on the turning template analysis performed to deter- mine if additional lane widening was necessary for the receiv- ing throat, the delineation between the two swept paths can be established. To provide positive guidance to road users, this delineation is often physically marked out through the intersection using skip striping. These turn guide lines in Florida are specified as a 2-ft skip with a 4-ft gap dotted line. At this location, skip striping was provided. Typical Design: Triple Left-Turn Lanes Context The triple left-turn lane intersection is in an urbanized area to the west of West Palm Beach, Florida (see Figure 4-13). The surrounding area is dominated by residential uses, with retail spaces along major arterial corridors. State Road 7, also known as Range Line Road, runs approximately north/south Figure 4-12. Reference turn angle for turning roadway designs (71). Angle of Turn (degrees) Design Vehicle 3-Centered Compound Curve Radii (ft) Symmetric Offset (ft) Curve Radii (ft) Asymmetric (ft) 90 P 100-20-100 2.5 ---- ---- SU 120-40-120 2.0 ---- ---- WB-40 120-40-120 5.0 120-40-200 2.0-6.5 WB-50 180-60-180 6.5 120-40-200 2.0-10.0 WB-62 400-70-400 10.0 160-70-360 6.0-10.0 WB-67 440-65-440 10.0 200-70-600 1.0-11.0 WB-100T 250-70-250 4.5 200-70-300 1.0-5.0 WB-109D 700-110-700 6.5 100-95-550 2.0-11.5 Table 4-4. Edge-of-traveled-way design for turns at intersections (71). Figure 4-13. State Road 7 (Range Line Road) and State Road 704 (Okeechobee Boulevard) in West Palm Beach, Florida. Source: Google Earth™ Mapping Service

53 at this location. To the north of this intersection, State Road 7 is a two-lane undivided arterial. However, as it approaches the intersection it widens to a four-lane divided arterial and, south of the intersection, widens again to a six-lane divided arterial. It has a posted speed limit of 45 mph and a bike lane in each direction. State Road 704, also known as Okeechobee Boulevard, runs approximately east/west at this location. It is an eight-lane divided arterial with a posted speed limit of 50 mph and a bike lane in each direction (see Figure 4-14). This intersection provides for triple left turns on two of the approaches—northbound State Road 7 and westbound State Road 704. Because the cross-section of State Road 7 changes from a six-lane to four-lane section north of the inter- section, one of the through lanes is dropped at the intersec- tion (sometimes referred to as a trap lane) as a left-turn lane for the south leg of the intersection forming one of the lanes of the triple left-turn provided on that approach (a Type B Triple Left Turn). On the east leg of the intersection (see Fig- ure 4-15) for the westbound State Road 704 approach, the triple left turn is fully developed as auxiliary lanes (a Type A Triple Left Turn). As such, this case study focused on the east leg of the intersection and the triple left turn provided for traffic traveling from westbound State Road 704 to south- bound State Road 7. The design speed for this approach is 60 mph (10 mph over the posted speed). Design Considerations The Florida Intersection Design Guide (71) details the requirements for the design of the auxiliary lanes to accom- modate both single and double left-turn lanes. While there is no specific design guidance on development of triple left-turn lanes, the concepts associated with both the single and double left-turn lanes can be adapted. Under this guid- ance, the auxiliary lane is defined by three key components: entering taper, deceleration length, and storage length. The minimum lengths associated with these key components are outlined in Table 4-3 and Figure 4-10. The entering taper requirement for a single left-turn lane is 50 ft, while it is 100 ft for double left-turn lanes. The short taper length is meant to “provide approaching road users with positive identification” of the upcoming auxiliary lane and to provide for the greatest length of full-width auxiliary lanes to the extent possible. This concept has been expanded for use with a triple left-turn condition by expanding the taper length to 150 ft at this location. To account for the actual path that road users will use to access the auxiliary lanes based on speed conditions, two clearance distances (L1 and L3) are identified (see Figure 4-10). These distances also help identify where to begin the striping of the lane lines for the auxiliary lanes. Based on a design speed of 60 mph, L1 has a value of 145 ft and L3 has a value of 230 ft. At this location, the striping for the outside turn lane begins at 175 ft and for both the middle and the inside turn lane begins at 230 ft from the beginning of the entering taper. The total deceleration length (shown as L in Figure 4-10) is “that needed for a safe and comfortable stop from the design speed of the highway.” This distance is made up of the clear- ance distance, L1, and the brake-to-stop distance, L2. The clearance distance identifies the point at which vehicles have entered the auxiliary lane. In urbanized areas, it is assumed a turning vehicle decelerates by 10 mph while traversing the through lane and this clearance distance. However, for design speeds over 50 mph, the average running speed is used. Thus, for a design speed of 60 mph, the assumed entry speed is 52 mph. The brake-to-stop distance then represents the distance to bring a vehicle to a stop at the assumed entry speed. At this location, L1 has a value of 145 ft and L2 has a value of 260 ft, resulting in a total deceleration length, L, of 405 ft. Figure 4-14. Close-up of triple left-turn lanes from northbound approach. Source: Google Earth™ Mapping Service Figure 4-15. Close-up of westbound approach. Source: Google Earth™ Mapping Service

54 Finally, the storage length accounts for the distance necessary to accommodate the number of vehicles likely to accumulate during a critical period. It is assumed that when two turning lanes are used, the storage length required is approximately half of what would be necessary for a single lane (note: not all states assume an even distribution of queue between the two pro- vided turn lanes). Expanding this approach to represent triple left-turns at this location, 285 ft of queue storage is provided in each turn lane, which represents a total storage length of 855 ft. Additional Considerations Because of the added length of the entry taper used to develop the triple left-turn auxiliary lanes, the entry area into these auxiliary lanes is larger than typical. To ensure that drivers receive positive guidance in this area, additional skip striping was used from the beginning of the entry taper to the beginning of the outside left-turn lane line. This skip striping provides a clear delineation of the inside through lane within this transition area. When triple left-turn lanes are provided, the design must provide intersection turning radii to accommodate vehicles turning simultaneously. For double left-turn lanes in Florida, most intersections on the state highway system must be able to accommodate at a minimum an SU truck and passenger car turning simultaneously. Depending on the design vehicles selected, the width of the receiving throat may need to be expanded to accommodate the necessary turning radii. At this location, no additional widening was necessary. Based on the turning template analysis performed to deter- mine if additional lane widening was necessary for the receiv- ing throat, the delineation between the swept paths can be established. To provide positive guidance to road users, this delineation is often physically marked out through the inter- section using skip striping. These turn guidelines in Florida are specified as a 2-ft skip with a 4-ft gap dotted line. At this location, skip striping was provided. The Florida Intersection Design Guide (71) identifies that intersections are defined by both their physical and func- tional areas. The functional area of an intersection extends both upstream and downstream from the physical area and includes auxiliary lanes and associated channelization. Drive- ways should not be within the functional area to improve both operational and safety characteristics of the inter section. At this location, access is limited to roadway segments outside the functional area. Typical Design: Double Left-Turn Lanes Context This intersection is in a rapidly urbanizing corridor south- east of Tallahassee, Florida (see Figure 4-16). Capital Circle is a non-limited access beltway that encircles approximately three-quarters of Tallahassee on the west, south, and east. This section of the arterial, Capital Circle SE, is also the align- ment of US Route 319 and Florida State Road 261 and runs approximately north/south at this location. Capital Circle SE cross-section is three through lanes and a bike lane in each direction with a posted speed of 45 mph. Blair Stone Road has a cross-section of two through lanes and a bike lane in each direction with a posted speed limit of 35 mph. This case study examined the east leg of the intersection and focused on the design of the double left-turn lanes pro- vided for traffic turning from westbound Blair Stone Road to southbound Capital Circle SE. The design speed for this approach is 40 mph (5 mph over the posted speed). However, all approaches at the intersection provide double left-turn lanes. Design Considerations According to the Florida Greenbook, “Storage (or decel- eration lanes) to protect turning vehicles should be provided, particularly where turning volumes are significant.” Based on the projected demand for turns at this intersection, double left-turn lanes were provided on each of the approaches. The Florida Intersection Design Guide (71) details the require- ments for the design of the auxiliary lanes to accommodate the double left-turn lanes. Based on this guidance, the auxiliary lane is defined by three key components: entering taper, decelera- tion length, and storage length. The minimum lengths associ- ated with these key components are outlined in Figure 4-10 and Table 4-3. The entering taper requirement is 100 ft for double left- turn lanes (see Figure 4-10). The short taper length is meant Source: Google Earth™ Mapping Service Figure 4-16. Capital Circle SE and Blair Stone Road, in Tallahassee, Florida.

55 to “provide approaching road users with positive identifica- tion” of the upcoming auxiliary lane and to provide for the greatest length of full-width auxiliary lanes to the extent pos- sible. At this location, a taper of 100 ft is provided. To account for the actual path that road users will use to access the auxiliary lanes based on speed conditions, two clear- ance distances (L1 and L3) are identified (see Figure 4-10). These distances also help identify the locations at which to begin the striping of the lane lines for the auxiliary lanes. Based on a design speed of 40 mph, L1 has a value of 80 ft and L3 has a value of 120 ft. At this location, the striping for the outside turn lane begins at 80 ft and for the inside turn lane begins at 120 ft from the beginning of the entering taper. The total deceleration length (shown as L in Figure 4-10) is “that needed for a safe and comfortable stop from the design speed of the highway.” This distance is made up of the clear- ance distance, L1, and the brake-to-stop distance, L2. The clearance distance identifies the point at which vehicles have entered the auxiliary lane. In urbanized areas, it is assumed a turning vehicle decelerates by 10 mph while traversing the through lane and this clearance distance. The brake-to-stop distance then represents the distance to bring a vehicle to a stop at the assumed entry speed. At this location, L1 has a value of 80 ft and L2 has a value of 75 ft, resulting in a total deceleration length, L, of 155 ft. Finally, the storage length accounts for the distance neces- sary to accommodate the number of vehicles likely to accu- mulate during a critical period. It is assumed that when two turning lanes are used, the storage length required is approxi- mately half of what would be necessary for a single lane (note: not all states assume an even distribution of queue between the two provided turn lanes). At this location, 65 ft of queue storage is provided in each turn lane, which represents a total storage length of 130 ft. Additional Considerations When double left-turn lanes are provided, the design must provide intersection turning radii to accommodate vehicles turning two abreast. In Florida, most intersections on the state highway system must be able to accommodate at a mini- mum an SU truck and passenger car turning simultaneously (see Figure 4-17). Depending on the design vehicles selected, the width of the receiving throat may need to be expanded to accommodate the necessary turning radii. At this location, no additional widening was necessary. Based on the turning template analysis performed to deter- mine if additional lane widening was necessary for the receiv- ing throat, the delineation between the two swept paths can be established. To provide positive guidance to road users, this delineation is often physically marked out through the inter- section using skip striping. These turn guidelines in Florida are specified as a 2-ft skip with a 4-ft gap dotted line. At this location, skip striping was provided. At this location, additional emphasis was undertaken to identify pedestrian crossing locations with pavement that has been both textured (to appear as brick) and colored. Supplementary Guidance When the case studies were examined to determine if supplementary guidance above and beyond that in the 2011 version of the Green Book would be beneficial, it was deter- mined that no supplementary guidance was necessary for island design, only minor supplementary guidance was necessary for multiple turn lanes to address the concept of a clearance distance, and supplementary guidance was needed for devel- opment of deceleration lanes for undivided roadways. This supplementary guidance is summarized below. Multiple Turn Lanes. Section 9.7 Auxiliary Lanes of the 2011 Green Book (2) provides needed guidance on identify- ing the functional area of an intersection and determining the deceleration length necessary for developing auxiliary lanes. This section also provides guidance on taper lengths and examples of taper design for development of the auxiliary lanes. Further, this section provides guidance on when dou- ble left-turn lanes should be implemented (where left-turn Figure 4-17. SU truck and passenger car turning simultaneously (71).

56 volumes exceed 300 veh/hr) and how to account for offtrack- ing and swept path widths and provides suggestions on the use of skip striping through the intersection for positive guid- ance. However, this section does not discuss the development of auxiliary lane pavement markings for use in multiple turn lane configurations. Although this may be viewed as a pavement marking issue, successful implementation through proper location of these pavement markings can affect the operational characteristics of the multiple turning lane configuration. Further, the 2009 Manual on Uniform Traffic Control Devices (72) does not pro- vide sufficient guidance for determining the beginning of turn lane lines within the transition area of the auxiliary lane. Without positive guidance for drivers within the approach transition to the auxiliary lane, proper vehicle alignment is more difficult, causing more turbulent flow, and, thus, may affect operational characteristics. The information presented in the 2011 Green Book identi- fies the full deceleration length as the sum of the taper dis- tance to begin deceleration and complete lateral movement and the distance traveled to complete deceleration to a stop. Florida DOT has identified a clearance distance to account for the actual path that road users will use to access the aux- iliary lane based on speed conditions. To account for this, it is suggested that Figure 9-49, Table 9-22, and the text for the subsection Taper Length of the 2011 Green Book be updated to identify and quantify this clearance distance. Figure 9-49 of the Green Book should be updated to show both single- and double-lane auxiliary lanes, instead of only a single-lane auxiliary lane. The complexity of this figure should be increased to identify the clearance distance. As shown in Figure 4-18, the clearance distances are called out as L1 and L3 and represent the location recommended for the beginning of lane line markings for the auxiliary lane. FDOT also provides guidance on the minimum clear- ance distances and deceleration distances based on design speed of the roadway (see Table 4-3). However, the values pre- sented are not as conservative as those presented in the Green Book. A direct comparison can be made for design speeds of 40 mph, 50 mph, and 60 mph (see Table 4-5) between AASHTO, North Carolina Department of Transportation (NCDOT), and FDOT minimum deceleration lengths. This shows that FDOT full deceleration distances are 46 to 77% higher than AASHTO values, and NCDOT values are 5 to 40% higher when compared to AASHTO values. As such, Table 4-5. Comparison of AASHTO, NCDOT, and FDOT full deceleration lengths. Speed (mph) Deceleration Distance (ft) Green Book (2) NCDOT (69) FDOT (70) 30 160 150 35 150 145 40 275 200 155 45 250 185 50 425 300 240/290 55 500 350 60 605 575 405 65 460 70 820 Single Left-Turn Lane Double Left-Turn Lane Figure 4-18. Components of an auxiliary lane (71).

57 sufficient width to accommodate the development of the auxiliary lane. Additional guidance is necessary for illustrat- ing the development of deceleration lanes for undivided road- ways, resulting in a flared intersection. Although the current guidance outlines the general characteristics of a flared inter- section in Section 9.3.2, Four-Leg Intersection, no specific guidance is given on how to provide the necessary widening. To account for this, a subsection on flared intersections could be added under Section 9.7 Auxiliary Lanes; the material for this subsection could be based on the corresponding infor- mation from the North Carolina Roadway Design Manual (69) described previously in this chapter. identifying values for use in quantifying the clearance dis- tance are not clear. The text presented in the 2011 Green Book for the subsection, Taper Length, could be updated to identify the concept of a clearance distance, but further investigation will be necessary to identify appropriate clear- ance distance values for use in updating Table 9-22 of the Green Book. Deceleration Lane Design. Section 9.7, Auxiliary Lanes, of the 2011 Green Book provides guidance on developing auxiliary lanes. However, the illustrated applications focus on those that occur on roadways that provide a median of