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29 4.1 Introduction Chapter 4 addresses geometric design and access management to accommodate trucks. With the exception of some industrial and port locations, where the traffic stream may consist almost exclusively of trucks, very few roadway segments and intersections are designed solely to serve trucks. At most locations, trucks constitute only a limited portion of the motor vehicle traffic stream, and other travel modesâincluding automobiles, bicyclists, pedestrians, and transitâmust also be served. There are inherent conflicts between the needs of these various modes such that designing solely for trucks might make travel inefficient or inconvenient for other modes. Thus, for the most part, roadways and intersections are not designed for trucks, but are designed to accommodate trucks, while also serving other travel modes. This chapter presents the design vehicles that are used to represent the maneuverability and turning paths of trucks in the design of intersections, interchanges, driveways, roadways, and work zones to accommodate trucks. Designs developed by each transportation agency should be reasonably consistent, with particular issues addressed in a consistent manner, to enhance driver expectancy for both motorists and truck drivers. 4.2 Design Vehicles One or more design vehicles is selected to guide the design of each road improvement project. The design vehicle for any given street or highway is not necessarily the largest vehicle that ever uses that street or highway but rather a vehicle that uses the street or highway with sufficient frequency to be chosen as a basis for design. In selecting appropriate design vehicle(s), roadway designers consider the vehicle mix likely to use the road and the character of the development served by the road. The primary purposes for which design vehicles are selected are as follows: â¢ To minimize encroachment by larger vehicles on curbs, shoulders, adjacent lanes, and opposing lanes as those vehicles traverse roadways and intersections. â¢ To estimate the storage space needed for larger vehicles in median opening areas and auxiliary lanes. 4.2.1 AASHTO Design Vehicles The AASHTO A Policy on Geometric Design of Highways and Streets, commonly known as the Green Book, presents dimensions and performance criteria for nine types of trucks that may be used as design vehicles for particular projects (AASHTO 2018). Table 1 contains a C H A P T E R 4 Geometric Design and Access Management to Accommodate Trucks
Design Vehicle Type Symbol Dimensions (ft) Overall Overhang WB1 WB2 S T WB3 WB4 Typical Kingpin to Center of Rear Tandem AxleHeight Width Length Front Rear Single-Unit Trucks Single-Unit Truck SU-30 11.0â13.5 8.0 30.0 4.0 6.0 20.0 â â â â â â Single-Unit Truck (three axle) SU-40 11.0â13.5 8.0 39.5 4.0 10.5 25.0 â â â â â â Combination Trucks Intermediate Semitrailer WB-40 13.5 8.0 45.5 3.0 4.5a 12.5 25.5 â â â â 25.5 Interstate Semitrailer WB-62* 13.5 8.5 69.0 4.0 4.5a 19.5 41.0 â â â â 41.0 Interstate Semitrailer WB-67** 13.5 8.5 73.5 4.0 4.5a 19.5 45.5 â â â â 45.5 âDouble-Bottomâ Semitrailer/Trailer WB-67D 13.5 8.5 72.3 2.3 3.0 11.0 23.0 3.0b 7.0b 22.5 â 23.0 Rocky Mountain Double Semitrailer/Trailer WB-92D 13.5 8.5 97.3 2.3 3.0 17.5 40.0 4.5 7.0 22.5 â 40.5 Triple-Semitrailer/Trailer WB-100T 13.5 8.5 104.8 2.3 3.0 11.0 22.5 3.0c 7.0c 22.5 22.5 23.0 Turnpike Double Semitrailer/Trailer WB-109D* 13.5 8.5 114.0 2.3 4.5a 12.2 40.0 4.5d 10.0d 40.0 â 40.5 *Design vehicle with 48.0-ft trailer as adopted in 1982 STAA. **Design vehicle with 53.0-ft trailer as grandfathered in with 1982 STAA. aThis is the length of the overhang from the back axle of the tandem axle assembly. bCombined dimension is typically 10.0 ft. c Combined dimension is typically 10.0 ft. dCombined dimension is typically 12.5 ft. â¢ WB1, WB2, WB3, and WB4 are the effective vehicle wheelbases, or distances between axle groups, starting at the front, and working toward the back of each unit. â¢ S is the distance from the rear effective axle to the hitch point or point of articulation. â¢ T is the distance from the hitch point or point of articulation measured back to the center of the next axle or the center of the tandem axle assembly. Table 1. Design vehicle dimensions (adapted from the Green Book).
Geometric Design and Access Management to Accommodate Trucks 31 summary of the characteristics of these truck design vehicles. The overall length of a truck is the distance from the front to the rear of the vehicle not including appurtenances, such as loading/ unloading devices, resilient bumpers, or aerodynamic devices. Table 1 includes the following design vehicles: â¢ Two single-unit truck design vehicles: the two-axle SU-30, with an overall length of 30 ft; and the three-axle SU-40, with an overall length of nearly 40 ft. â¢ Three tractor-semitrailer trucks: the four-axle WB-40 with a trailer length of 33 ft; the five- axle WB-62, with a trailer length of 48 ft; and the five-axle WB-67 with a trailer length of 53 ft. â¢ One twin âdouble-bottomâ tractor-semitrailer-trailer truck: the five-axle WB-67D with two 28.5-ft trailers. â¢ Three longer combination vehicles (LCVs): the seven-axle tractor-semitrailer-trailer WB-92D with one 48-ft and one 28.5-ft trailer, known as a âRocky Mountain Doubleâ; the seven-axle tractor-semitrailer-trailer WB-100T triple-trailer truck; and the nine-axle tractor-semitrailer- trailer WB-109D with two 48-ft trailers, known as a âTurnpike Double.â The sizes of the WB-62, WB-67, and WB-67D design vehicles are largely set by the 1982 STAA. Although the STAA does not control the overall length of trucks, it does require states to permit maximum trailer lengths of at least 48 ft for tractor-semitrailer trucks on the NN of truck routes (designated by the U.S. Secretary of Transportation in consultation with the states), and access routes to and from the NN. The majority of states permit tractor-semitrailer trucks with 53-ft trailers to operate on the NN. Some states allow tractor-semitrailer trucks with 53-ft trailers but limit the KPRA to 41 ft or less, which limits the space the truck needs to make a turning maneuver. Many tractor-semitrailer trucks have the capability to move their rear axles forward and back to accommodate such regulations. The STAA set a maximum trailer length of 28.5 ft for tractor-semitrailer-trailer trucks. Trucks with two or more trailers that exceed the STAA trailer length limits, including all triple-trailer trucks, are classified as LCVs, and are generally allowed to operate only where they operated prior to enactment of the STAA. Only limited exceptions to the STAA restrictions on LCV operation have been made since 1982. For a more detailed discussion of the NN and its access routes, see Section 3.2 on truck routing. The STAA also established a uniform nationwide maximum truck weight limit of 80,000 lb for the NN and its access routes, with maximum weights of 20,000 lb on any single axle and 34,000 lb on any double or tandem axle. With limited exceptions, these weight limits can only be exceeded where higher limits were in place when the STAA was enacted in 1982. Up through the 1994 edition, the Green Book included a design vehicle known as the WB-50 that was intermediate in size between the WB-40 and the WB-62 design vehicles. Trucks like the WB-50 were once common on U.S. highways, but the WB-50 has been dropped from AASHTO policy because trucks of this size no longer exist, having been replaced by trucks like the WB-62 and WB-67. For this reason, the WB-50 is no longer appropriate for use in design. 4.2.2 Selection of Design Vehicles The selection of an appropriate design vehicle is generally a project-level design decision. The Green Book states that, in selecting the design vehicle for any roadway, the designer should con- sider the largest design vehicle that is likely to use that facility with considerable frequency or a design vehicle with particular characteristics appropriate to a particular location (AASHTO 2018). Some state transportation agencies have a formal process for the selection of design vehicles.
32 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide Truck count data, gathered as part of the transportation planning process, can assist agencies in the selection of appropriate design vehicles for particular projects or corridors. The FHWA Traffic Monitoring Guide (FHWA 2016) requires vehicle classification counts to address 13 specific vehicle classes (see Figure 8). While the 13 vehicle classes do not correspond directly to the AASHTO design vehicles, classification count data can provide information on what types of vehicles can be expected for a specific project or corridor. Truck routes may justify the choice of a larger design vehicle than other routes. The WB-67 tractor-semitrailer truck with a 53-ft trailer may be the appropriate design vehicle for many truck routes. Given the STAA legislation, the WB-67 has become the most common size truck on many truck routes throughout the United States, not just on freeways but on arterial roads and streets in both rural and urban areas as well. For example, most grocery stores and other Figure 8. Thirteen vehicle categories required by FHWA for vehicle classification counts (FHWA 2016).
Geometric Design and Access Management to Accommodate Trucks 33 retail businesses throughout the United States get their deliveries from WB-67 trucks, although WB-62 trucks or WB-67 trucks with the rear axles pulled forward for a 40- or 41-ft KPRA distance may be used in some states. As for the smaller semitrailer design vehicles, WB-40 trucks are specialized vehicles used in transporting some freight containers, while WB-62 trucks with 48-ft trailers have largely been replaced by WB-67 trucks. On the other hand, where agencies require WB-67 trucks to operate with a maximum 40-ft KPRA distance, the WB-67 truck will maneuver much like a WB-62. Generally, a specific design vehicle is selected to guide the design of each project. However, if a project includes multiple roadways that vary in the types of truck traffic they serve, it may be appropriate to use a different design vehicle for each roadway. Furthermore, it is not necessary that every turning movement at every intersection be designed to accommodate the largest design vehicle considered in the design of the roadway, especially when that largest design vehicle is a WB-62, a WB-67, or an LCV. Every intersection of a truck route should be designed to accom- modate through movements by the design vehicle; however, the largest design vehicles need to be considered in the design for turning movements at intersections only where large trucks are likely to make turning movements, such as at intersections with other truck routes. Where the minor road at an intersection on a truck route does not frequently serve larger trucks, a smaller design vehicle such as a SU-20, SU-30, or WB-40 may be appropriate. For example, if the minor road at an intersection on a truck route is a residential street, the appropriate design vehicle may be a single-unit truck of a size representative of garbage-collection and delivery trucks that use that street. While every residential street occasionally serves larger trucks, such as moving vans, there is no need to consider design vehicles that are not expected to use the street on a daily basis. Every intersection should be designed to accommodate emergency vehicles, such as fire trucks, that may occasionally need to make turning maneuvers but, in designing for fire trucks, it is not necessary to minimize encroachment of the emergency vehicles on adjacent or opposing lanes. Similarly, projects on roadways that serve OSOW permit vehicles should be designed so that they do not eliminate the capability of the roadway to accommodate permit vehicles that currently use the roadway; since permit vehicle movements are generally occasional, rather than frequent, permit vehicles may need to encroach on adjacent or opposing lanes. The largest OSOW permit vehicle that uses a particular roadway should be used as a âcheck vehicleâ to verify that any modified design can still accommodate OSOW vehicles that use the existing roadway. 4.2.3 Roadway Design for a Selected Design Vehicle The design vehicle selected for a specific project or turning movement is used in the design of critical features, such as curb return radii at intersections, radii of turning roadways, and radii and width of driveways. These features are typically designed to accommodate the turning radius and swept path width of a specific design vehicle. The minimum turning radii of the Green Book design vehicles are shown in Table 2. The minimum turning radius is the smallest radius through which the front axle of a truck can turn. Axles or axle sets farther to the rear of a truck do not follow the path of the front axle. This phenomenon is known as offtracking. At low speeds (which generally serve as the basis for intersection design), the rear axles of a truck follow a path inside of the path of the front axles. The area between the path of the innermost edge of the vehicle body and the outermost edge of the vehicle body of the truck during a turn is known as the swept path of the truck. This is typically slightly wider than the area between the path of the inside front axle and the outside rear axle. The maximum width of the swept path at any point during a turn is
34 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide known as the swept path width. Figure 9 illustrates offtracking and swept path width. Both the minimum turning radius and the swept path width are considered in designing an intersection to determine whether a specific truck design vehicle can maneuver through an intersection without encroaching on a curbline or shoulder area, an adjacent lane, or the opposing direc- tion of travel. Turning templates are used to incorporate consideration of minimum turning radius and swept path width in design. Turning templates show the offtracking and swept path width that will occur for a specific truck making a turning maneuver at very low speeds; for this Design Vehicle Type Single-Unit Truck Single-Unit Truck(Three Axle) Articulated Bus Intermediate Semitrailer Symbol SU-30 SU-40 A-BUS WB-40 Minimum Design Turning Radius (ft) 41.8 51.2 39.4 39.9 Centerlinea Turning Radius (CTR) (ft) 38.0 47.4 35.5 36.0 Minimum Inside Radius (ft) 28.4 36.4 21.3 19.3 Design Vehicle Type Interstate Semitrailer âDouble-Bottomâ Combination Rocky Mtn Double Triple- Semitrailer- trailers Turnpike Double- Semitrailer-trailer Symbol WB-62* WB-67** WB-67D WB-92D WB-100T WB-109D* Minimum Design Turning Radius (ft) 44.8 44.8 44.8 82.0 44.8 59.9 Centerlinea Turning Radius (CTR) (ft) 41.0 41.0 40.9 78.0 40.9 55.9 Minimum Inside Radius (ft) 7.4 1.9 19.1 55.6 9.7 13.8 *Design vehicle with 48-ft trailer as adopted in 1982 STAA. **Design vehicle with 53-ft trailer as grandfathered in with 1982 STAA. aThe turning radius assumed by a designer when investigating possible turning paths and set at the centerline of the front axle of a vehicle. If the minimum turning path is assumed, the CTR approximately equals the minimum design turning radius minus one-half the front width of the vehicle. Table 2. Minimum turning radii of design vehicles (AASHTO 2018). (a) Offtracking (b) Swept Path Width Figure 9. Illustration of truck offtracking and swept path width during a turning maneuver (Harwood et al. 2003).
Geometric Design and Access Management to Accommodate Trucks 35 reason, turning templates are said to illustrate low-speed offtracking. Turning templates are needed because offtracking and swept path width are not constant throughout a truck-turning maneuver. Offtracking and swept path width develop gradually as a truck enters a turn and may eventually reach a constant or steady-state value. An example of a turning template from the Green Book for the WB-67 design vehicle is shown in Figure 10. Turning templates like these, drawn to scale, were formerly used manually in the development of intersection plans. Today, commercially available software can be used in conjunction with CADD software to develop designs to accommodate specific trucks. The software user may specify the path of the front axles for a turning maneuver (or series of turning maneuvers) by any design vehicle, and the software will show the path of the rear axles, the inside and outside of the vehicle body, and other specified points on the vehicle. Low-speed offtracking and swept path width increase with the distances between axles of a single-unit truck and between axles and hinge points of a combination or multi-unit truck. Hinge points include the kingpin that connects a tractor to a following semitrailer and the hitch point that connects a tractor or semitrailer to full trailers that follow. Rear swingout is the phenomenon by which the rear outside corner of a truck follows a path outside the rear outside axle of a truck during a turn. Rear swingout increases as the distance from the rear axle to the rear of the truck, known as rear overhang, increases. However, turning plots show that, while the outside rear corner of the trailer of a combination truck follows a path outside the rear trailer wheels, it is inside the swept path of the truck. For this reason, rear swingout is rarely a concern to other vehicles unless they are making a parallel turn, which might be the case at double or triple turn lanes. None of the Green Book design vehicles have rear swingout that exceeds 0.69 ft for a turn with a radius of 50 ft, even with the rear axles pulled forward to maintain a KPRA distance of 41 ft (Harwood et al. 2003). 4.2.4 Accommodating Large Design Vehicles on the Roadway System While the WB-67 design vehicle may be the appropriate design vehicle for most truck routes, not all arterial roads and streets, especially in urban areas, can fully accommodate turning maneuvers by such a large vehicle. Adjacent development may limit making the roadway wide enough and/or the turning radii large enough to accommodate a WB-62 or WB-67 truck. Thus, as much as designers would like to fully accommodate an appropriate design vehicle, there are locations where this is not possible without interfering with adjacent develop ment. Therefore, in restrictive conditions, it may be desirable to consider (if the applicable state vehicle code allows) occasional encroachment on opposing lanes rather than incur the social costs of interfering with existing development or the economic costs of prohibiting trucks. Mitigation strategies could include rerouting trucks away from the intersection when acceptable alternative routes are available. The curb return radius is the curved curb connecting the curblines along the outside of the two intersecting streets at each corner of an intersection. Figure 11 illustrates a typical curb return radius. Where trucks make right turns at intersections, it is desirable to provide a curb return radius sufficiently large that a truck can make a right turn without encroaching on the curbline or on an adjacent or opposing lane. A turning template for a specific design vehicle, or an automated version of a turning template in CADD software, can be used to assess whether truck encroachment on the curbline or another lane will occur. It should also be recognized that, where a pedestrian crosswalk is present, longer curb return radii can increase the crossing distance for pedestrians (see AASHTO 2004; AASHTO 2018). Longer curb return radii may also result in locating the stop line for vehicles approaching the intersection further from the curbline, which may reduce intersection sight distance. The tradeoffs between accommodating trucks and pedestrians are examined in Section 5.1.
36 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide Figure 10. Dimensions and minimum turning path for the Interstate semitrailer (WB-67) design vehicle (AASHTO 2018).
Geometric Design and Access Management to Accommodate Trucks 37 Figure 11 shows that, where parking lanes or other features, such as curb extensions are present, there is an effective radius for right-turn maneuvers that is larger than the actual curb return radius. Turning templates for specific design vehicles, or the software equivalent of such templates, are also used to assess median opening designs. These assessments can evaluate whether turning trucks will encroach on the median, whether left-turning trucks will encroach on adjacent turn lanes (for double and triple left-turn lanes), or whether the areas required for opposing left-turning trucks within the intersection overlap. Indirect left-turn treatments, such as those discussed in Section 4.3.6, should accommodate the larger design vehicles if large trucks frequently make the left-turn maneuver in question. There are many intersections on truck routes along urban arterials and in small towns where, even though it may be desirable to design the intersection based on a WB-62, WB-67, or LCV, it is infeasible to provide intersection geometrics that minimize encroachment on adjacent or opposing lanes without widening the roadway in a manner that would interfere with surrounding development. From a community impact standpoint, it is not generally desirable to acquire right of way and remove buildings or other development that are important to the community, solely to accommodate large trucks. Alternatives that may be considered in such situations include the following: â¢ Reconfigure the intersection to accommodate truck-turning maneuvers without encroach- ment on adjacent or opposing lanes. This alternative may involve right-of-way acquisition and have undesirable impacts on surrounding development. â¢ Keep the current geometrics and accept occasional encroachment by turning trucks on adjacent or opposing lanes. This alternative may limit the operational efficiency of the inter- section, but other motorists can usually respond to occasional truck encroachments without creating safety concerns. This alternative is explicitly encouraged by some agencies (see Section 4.3.1) but may be unacceptable in jurisdictions that prohibit encroachment by vehicles on adjacent or opposing lanes by law. Figure 11. Typical intersection corner with actual curb return radius and effective radius (adapted from the Green Book).
38 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide â¢ Where the volume of turning movements by large trucks is so high that interference with traffic operations would be more than occasional, rerouting truck traffic may be appropriate. Rerouting of trucks needs to be considered in consultation with the local community. 4.3 Intersections Section 4.3 presents a range of intersection features and types that may be used in better accommodating trucks on the road network. Many of these intersection features and types have been used extensively for many years and are familiar to motorists and truck drivers. The discussion also includes some innovative intersection types that have come into general use only in recent years. Continuing public education may be desirable to familiarize motorists with these innovative intersection types and how trucks operate on them. 4.3.1 Curb Return Radius The curb return radius is the curved curbline connecting the curblines along the outside of the two intersecting streets at each corner of an intersection. Figure 11 illustrates a typical curb return radius. Where trucks make right turns at intersections, it is desirable to provide a curb return radius sufficiently large that a truck can make a right turn without encroaching on the curbline or on an adjacent or opposing lane. A turning template for a specific design vehicle, or an automated version of a turning template in CADD software, can be used to assess whether truck encroachment on the curbline or another lane will occur. It should also be recognized that, where a pedestrian crosswalk is present, longer curb return radii can increase the crossing distance for pedestrians (AASHTO 2004; AASHTO 2018) and can result in a stop line loca- tion further from the curbline on the intersection approach, which may reduce intersection sight distance. Figure 12 illustrates the tradeoff between curb return radius and pedestrian crossing distance. The curb return radius for a right turn at an at-grade intersection should be selected to accom- modate the applicable design vehicle for a right-turn maneuver, which, as described in Section 4.2, might be anything from a single-unit truck to a WB-67 or larger, depending on the character of truck traffic on the minor road at the intersection. An SU-30 or SU-40 design vehicle might be used for right turns to minor residential streets, while a WB-62 or WB-67 design vehicle might be used for right turns to other truck routes or industrial areas. As explained in Section 4.2, it is not always feasible to accommodate the desirable curb return radius because of the presence of adjacent development or the desire to reduce pedestrian crossing length. Some agencies are constrained by law to develop designs so that turning trucks will not encroach on adjacent or opposing lanes. Other agencies may find it desirable to accept occasional encroachment on adjacent or opposing lanes, especially where larger design vehicles turn right only occasionally. Figure 13 shows the turning paths of specific design vehicles for right turns at an intersection with a 30-ft curb return radius and a four-lane receiving roadway. The WB-40 will encroach slightly on an adjacent lane and the WB-62 and WB-67 will encroach on an opposing lane. If there were only two lanes on the receiving roadway, the truck might need to encroach on an adjacent or opposing lane on the approach roadway to avoid encroaching on the curbline. Other traffic at the intersection will likely adapt to such maneuvers if they occur only occasionally. Figure 14 illustrates the difference between âdesigning forâ and âaccom- modatingâ large truck movements at intersections. Some agencies have used mountable curb for smaller curb return radii, recognizing that some large trucks may encroach on the curbline. This practice is not desirable where pedestrians are present since pedestrians use the sidewalk adjacent to the curb return radius. Other agen- cies maintain a paved shoulder between the traveled way and the curb return radius at some
Geometric Design and Access Management to Accommodate Trucks 39 locations to minimize the potential for truck encroachment on the curbline. This may be effective in minimizing truck encroachment but, where pedestrians are present, it increases the pedes- trian crossing distance by the width of the shoulder provided. Where pedestrians cross the roadway in substantial numbers, it may not be desirable to provide the larger curb return radius, because this will increase the pedestrian crossing distance on the intersecting roadways. (See further discussion of resolving conflicts between pedes- trians and trucks in Section 5.1.) In such cases, it may be desirable to use a smaller curb return radius that provides a shorter pedestrian crossing distance, as shown in Drawing A in Figure 12, or to keep the larger curb return radius but provide a channelized right-turn roadway (see Section 4.3.5). Provision of a channelized right-turn roadway can limit the pedestrian crossing distance, because a two-stage crossing is provided, but channelized right-turn roadways are undesirable for pedestrians with vision disabilities, who may not be able to detect approaching vehicles from unexpected angles when crossing the right-turn roadway (Potts et al. 2011). Figure 12. Variations in pedestrian crossing distance with different curb radius (adapted from the Green Book).
40 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide Figure 13. Turning requirements for specific design vehicles (adapted from City of Portland, Oregon, 2008). Figure 14. Comparison of âdesigning forâ and âaccommodatingâ large truck movements at an intersection (City of Portland, Oregon, 2008).
Geometric Design and Access Management to Accommodate Trucks 41 4.3.2 Median End Treatments Median openings on divided highways at intersection and nonintersection locations are typically designed in accordance with guidance in Chapter 9 of the Green Book. The vehicle paths through the median opening for left-turning vehicles are typically laid out with turning template software, as discussed in Section 4.2.3 of this Guide. Median ends at median openings on roads with narrow medians are typically designed with a semicircular shape. However, where the median width exceeds 10 ft, the Green Book indicates that a median end with a bullet nose shape (shown in Figure 15) is preferred to more closely accommodate the turning path of truck design vehicles (AASHTO 2018). Turning radii of 40 ft can be used in design of median openings with minimum design low-speed turning maneuvers; where turning speeds exceed 15 mph, turning radii of 50 ft should be accommodated in above-minimum designs, and the median end treatments should be designed with dimensions shown in Figure 15. 4.3.3 Left-Turn Lanes A left-turn lane is a separate, full-width lane provided exclusively for vehicles that are making a left turn from the roadway (Dixon et al. 2016), as illustrated in Figure 16. Left-turn lanes are used primarily at intersections and ramp terminals but may also be used at driveways Width of Median, M (ft) Dimensions In Feet When R1 = 90 ft R1 = 170 ft R1 = 230 ft L B L B L B 20 58 65 66 78 71 90 30 48 68 57 85 63 101 40 40 71 50 90 57 109 50 â â 44 95 51 115 60 â â â â 46 122 70 â â â â 41 128 Figure 15. Above-minimum design of median openings with typical bullet nose ends (AASHTO 2018).
42 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide and median openings. Left-turn lanes provide both traffic operational and safety benefits. Figures in this Guide, such as Figure 16, are presented to illustrate specific design features (in this case, left-turn lanes) and are not presented as complete guidance for every design feature that might be needed or provided at specific intersections. Traffic Operational Benefits of Left-Turn Lanes By removing left-turning vehicles from the through-traffic stream, left-turn lanes reduce delay to through vehicles (caused by stopping or slowing down behind the left-turning vehicle). Trucks benefit from this reduced delay as well. A truck accelerates more slowly than a passenger car; therefore, when a truck has to stop behind a left-turning vehicle, it causes addi- tional delay to the vehicles following the truck. The traffic operational benefits of left-turn lanes can be quantified with Highway Capacity Manual: A Guide for Multimodal Mobility Analysis (2016) procedures and traffic simulation models. Safety Benefits of Left-Turn Lanes Removing left-turning vehicles from the through-traffic stream provides safety benefits as well by reducing the potential for rear-end collisions in which through vehicles strike the rear of left-turning vehicles. In addition, because left-turn lanes provide a sheltered location for drivers to wait for a gap in opposing traffic, left-turning drivers may be more selective in choosing a gap to complete their left-turn maneuver, which may reduce the potential for collisions between left-turn vehicles and opposing through vehicles. Drivers of trucks are likely even more selective than passenger-car drivers in choosing a gap in opposing traffic to complete a left-turn maneuver because trucks need a larger gap in opposing traffic than passenger cars. Therefore, provision of a left-turn lane is particularly important at locations where trucks frequently make left-turn maneuvers. Table 3 presents the estimated reduction in crashes after installation of a left-turn lane (AASHTO 2010). Figure 16. Four-leg intersection with left-turn lane on each approach.
Geometric Design and Access Management to Accommodate Trucks 43 Deciding Where to Install a Left-Turn Lane The decision to install a left-turn lane at a particular location depends on a number of factors, including overall traffic volume, left-turn volume, and operating speed. Many transportation agencies have adopted their own left-turn lane recommendations and warrants based on one of the following references: â¢ Harmelink warrants (Harmelink 1967). â¢ Green Book guidance (AASHTO 2018). â¢ ITE guidelines for unsignalized left-turn lanes (Institute of Transportation Engineers 2011). â¢ Threshold traffic volumes for signalized and unsignalized left-turn lanes (Harwood et al. 2002). â¢ Warrants recommended in NCHRP Report 279 (Neuman 1985). â¢ Warrants recommended in NCHRP Report 745 (Fitzpatrick et al. 2013). Chapter 21 of the AMAG presents an overview of each of these left-turn lane warrants and recommendations, and Chapter 16 of the TRB Access Management Manual (AMM) (Williams et al. 2014) presents examples of several transportation agenciesâ left-turn lane warrants. Geometric Design Guidelines The Green Book provides guidance on the geometric design of left-turn lanes. Left-turn lanes are typically 10 to 12 ft in width. Where substantial truck traffic turns left, a 12-ft width is prefer- able for left-turn lanes. The Green Book also provides guidance on determining the length of left-turn lanes as the sum of a taper length, a deceleration length, and a storage length. The taper length and deceleration length are based on the assumption that a vehicle entering a left-turn lane should decelerate by no more than 10 mph in the through-traffic lanes before entering a left-turn lane. The storage length is based on consideration of peak-hour left-turn volumes to define the number of vehicles that typically need to be stored in the left-turn lane. Because of the extra length of trucks, longer left-turn lanes are a consideration to accommodate truck storage. The AMM and the AMAG Reduction in Crashes (%) Intersection Characteristic Turn Lanes Added to OneApproach Turn Lanes Added to Both Approaches Left-turn lane: rural Three-leg Stop signs 44 NA Traffic signals 15 NA Four-leg Stop signs 28 48 Traffic signals 18 33 Left-turn lane: urban Three-leg Stop signs 33 NA Traffic signals 7 NA Four-leg Stop signs 27 47 Traffic signals 10 19 NOTE: NA = Not Applicable. Table 3. Estimated reduction in crashes after left-turn lane installation on major-road approaches to at-grade intersections (AASHTO 2010).
44 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide identify the additional need to consider the volume (or percentage) of large vehicles and their extra vehicle length in estimating the storage length for left-turn lanes. The AMM presents an equation for estimating the left-turn storage length as follows: (2)L V N k s( )= Ã Ã where: L = design length of left-turn storage (ft). V = estimated left-turn volume (vph). N = number of cycles per hour. k = factor that is the length of the longest queue divided by the average queue length. s = average length per vehicle, including the space between vehicles. For k in the previous equation, a value of 2.0 is commonly used for major arterials, while a value of 1.5 to 1.8 might be considered for an approach on a minor street or on a collector. Also, s is generally assumed to be 25 ft. However, s can be adjusted based on the presence of trucks. Table 4 provides queue storage length adjustments for trucks that can be used for s based on the percentage of trucks in the left-turn lane. The desirable storage length for left-turn lanes should also be a consideration on truck routes, even at locations where trucks are not likely to make left-turn maneuvers. Efficient through travel for trucks is important, and delay to trucks from the need to stop behind queues of left-turning vehicles spilling into the through lanes can hinder on-time deliveries. Offset Left-Turn Lanes Vehicles in opposing left-turn lanes can each limit their driverâs view of opposing traffic [see Figure 17(a)]. Truck drivers, with their increased eye height, may have an advantage over passenger-car drivers in this situation, because they may be able to see over an opposing passenger-car. However, in some situations, a truck driverâs view of opposing traffic may be just as limited as a passenger-car driverâs view. The presence of a large truck in a left-turn lane also decreases the ability of the driver of an opposing left-turn vehicle to judge gaps in oncoming traffic. Offset left-turn lanes are implemented by moving each of the two opposing left-turn lanes to the left (creating space between the through lane and left-turn lane) so that the opposing left-turn lane is located to the right of the driverâs field of view [see illustration in Figure 17(b)]. This design treatment, which opens up the view of opposing traffic to the left-turning driver, is particularly beneficial at wide medians. The restriction on the available sight distance for drivers of opposing left-turn vehicles is dependent on the amount and direction of the offset between the opposing left-turn lanes, as well as the sizes of the vehicles. NCHRP Report 745 Trucks (%) Average Length per Vehicle for Determining Queue Storagea (ft) â¤ 2 25 5 28 10 32 15 35 20 38 25 41 aBased on a WB-67 truck. Table 4. Queue storage length adjustments for trucks, used in Equation (2) for s (Williams et al. 2014).
Geometric Design and Access Management to Accommodate Trucks 45 provides guidance concerning the appropriate offset distance for opposing left-turn lanes (Fitzpatrick et al. 2013). Double and Triple Left-Turn Lanes Double and triple left-turn lanes, as shown in Figure 18 are provided at intersections to increase the traffic operational efficiency of the intersection, limit the length of left-turn lanes, and limit the length of left-turn signal phases. Double and triple left-turn lanes are used almost exclusively at signalized intersections. Under certain conditions, double left-turn lanes accompa- nied by a separate left-turn signalization phase can accommodate up to approximately 180% of the volume that can be served by a single left-turn lane with the same available green time in the signal cycle. While double and triple left-turn lanes are often provided based on consideration of passenger cars alone, they can also enhance left-turn operations for trucks. Turning maneuvers are one of the key challenges faced by trucks on urban and suburban arterials. Many of the transportation agencies surveyed in NCHRP Project 15-62 (Potts et al. 2019) indicated they have provided double or triple left-turn lanes specifically to facilitate left turns by trucks. Trucks seldom turn side by side in double or triple left-turn lanes. Instead, trucks tend to use the outside left-turn lane (i.e., the left-turn lane farthest to the right), which has the largest turning radius, or to stagger themselves such that trucks use all available lanes, but do not turn side by side. In some cases, WB-62 or WB-67 trucks turning left in the outside left-turn lane may encroach on the inside left-turn lane. The avoidance of side-by-side turning maneuvers limits the possibility of collisions between adjacent trucks but results in a reduction in the operational efficiency of the double or triple left-turn lanes. Nevertheless, even with some reduction in traffic operational efficiency due to avoidance of side-by-side turning maneuvers by trucks, the traffic operational efficiency of the intersection is still better than it would be if only a single left-turn lane were provided. Figure 19 shows a photograph of a double left-turn lane in which trucks are in staggered position in the two left-turn lanes preparing to make a left- turn maneuver. Computerized swept path width software can be used to mark turning paths through the intersection appropriate for vehicles turning side by side. 4.3.4 Right-Turn Lanes A right-turn lane is a separate, full-width lane provided exclusively for vehicles that are making a right turn from the roadway (Dixon et al. 2016) (see Figure 20). Right-turn lanes are used primarily at intersections and ramp terminals but may also be used at driveways. Right-turn lanes provide both traffic operational and safety benefits. (a) Opposing Left-Turn Lanes with Negative Offset (b) Opposing Left-Turn Lanes with Positive Offset Figure 17. Opposing left-turn lanes with and without positive offset (AASHTO 2018).
46 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide (a) Double Left-Turn Lanes (b) Triple Left-Turn Lanes Figure 18. Typical double and triple left-turn lanes. (Photo source: Google Maps.) Traffic Operational Benefits of Right-Turn Lanes By removing right-turning vehicles from the through-traffic stream, right-turn lanes reduce delay to through vehicles resulting from the need for the through vehicles to stop or slow behind the right-turning vehicle. Trucks benefit from the reduced delay provided by right-turn lanes as well. Trucks accelerate more slowly than passenger cars; therefore, when a truck has to stop behind a right-turning vehicle, it causes additional delay to the truck and to the vehicles following the truck. Some transportation agencies have indicated that right-turn lanes are often needed at entrances into developments generating truck traffic, particularly to accommodate truck deceleration. The traffic operational benefits of right-turn lanes can be quantified with Highway Capacity Manual procedures and traffic simulation models.
Geometric Design and Access Management to Accommodate Trucks 47 Figure 19. Trucks in staggered position preparing to turn left in a double left-turn lane. (Photo source: MRIGlobal.) Figure 20. Four-leg intersection with right-turn lane on two approaches.
48 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide Safety Benefits of Right-Turn Lanes Removing right-turning vehicles from the through-traffic stream provides safety benefits as well, by reducing the potential for rear-end collisions in which through vehicles strike the rear of right-turning vehicles. Table 5 provides crash modification factors for right-turn lanes, as presented in the AASHTO Highway Safety Manual (HSM). Deciding Where to Install a Right-Turn Lane The decision to install a right-turn lane at a particular location depends on a number of factors including the following: â¢ Through- and right-turn traffic volumes. â¢ Right-turn truck volumes. â¢ Posted and/or operating speed. â¢ Available sight distance. â¢ Crash history. â¢ Construction cost. â¢ Pedestrian and bicycle volumes. Potts et al. developed an economic analysis procedure that can identify where installation of right-turn lanes at unsignalized intersections and major driveways would be cost-effective (Potts et al. 2006). The economic analysis procedure can be used to develop plots that indi- cate combinations of through-traffic volumes and right-turn volumes for which provision of a right-turn lane would be recommended. The economic analysis procedure can be applied by transportation agencies using site-specific values for average daily traffic, turning volumes, crash frequency, and construction cost for any specific location of interest. Geometric Design Guidelines The Green Book provides guidance on the geometric design of right-turn lanes. Right-turn lanes are typically 10 to 12 ft in width. Where substantial truck traffic turns right, a 12-ft width is preferable for right-turn lanes. The Green Book also provides guidance on determining the length of right-turn lanes as the sum of a taper length, a deceleration length, and a storage length. The taper length and deceleration length are based on the assumption that a vehicle entering a right-turn lane should decelerate by no more than 10 mph in the through-traffic lanes before entering a right-turn lane. The storage length is based on consideration of peak-hour right-turn volumes to define the number of vehicles that typically need to be stored in the right-turn lane. Because of the extra length of trucks, longer right-turn lanes are a consideration to accommodate truck storage. Sections 4.2.4 and 4.3.1 present guidelines on the design of curb return radii for right-turn maneuvers. Sections 5.1 and 5.2 discuss compatibility of designing to accommodate trucks in relation to pedestrians and bicyclists, respectively. Major Roadway Type Three-Leg Intersection Four-Leg Intersectiona Unsignalized Signalized Unsignalized Signalized Rural two-lane highway 0.86 NA 0.86n 0.86n Rural multilane highway 0.86 NA 0.86n NA Urban-suburban arterial 0.86 0.96 0.86n 0.96n NA = Not Available. a n = number of intersection approaches without stop-sign control that have right-turn lanes. For example, a four-leg unsignalized intersection with right-turn lanes on both major street approaches will have a crash modification factor of (0.86)2, or 0.74. Table 5. Crash modification factors for right-turn lanes (AASHTO 2010).
Geometric Design and Access Management to Accommodate Trucks 49 Double and Triple Right-Turn Lanes Turning maneuvers are one of the key challenges faced by trucks on urban and suburban arterials, and some transportation agencies have indicated that right turns at intersections are most problematic for trucks. Double right-turn lanes not only increase the capacity of right-turn movements, but the outside (left) right-turn lane can provide a larger turning radius for trucks. As an example, the Port Authority of New York and New Jersey (PANYNJ) implemented dual right-turn lanes to accommodate trucks leaving Port Newark. In constructing the double right- turn lanes, PANYNJ was able to construct each of the two right-turn lanes to be extra wide to accommodate truck swept path widths, as illustrated in Figure 21. This design worked effectively at this location because no pedestrians were present. Triple right-turn lanes have also been used at a few locations, particularly for turns from a freeway off ramp to an arterial crossroad. Offset Right-Turn Lanes A challenge with providing right-turn lanes at unsignalized intersections is that vehicles, particularly trucks, in a right-turn lane on the major road may potentially block the view of drivers stopped on the minor-road approach and prevent them from being able to see traffic approaching from the left on the major road. This can lead to crashes between vehicles turning onto or crossing the major road and through vehicles on the major road. To mitigate this concern, a right-turn lane can be offset by separating it from the adjacent through lane so that vehicles in the right-turn lane no longer obstruct the view of the minor road driver, as illustrated in Figure 22. One transportation agency has indicated they have implemented an offset right- turn lane at a gas station entrance with substantial truck activity to prevent entering trucks from blocking the sightline of exiting passenger cars. Figure 21. Double right-turn lanes for trucks leaving Port Newark. (Photo source: Google Earth.) Figure 22. Typical offset right-turn lane.
50 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide 4.3.5 Channelized Right-Turn Roadways Right-turn maneuvers at intersections are a key challenge faced by trucks on urban and suburban arterials because of the potential for trucks to either encroach into the adjacent lane or mount the curb to negotiate a right turn. Channelized right-turn roadways can provide a larger curb return radius to accommodate turning vehicles, including large trucks, without unnecessarily increasing the intersection pavement area and the pedestrian crossing distance. Channelization can be provided in a variety of forms, including painted pavement areas and curbed islands. Figure 23 illustrates a typical channelized right-turn roadway. Channelized right-turn roadways may be appropriate in some quadrants of an intersection but not in others, depending on intersection geometry and traffic demands. Channelized right-turn roadways provide advantages for trucks, in that a larger curb return radius can be used. The channelized right-turn roadway design provides a two-stage crossing for pedestrians, rather than increasing the pedestrian crossing distance at the main intersection. However, channelized right-turn roadways provide a disadvantage for pedestrians with vision disabilities who may have difficulty determining the correct path for crossing the channelized right-turn roadway. Some transportation agencies have indicated that channelized right-turn roadways can be challenging for trucks and, in some cases, they have removed the channelizing island at an intersection. In these situations, it is likely that the channelized right-turn roadway was too narrow for trucks to traverse. Figure 23. Typical channelized right-turn roadway.
Geometric Design and Access Management to Accommodate Trucks 51 4.3.6 Indirect Left-Turn Movements Certain intersection designs remove direct left-turn paths and route left-turning traffic indirectly. Where medians are installed in the roadway to prevent direct left-turn movements, alternative routes should be provided to complete left-turn maneuvers. In some cases, these alternative routes include U-turn roadways through the roadway median. Such U-turn roadways should be designed to accommodate trucks that need to make the specific left-turn maneuver. Examples of intersections with indirect left-turn movements, including RCUT, MUT, continuous flow, and jughandle intersections, are provided in this section, with guidance on how they should be used on truck routes. RCUT and MUT Intersections RCUT and MUT intersections are innovative intersection types that are used in place of conventional at-grade intersections. At RCUT intersections, left-turn movements from a side street are required first to make a right turn onto the major street, followed by a U-turn in order to complete the left turn. Path B in Figure 24 shows the indirect left-turn path from the side street at an RCUT intersection. Through movements from the side street at an RCUT inter- section follow a similar path (see Path A in Figure 24). The U-turn roadways of an RCUT inter- section are often referred to as J-turns. At MUT intersections, all left-turn movements are prohibited at the crossing of the intersecting roadways. Left turns are accomplished by either going through the intersection and making a U-turn followed by a right turn (Path A in Figure 25) or first turning right onto the intersecting Figure 24. Illustration of side-street left-turn and through paths at RCUT intersection (FHWA 2018a). Figure 25. Illustration of indirect left-turn paths at MUT intersection (FHWA 2018a).
52 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide roadway followed by a U-turn (Path B in Figure 25). This design has been used extensively in Michigan and has been referred to as a Michigan U-turn intersection. RCUTs and MUTs improve the operational performance of signalized intersections by reducing the number of critical phase-pairs. RCUTs and MUTs are often used to decrease delay and travel time on high-demand corridors. RCUTs are also used on rural high-speed divided roadways as a safety treatment to remove crossing maneuvers from the side-street approaches. RCUTs and MUTs can be used in conjunction with driveways and side streets that are limited to right-in/right-out operation, because these intersection types have U-turn roadways that can combine with the right-in/right-out maneuvers to create an indirect left-turn movement. During the course of this research, some transportation agencies indicated that RCUT and MUT intersections can be designed to operate effectively for large trucks. Other agen- cies indicated they avoid the need for U-turns by large trucks by adjusting driveways and by encouraging effective onsite circulation design and effective truck delivery routes (Potts et al. 2019). The U-turn maneuver at RCUT and MUT intersections can be challenging for trucks to traverse. The Green Book provides guidance, presented in Table 6, on the minimum median width to accommodate U-turn movements on four-lane divided arterials by specific design vehicles, including WB-40, SU-30, SU-40, WB-62, and WB-67 trucks. Three cases are addressed: â¢ U-turns from inner lane to inner lane â¢ U-turns from inner lane to outer lane â¢ U-turns from inner lane to shoulder (which also addresses U-turns from the inner lane to the outer lane of a six-lane divided arterial) On a four-lane divided arterial with a median width such that a truck would run onto the shoulder of the receiving roadway while making a U-turn, a loon can be provided. A loon is a widened and strengthened portion of the paved shoulder marked specifically for use by trucks in making U-turn maneuvers. Figure 26 illustrates a typical loon on the shoulder opposite a median U-turn roadway. U.S. Customary Type of Maneuver MâMinimum Width of Median (ft) for Design Vehicle P WB-40 SU-30 BUS SU-40 WB-62 WB-67 Length of Design Vehicle (ft) 19 50 30 40 40 63 68 Inner Lane to Inner Lane 30 61 63 63 76 69 69 Inner Lane to Outer Lane 18 49 51 51 64 57 57 Inner Lane to Shoulder 8 39 41 41 54 47 47 Table 6. Minimum median width to accommodate U-turn movements (AASHTO 2018).
Geometric Design and Access Management to Accommodate Trucks 53 The following guidelines should be used when considering the design and placement of the loon: â¢ At U-turn roadways where trucks are present, provide loons where needed to accommodate truck-turning paths in completing a U-turn maneuver. â¢ Use turning templates in CADD software to ensure adequate design is provided for the design vehicle making a U-turn. â¢ Corridors do not necessarily need loons at every median U-turn roadway. Carefully consider which median U-turn locations will be used by trucks, and provide loons where needed at those locations. Other median U-turn locations may just serve smaller vehicles and may not need a loon. â¢ On corridors with limited right of way on one side of the roadway, jughandles can be used in lieu of loons. Continuous Flow Intersections Continuous flow intersections (CFIs) are an innovative intersection design that can be used in place of conventional at-grade intersections. CFIs consist of displaced left-turn movements that cross the opposing through movements upstream of the main intersection. CFIs can have displaced left-turn movements on all four approaches of the intersection; however, it is most common to have displaced left-turn movements on the major crossing roadway. Figure 27 shows a typical CFI with displaced left-turn movements on two of the four approaches. The indirect left-turn paths are shown in the figure. CFIs improve the operational performance of signalized intersections by reducing the number of critical phase-pairs. CFIs are often used to decrease delay and travel time on high-demand corridors. The left-turn crossover must be placed far enough upstream of the main intersection to allow for adequate storage space for left-turning vehicles. Displaced left turns with high truck volumes must be designed to accommodate truck movements and to prevent queue spillback into the crossover intersection. Transportation agencies representatives interviewed during the course of this research indicated that this can be done effectively (Potts et al. 2019). Strategic signal timing can be used to coordinate the crossover and main intersections during times of day in which heavy truck volumes create the potential for sustained spillback. Jughandles A jughandle is a particular type of turning roadway used at at-grade intersections to route left- and right-turning traffic from the major roadway onto the minor roadway or to provide Figure 26. Illustration of a loon at a median U-turn roadway.
54 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide Case Study: Selected Use of Loons at U-Turn Locations Along a Truck Route Michigan DOT (MDOT) makes extensive use of the MUT design in which all left turns are prohibited at the main intersections where two streets cross and left turns are accomplished indirectly by means of median U-turn roadways (âcrossoversâ). Even on six-lane divided roadways, U-turns by the largest trucks that operate in Michigan can typically be accomplished only where loons are provided on the shoulder of the opposing roadway opposite the U-turn roadway. In one particular corridor on a suburban four-lane divided arterial with spacing of approximately 1 mi between signalized intersections, MDOT has made a careful study of which intersections have (and which intersections do not have) substantial left-turn demands. Where truck left-turn demands are present, loons are provided at the adjacent median U-turn roadway in each direction of travel to facilitate left turns by trucks. Where truck left-turn demands are not present, loons are not provided at the adjacent median U-turn roadway, and U-turns by trucks are prohibited by black-on-white signs that read NO LARGE TRUCKS THIS CROSSOVER (see photograph). If a truck desires to turn left using a U-turn roadway that prohibits truck use, it is typically no more than 1 mi to a median U-turn roadway with a loon that permits truck use. (Photo source: MRIGlobal.) Loon Provided No Loon Provided (Photo source: adapted from Google Earth.) a U-turn movement for the roadway. Nearside jughandles provide an indirect turning path upstream of the intersection (see Figure 28), while farside jughandles provide an indirect left- turning path downstream of the intersection (see Figure 29). Farside jughandles are also known as indirect left-turn loops. Jughandle intersections have been used extensively in New Jersey and are now in use in other states as well. Jughandles can be beneficial for providing a path for trucks to make U-turns and for providing a larger turning radius for turning trucks. Jughandles with inadequate storage can be challenging for trucks to navigate. Transportation agencies report that farside jughandles are more effective in facilitating truck movements than nearside jughandles (Potts et al. 2019).
Geometric Design and Access Management to Accommodate Trucks 55 The following guidelines should be used when considering jughandles for truck accommodation: â¢ For left-turn movements with high truck demand, use of farside jughandles is recommended. â¢ On corridors where left turns are prohibited, jughandles can work well for providing U-turns. Midblock locations and three-leg intersections are better suited than four-leg intersections as locations for jughandles that provide for U-turn movements by trucks. â¢ Nearside jughandles are not well suited for use at intersections where left-turning truck volumes are high because of the limited storage space between the jughandle intersection with the side street and the main intersection. Figure 27. Illustration of CFI with displaced left turns on two of the four approaches (FHWA 2018b). Figure 28. Illustration of a four-leg intersection with nearside jughandles.
56 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide Jughandles are not ideal on facilities that accommodate pedestrians, because they provide an additional roadway that pedestrians must cross. If possible, it is best to provide pedestrian and bicycle facilities on parallel streets to eliminate conflicts between jughandle traffic and nonmotorists. Quadrant Intersections A quadrant intersection is an innovative intersection that is used in place of a conven- tional at-grade intersection. At quadrant intersections, there is a connector roadway that is positioned in one quadrant of the intersection, as shown in Figure 30. All left-turn movements at the main intersection are prohibited and must use the quadrant roadway to complete a left- turn maneuver. Quadrant intersections improve the operational performance of signalized inter- sections by reducing the number of critical phase-pairs. Similar to RCUTs and MUTs, quadrant inter sections are often used to decrease delay and travel time on high-demand corridors. The quadrant roadway must be placed far enough away from the main intersection to allow for adequate storage space for vehicles on both the main roadways and the quadrant roadway. Quadrant intersections with heavy truck volumes must be designed to accommo- date truck movements. Strategic signal timing can be used to coordinate the main intersection with the two-quadrant roadway intersections during times of day in which heavy truck volumes create the potential for sustained spillback. Pedestrians may find it easier to traverse a quadrant intersection rather than a conventional intersection because shorter cycle lengths at a quadrant intersection may reduce pedestrian Figure 29. Illustration of a four-leg intersection with farside jughandles.
Geometric Design and Access Management to Accommodate Trucks 57 delay. Crosswalk lengths may also be shorter at quadrant intersections because there are no left-turn lanes at the main intersection. Other Special Considerations In some cases, it may be necessary to prohibit trucks from making left turns at intersections as a result of inadequate turning radii or storage space, while still allowing other vehicles to make the direct left turn. At some locations, an indirect left turn should be provided down- stream so that trucks have an alternate route to the desired roadway. Roundabouts can be used on corridors to replace conventional intersections. This may be particularly appropriate where the corridor is being converted from an undivided to a divided cross section. In this situation, all vehicles that desire to turn left from driveways must now turn right and use a downstream roundabout to make a U-turn, thus creating an indirect left turn. This strategy can be beneficial on corridors with several driveways frequently used by trucks. An example of such a corridor is presented in the case study âReplacement of Closely Spaced Signalized Intersections with Roundabouts on Arterial Corridor.â 4.3.7 Roundabouts A roundabout is a form of circular intersection in which traffic travels around a central island and at which entering traffic must yield to circulating traffic (i.e., traffic already within the roundabout). Figure 31 presents the key features of a roundabout that make it operate effectively (Rodegerdts et al. 2010). The Roundabouts: Informational Guide, Second Edition (Rodegerdts et al. 2010) separates roundabouts into three basic categories according to their size and number of lanes: â¢ Mini-roundaboutsâsmall roundabouts with a fully traversable central island, most commonly used in low-speed urban environments. â¢ Single-lane roundaboutsâroundabouts with a single-lane entry at all legs and one circula- tory lane. Operating speeds may be slightly higher than at mini-roundabouts, and the central island is non-traversable. Figure 30. Paths of left turns at a quadrant intersection (Hughes et al. 2010).
58 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide Case Study: Alternative Paths for Passenger Cars and Trucks MDOT uses some diamond interchanges with slip ramps to continuous frontage roads, similar to the interchange design that is also used commonly in Texas. At one location, MDOT found that the space between the crossroad ramp terminals was sufficient to store passenger cars making a direct left turn from one frontage road, but that large trucks turning left could not be stored in the space between the crossroad ramp terminals. In this case, MDOT decided to let passenger cars continue to make the direct left-turn maneuver, but to prohibit direct left turns by trucks. As an alternative, a U-turn roadway was constructed to provide an indirect left-turn routing for trucks (see photograph). This unique treatment may be applicable in only a limited number of sites, but it illustrates that separate turning paths can be used for passenger cars and trucks where appropriate. (Photo Source: adapted from Google Earth.) â¢ Multilane roundaboutsâroundabouts having at least one entry with two or more lanes. Geometric design includes raised splitter islands, a non-traversable central island, and appro- priate entry path deflection. Single-lane roundabouts are an effective safety treatment for at-grade intersections because they eliminate the right-angle and left-turn conflicts associated with traditional intersections. Furthermore, at single-lane roundabouts, drivers have no lane-use decisions to make, pedestrians only have to cross one lane of traffic at a time, and operating speeds are typically low enough to allow for bicyclists and motor vehicles to share the road comfortably. Multilane roundabouts have an increased number of conflicting and interacting move- ments and, therefore, may not achieve the same safety performance as single-lane roundabouts. Drivers, pedestrians (particularly pedestrians with vision disabilities), and bicyclists each face more complex decisions and scenarios in multilane roundabouts. However, their overall safety performance is often better than comparable signalized intersections, particularly in terms of fatal and injury crashes (Rodegerdts et al. 2010).
Geometric Design and Access Management to Accommodate Trucks 59 Case Study: Replacement of Closely Spaced Signalized Intersections with Roundabouts on Arterial Corridor A heavily traveled arterial corridor in Louisiana, with high truck volumes, experienced traffic operational and safety issues because of a series of closely spaced signalized and unsignalized intersections and driveways in the vicinity of an Interstate interchange. The high volumes of trucks, midblock left-turn movements, and closely spaced driveways and intersections contributed to congestion along the corridor. In addition, long queues at signalized intersections contributed to congestion and delays at unsignalized access points. Also contributing to the problem was a truck stop and truck wash, located within approximately 300 ft of the interchange, which led to traffic operational and safety issues related specifically to trucks turning into and out of these businesses. To address the traffic operational and safety issues along the corridor, the Louisiana DOTD (LaDOTD) decided to replace three closely spaced signalized intersections (two located at the interchange with I-10 and one located approximately 1,000 ft from the interchange) with roundabouts. They also installed a raised median along this corridor, preventing left turns in and out of driveways. The operation of the corridor has improved significantly since the construction of the roundabouts, and the raised median has eliminated several conflict points. Trucks are able to circulate around the roundabouts without issue, using the truck aprons as needed. (Photo source: adapted from Google Earth.) (Photo source: adapted from Google Earth.)
60 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide Research has shown that roundabouts can provide a reduction of 48% in all crashes and 78% in injury crashes where a signalized intersection is converted to a roundabout, and a reduc- tion of 44% for all crashes and 82% in injury crashes where an intersection with minor-road Stop-control is converted to a roundabout (AASHTO 2010). These substantial safety benefits make roundabouts an attractive access management technique for transportation agencies to implement at these types of intersections. For crash reduction effectiveness estimates for conversion of other intersection types to roundabouts, see the AASHTO HSM or the FHWA Crash Modification Factors Clearinghouse (www.cmfclearinghouse.org). Roundabouts can also be used as part of an overall access management strategy. Where roundabouts are implemented at key intersections and driveways to facilitate major movements, access points between roundabouts can be designed with partially restricted turning movements (e.g., right-in/right-out operation), because the roundabouts provide U-turn opportunities. Geometric Design Considerations at Roundabouts Figure 32 defines the basic geometric elements of a roundabout. The typical dimensions of geometric features for a roundabout are shown in Table 7. Entry widths up to 18 ft are typical for roundabouts. Exit widths and apron widths should be based on CADD analysis of design vehicle swept paths. Truck aprons surrounding the central island are typically designed so that the front wheels of the truck tractor remain in the circulating roadway, while the rear wheels may utilize the apron where needed. Truck drivers have indicated that they like roundabouts because they improve safety and traffic operations (Potts et al. 2019); however, where truck traffic is anticipated, roundabouts need to be designed to accommodate trucks. Since roundabouts are designed to intentionally reduce vehicle speeds, they typically include narrow cross sections and tight turning radii. However, if lane widths are too narrow and turning radii are too tight, this can be problematic for large trucks traversing the roundabout. An appropriate design vehicle needs to be selected for each roundabout and, if the design vehicle is a truck, its turning path requirements will determine many of the roundaboutâs dimensions, including the exit radius. Figure 31. Key features of a roundabout (Rodegerdts et al. 2010).
Geometric Design and Access Management to Accommodate Trucks 61 At single-lane and multilane roundabouts, transportation agencies often use a traversable apron around the perimeter of the central island to provide the additional width needed to accommodate the swept path width of large vehicles. Initially, truck drivers did not like using these raised aprons because of concerns with truck rollover and tire blowouts. In fact, one transportation agency uses a wider single circulating roadway because the trucking industry in their state does not like using the apron surrounding the center island of a roundabout. Recent adjustments to roundabout design to accommodate trucks, however, have included flatter cross slopes and lower mountable curbs on the aprons to make this design better accom- modate trucks. In many states, truck drivers seem to use aprons more fully now, and some truck drivers have indicated a desire for mountable aprons on the outside of roundabouts as well. This design, however, is not desirable where pedestrian activity is present. Figure 33 illustrates a single-trailer combination truck traversing a roundabout, using the apron surrounding the central island. Figure 34 shows a double-trailer combination truck traversing a roundabout without using the apron, at least to this point in the turning maneuver. Roundabouts may also be designed with a traversable center island in locations where OSOW permit trucks are common. One state has provided a roadway traversing the center island of a roundabout with lockable gates that can be opened by the transportation or law enforcement agency when the roadway is used for specific OSOW permit truck movements. Case Study: Full-Scale Roundabout Demonstration Many transportation agencies have encountered initial public resistance to installation of roundabouts. One frequently encountered concern from operators of large trucks and large farm equipment was that a roundabout would not be able to accommodate their vehicles. A California Department of Transportation (Caltrans) district office met these concerns by marking a full-scale roundabout on an unpaved area in a farm field and traversing the demonstration roundabout with typical large trucks and farm equipment. The demonstration showed clearly that the roundabout could accommodate the largest vehicles that were likely to use the roundabout without encroaching on the outside curb or the center island. An example of this demonstration, shown in the photograph below, can be seen in a video at https://www.youtube.com/watch?v=JoaB-gQCsD8. (Photo source: Caltrans.)
62 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide Figure 32. Basic geometric elements of a roundabout (AASHTO 2018). Design Element Mini-Roundabout Single-Lane Roundabout Multilane Roundabout Desirable maximum entry design speed 15 to 20 mph 20 to 25 mph 25 to 30 mph Maximum number of entering lanes per approach 1 1 2+ Typical inscribed circle diametera 45 to 90 ft 90 to 180 ft 150 to 300 ft Central island treatment Mountable Raised Raised Typical daily service volumes for a four-leg roundabout below which the roundabout may be expected to operate without needing a detailed capacity analysisb 0 to 15,000 0 to 20,000 0 to 45,000 for a two-lane roundabout aSee Figure 32 for the definition of inscribed circle diameter. bOperational analysis is needed to verify upper limit for specific applications or for roundabouts with more than two lanes or four legs. Table 7. Comparison of typical geometric features by roundabout type (Rodegerdts et al. 2010; AASHTO 2018).
Geometric Design and Access Management to Accommodate Trucks 63 Figure 33. Single-semitrailer truck using apron that surrounds the central island of a roundabout. (Photo source: Rodegerdts et al. 2010.) Figure 34. Double-trailer truck traversing a roundabout. (Photo source: MRIGlobal.) To better accommodate trucks at roundabouts, some transportation agencies use slip ramps (bypass lanes) that allow vehicles to make a right-turn maneuver without ever entering the circulating roadway of the roundabout (see Figure 35). This treatment is desirable only where there is a high-volume right-turn movement with substantial congestion, constrained geometrics on the circulating roadway, or a documented pattern of right-turn crashes. Horizontal clearances to roadside objects should be designed considering the capability of the roundabout to accommodate oversize permit vehicles. Selection of cross slopes for the entry and circulating roadways should consider the need to accommodate trucks with low vehicle undercarriage clearance, such as low-boy trailers. Multilane roundabouts can be designed so that trucks can traverse the roundabout while remaining within a single lane, typically the outermost lane of the circulating roadway. How- ever, this may result in a larger radius for the roundabout than is typically considered desirable. Because large trucks are often unable to avoid encroaching on adjacent lanes while turning, many multilane roundabouts are designed with the expectation that large vehicles will track across more than one lane while entering, circulating, and exiting the roundabout. Where multi- lane roundabouts are designed to allow tracking across more than one lane, public education is needed so that other drivers are aware of the need for trucks to use more than one lane.
64 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide In some cases, roundabouts have been designed with aprons or gated roadways through the central island to accommodate oversize trucks and emergency vehicles (see further discus- sion in Section 3.6 on OSOW permits for trucks). The needs of oversize trucks that are likely to use a particular roundabout should be considered in its design. For example, the location of poles, posts, and signs needs to be carefully considered so that oversize trucks can negotiate the roundabout without hitting them. Pedestrian and Bicycle Usage Roundabouts provide safer and more efficient operations with reduced crashes for motor vehicles, including passenger cars, trucks, and transit vehicles; however, roundabouts do present challenges for pedestrians and bicyclists. Roundabouts increase the length of the pedes- trian travel path for through movements by pedestrians and result in crosswalk locations where nearly all motor vehicles are engaged in a turning maneuver. Roundabouts are particularly challenging for pedestrians with vision disabilities because vehicles approach from unexpected directions. Bicyclists using the circulating roadway encounter conflicts with motor vehicles that differ from those at conventional intersections. At single-lane roundabouts, operating speeds are typically low enough to allow for bicyclists and motor vehicles to share the road comfortably. Multilane roundabouts, with increased radii, higher volumes, and higher speeds, may be more difficult for bicyclists to traverse. Useful guidance on accommodation of pedes- trians and bicyclists at roundabouts can be found in Roundabouts: An Informational Guide, Second Edition (Rodegerdts et al. 2010). 4.4 Crossroad and Ramp Terminal Design as Affected by Interchange Configuration This section presents a range of interchange types that may be used in better accommo dating trucks on the road network. Many of these interchange types have been used extensively for many years and are familiar to motorists and truck drivers. The discussion also includes some innovative interchange types that have come into general use only in recent years. Continuing public education may be desirable to familiarize motorists with these innovative interchange types and how trucks operate on them. Figure 35. Typical roundabout with right-turn bypass lane. (Photo source: New Hampshire DOT.)
Geometric Design and Access Management to Accommodate Trucks 65 4.4.1 Conventional Diamond Interchanges and Closely Related Interchange Configurations A conventional diamond interchange consists of four diagonal ramps, two exit ramps and two entrance ramps on either side of a freeway, that connect the freeway to an arterial cross- road. Figure 36 illustrates a typical conventional diamond interchange. For rural or low-volume interchanges, the two exit ramps may be Stop-controlled at the crossroad ramp terminals. For higher volume interchanges, the crossroad ramp terminals are often signalized. Keys to design of effective conventional diamond interchanges include the following: â¢ Provision of an appropriate number of lanes on the exit ramps to serve the volume of traffic exiting the freeway and to store queues of stopped vehicles on the ramp approaches to the crossroad ramp terminal. â¢ Provision of an appropriate number of through and turning lanes on the arterial crossroad to serve traffic exiting the freeway, entering the freeway, and passing through the intersection on the crossroad. â¢ Provision of sufficient spacing between the crossroad ramp terminals to provide sufficient storage space within the interchange on the arterial crossroad. At higher volume interchanges, effective operation of the interchange is dependent on appro- priate phasing and timing of the traffic signals at the crossroad ramp terminals. Figure 36. Typical conventional diamond interchange.
66 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide Key truck considerations in the design of conventional diamond interchanges include the following: â¢ Design of the ramp terminals to accommodate left and right turns by trucks (see Section 4.2 on design vehicles). â¢ Design of any double or triple left-turn lanes to accommodate trucks turning from the exit ramps to the crossroad and from the crossroad to the entrance ramps, including the potential for side-by-side turning maneuvers by trucks (see Section 4.3.3 on double and triple left-turn lanes). â¢ Design of the spacing between the ramp terminals and the cross section of the arterial cross- road to avoid spillback through the ramp terminals and spillback onto the freeway. â¢ Design of entrance ramps to avoid spillback onto the arterial crossroad and to accommodate any ramp-metering system that may be in place. Each of these key design issues for accommodating trucks is traffic-volume dependent and is, therefore, dependent on appropriate analysis procedures rather than on formal design criteria. For freeways with continuous frontage roads, full diamond interchanges can be designed with slip ramps to and from the parallel frontage roads, as illustrated in Figure 37. This design can have several advantages over a conventional diamond that can benefit trucks. First, the slip ramps can be located further from the crossroad, providing more storage space on the approach to the crossroad ramp terminal, if needed. Second, U-turn roadways between the frontage roads on opposite sides of the freeway can be provided either in advance of the crossroad or Figure 37. Diamond interchange with slip ramps to and from parallel frontage roads.
Geometric Design and Access Management to Accommodate Trucks 67 beyond the crossroad or both, thus minimizing the volume of left-turn movements that use the crossroad and the storage space needed between the ramp terminals on the crossroad. The same key design and truck considerations that apply to conventional diamond inter- changes also apply to tight diamond interchanges and âfolded diamondâ interchanges (also known as partial cloverleaf interchanges). Figure 38 illustrates a typical tight diamond inter- change. Tight diamond interchanges have ramps closer to the freeway than the ramps in a conventional diamond and shorter spacing between the ramp terminals on the arterial cross- road. Tight diamond interchanges are used primarily in urban areas where available right of way is limited, and the interchange must be fitted into a constrained space. Truck considerations for tight diamond interchanges are more challenging than for conventional diamond inter- changes because the ramps are often shorter and because there is less storage space on the arterial crossroad between the ramp terminals. Potential truck volumes and storage space needs should be considered by designers in making the decision between providing a conventional diamond or a tight diamond interchange. A folded diamond or partial cloverleaf interchange is similar in concept to a conventional diamond except that one or more diagonal ramps are replaced by loop ramps. Figure 39 illus- trates several typical configurations for partial cloverleaf interchanges. Partial cloverleaf inter- changes are often used where ramp construction in one or more quadrants of the interchange is infeasible (e.g., due to a railroad or river parallel to the arterial crossroad) or where more than one exit or entrance ramp is needed to serve the traffic demands. The two-quadrant partial cloverleaf designs typically have Stop-sign or traffic-signal control at the crossroad ramp terminals, and thus, operate similarly to conventional diamond interchanges. Design and truck consider- ations for two-quadrant partial cloverleaf interchanges are essentially the same as for conven- tional diamond interchanges. Four-quadrant partial cloverleaf designs are essentially the same as a conventional diamond interchange with loop ramps added in one or more quadrants of the interchange. Such loop ramps operate with free-flow movement to or from the arterial crossroad, and thus, add design considerations similar to those for a full cloverleaf interchange. Figure 38. Crossroad area of a typical tight diamond interchange.
68 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide 4.4.2 Single-Point Diamond Interchanges SPDIs sometimes referred to as single-point urban interchanges (SPUIs) or just as single- point interchanges, are a modified form of a diamond interchange where opposing left turns from the ramps can flow simultaneously. The same is true for the left turns from the arterial crossroad. The ramp terminals that are controlled by separate, but coordinated, signal instal- lations in a conventional diamond interchange are controlled as a single signalized intersection in a SPDI. Figure 40 shows the layout of a typical SPDI. SPDIs are beneficial for locations with high turning volumes as well as locations with restricted right of way. Larger bridge structures are required compared with other diamond interchange types, which may result in higher construction costs. Agencies generally agree that SPDIs work well for trucks. SPDIs are preferred in situations where there are high volumes of left-turning trucks. Since there is only a single ramp terminal, there is no queuing or storage of left-turning trucks within the interchange. SPDIs also work well for turning trucks because the left- and right-turn radii are typically larger than those at conventional diamond interchanges. The left turns within a SPDI also typically work well, even for oversized permit trucks, because of the larger turning radii. CADD systems can be used to ensure the turning paths of trucks are accommodated in the design of a SPDI. Queue storage on both the ramps and the crossroad needs to be adequate to prevent trucks from either spilling back onto the freeway facility or spilling back into upstream intersections on the crossroad. Because of the traffic-signal phasing scheme of a SPDI, it is difficult to provide signalized pedestrian crossings across the crossroad at the interchange. However, this can be accomplished if two-stage pedestrian crossings are used and an adequately sized pedestrian refuge area is provided in the median of the crossroad. Many agencies opt not to provide a pedestrian cross- ing across the crossroad at the interchange, and instead, accommodate pedestrian crossings at another location along the crossroad. Figure 39. Typical partial cloverleaf interchanges. (a) (b)
Geometric Design and Access Management to Accommodate Trucks 69 4.4.3 Full Cloverleaf Interchanges A typical full cloverleaf interchange is illustrated in Figure 41. In this interchange configu- ration, all of the crossroad ramp terminals have free-flow operation, rather than Stop signs or traffic signals. The arterial crossroad is typically characterized by a free-flow ramp terminal with a loop exit ramp from the freeway followed by a free-flow ramp terminal with a loop entrance ramp to the freeway. This configuration creates a weaving area between the successive loop ramp terminals that can present traffic operational challenges on the arterial crossroad even in the absence of trucks. Substantial truck volumes on the loop ramps and the arterial crossroad can magnify the traffic operational problems. The most common treatment to ease traffic operational problems in such a weaving area is to include one or more auxiliary lanes extending from the loop exit ramp to the loop entrance ramp so that weaving maneuvers can be made more readily. It is also possible to include a collector-distributor (C-D) road along the arterial crossroad to which both loop ramps connect, although this treatment is more common at the freeway end of the ramps than at the crossroad end. The free-flow ramp termi- nals on the crossroad at the outer connection exit and entrance ramps are less likely to experi- ence traffic operational problems on the crossroad unless there are intersections or driveways located very close to the ramp terminals, or the merge areas are not appropriately designed to accommodate merging maneuvers by trucks. 4.4.4 Diverging Diamond Interchanges DDIs, also known as double crossover diamond interchanges, are a unique type of diamond interchange in which a portion of each direction of the arterial crossroad is positioned to the left of the centerline relative to their direction of flow. The two crossover points on the arte- rial crossroad occur at each ramp terminal. DDIs have fewer conflict points and provide greater capacity than conventional diamond interchanges. Each crossover point requires signalization; however, each signal only has two critical phases versus the three critical phases Figure 40. Typical layout of a single-point diamond interchange.
70 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide of a conventional diamond ramp terminal. Figure 42 shows a basic plan view of the traffic flows at a DDI. Transportation agencies and field observations indicate that DDIs work well for trucks (Potts et al. 2019). The primary reason for the success of DDIs in accommodating trucks is that queuing on the arterial crossroad between the ramp terminals is minimal. Traffic from the crossroad making a left turn onto the freeway facility has a free-flow or yield movement onto the ramp, preventing queuing onto the segment between the terminals. Also, because signal cycle lengths can be relatively short compared with those of a conventional diamond interchange, delay for traffic turning left from the ramp onto the arterial crossroad is reduced, which also minimizes long queues between the ramp terminals. Figure 43 shows trucks traveling on the roadway segment between the crossover points. The horizontal curves at the ramp terminals of current DDI designs typically have sufficient radii that trucks can traverse the curves while remaining within their lane of traffic. DDIs that are expected to accommodate trucks should be designed such that the turning paths of trucks are fully contained within the marked lane, especially on multilane approaches. This can be verified during the design process in CADD systems. Signal timing is another important aspect to consider. Coordination of the two signals will keep traffic movements with heavy truck volumes flowing without producing long queues between the ramp terminals. DDIs commonly serve other modes of transportation including pedestrians and bicyclists. The Diverging Diamond Interchange Informational Guide (Schroeder et al. 2014) provides Figure 41. Typical full cloverleaf interchange.
Geometric Design and Access Management to Accommodate Trucks 71 a good resource for design alternatives to accommodate pedestrians and bicyclists as well as transit and heavy vehicles at a DDI. 4.4.5 Roundabout Interchanges Roundabout interchanges are interchanges with roundabouts, rather than with signalized or Stop-controlled intersections at the ramp terminals, as illustrated in Figure 44. The most common type of roundabout interchange is a traditional diamond interchange with a roundabout Figure 42. Basic schematic of traffic flows in DDI (FHWA 2018b). Figure 43. Trucks traversing segment between ramp terminals approaching the crossover at a DDI. (Photo source: MRIGlobal.)
72 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide Figure 44. Typical roundabout interchange. (Source: Applied Technology and Traffic Analysis Program, Maryland State Highway Administration.) AR TE R IA L CROSS STREET on the arterial crossroad at each ramp terminal. The absence of traffic signals and Stop signs allows traffic to flow freely to, from, and along the arterial crossroad, reducing delays and congestion at the interchange. Providing roundabouts at ramp terminals can reduce traffic crashes by eliminating right-angle and left-turn conflicts, just as they do at noninterchange locations (see Section 4.3.7). A roundabout interchange can be beneficial where the interchange experiences high left-turn volumes from the exit ramps, entrance ramps, or both, and where only limited space is available for queue storage on the bridge crossing, exit ramp, or arterial approaches. Roundabout inter- changes have become more common in recent years because they have the potential to improve safety and traffic operations and eliminate the need to widen bridges. The roundabouts used in a roundabout interchange can have either a circular central island or a teardrop-shaped central island. Figure 45 illustrates a roundabout interchange with teardrop-shaped central islands. The circular central island is preferable where it is desirable to allow U-turns at each roundabout or to provide access to legs other than the cross street and ramps (Rodegerdts et al. 2010). Interchanges with teardrop-shaped roundabouts help
Geometric Design and Access Management to Accommodate Trucks 73 prevent wrong-way movements onto the freeway. Because U-turns at interchanges with teardrop-shaped roundabouts require navigating two roundabouts, these interchanges are preferable where minimal U-turns are expected. 4.5 Driveways Driveways are essentially private roads that provide access between public roadways and abutting development. However, the term driveway typically refers to just a part of a drivewayâ the area where the driveway intersects the public highway or street (Gattis et al. 2010). The Green Book refers to the point at which the driveway joins a public highway or street as a driveway terminal, and states that driveway terminals are, in effect, at-grade intersections. Access points, including driveways, introduce conflicts and friction into the traffic stream. Vehicles entering and leaving the main roadway often slow the through traffic, and the difference in speeds between through and turning traffic increases crash potential. The design of a driveway varies and depends on several factors, including the following: â¢ Type of development. â¢ Design vehicle. â¢ Traffic volume. â¢ Proximity to intersection. â¢ Exposure to bicyclists and pedestrians. Some driveway design practices create challenges for pedestrians and bicyclists, while others are problematic for motorists. Where trucks are expected to use a driveway, particular consideration should be given to truck needs when designing various elements of the driveway. Koepke and Levinson (1992) note that for properties expecting moderate volumes of large truck traffic, it is desirable to provide one well-designed service or truck driveway to accommo- date such vehicles, allowing only passenger-type vehicles to use other appropriately designed Figure 45. Roundabout interchange with teardrop-shaped central islands at each roundabout. (Photo source: Missouri DOT.)
74 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide driveways within the development. Gattis et al. present design guidance on all aspects of drive- ways. Several driveway design elements that deserve particular attention when accommodating trucks are discussed as follows: 4.5.1 Driveway Throat Width Koepke and Levinson recommend that, where a driveway is to be used by larger vehicles (e.g., farm equipment or trucks), a driveway width of at least 20 ft should be provided and entrances up to 30 ft wide may be permitted. Gattis et al. recommend that, for driveways often used by large trucks, driveway width should be at least 26 ft. At driveways with high existing or anticipated pedestrian activity, the driveway width should reflect the needs of both motor vehicle and pedestrian traffic. 4.5.2 Driveway Throat Length The driveway throat length represents the distance from the outside edge of the traveled way to the first point along the driveway at which there are conflicting traffic movements (Gattis et al. 2010). Figure 46 illustrates driveway throat length. The driveway throat needs to be long enough for vehicles to enter, exit, or circulate on the site without interfering with each other or with through traffic on the abutting roadway. A longer throat length may be needed based on the volume and length of trucks projected for the site. 4.5.3 Driveway Profile The vertical profile of a driveway should be designed such that the undercarriage of a truck or trailer does not hang up on the pavement. Williams et al. (2014) recommend consider- ation of trucks and their trailers when determining driveway vertical profiles, and Gattis et al. present guidelines on driveway vertical profiles, based on driveway type and intensity of use. At industrial driveways or other driveways frequently used by larger vehicles, and where low-body trailers are expected, crest breakover grade differences without a vertical curve should be limited to 3.5%. Gattis et al. also illustrate potential problems related to poorly designed crest and sag vertical curves at driveways (see Figure 47). 4.5.4 Curb Termination Treatments Where a curb is present on either side of a driveway, the curb may be terminated in a variety of ways, including terminated with an abrupt end, terminated by means of a drop-down curb, or terminated by means of a return curb. Figure 48 illustrates each of these curb termination treatments. Figure 46. Definition of driveway throat length (Gattis et al. 2010).
Geometric Design and Access Management to Accommodate Trucks 75 An abrupt end to a curb is undesirable because it is likely to snag a vehicle tire. The vertical face of a return curb provides entry-edge definition for an approaching motorist. Some truck drivers have suggested that flat or mountable curbs be provided for a short distance on either side of the driveway to facilitate trucks turning into and out of the driveway. 4.5.5 Curb Return Radii for Driveways According to the survey conducted during this research, transportation agencies identi- fied curb return radius as one of their most common concerns (Potts et al. 2019). Gattis et al. recommend using a curb return radius ranging from 40 to 75 ft for driveways with large trucks as the design vehicle; however, they note that accommodating trucks with a simple radius design (with sufficient radius to prevent encroachment on the curb) may result in a very wide driveway opening. To better accommodate the wheel paths of turning trucks without paving such a wide area, Gattis et al. recommend referring to the Green Book discussion of designing simple curves with a taper and designing three-centered compound curves. Figure 49 illus- trates the geometry of a three-centered curve at a 90-degree curb return radius. Koepke and Levinson (1992) also state that the most efficient design for a large vehicleâs turning transition can be made by constructing a curb return with a series of compound curves or by using a simple curb radius with transitioning tapers. 4.5.6 Medians in Driveways Where medians are provided in driveways, the half-bullet nose design provides a larger turning radius, which in turn accommodates the swept path of a large truck (see Figure 50). If an island is located at the end of the driveway, mountable curbs and reinforced pavement Figure 47. Design of driveway profiles to accommodate low-body vehicles (Gattis et al. 2010). Figure 48. Methods to terminate the curb at a driveway (Gattis et al. 2010).
76 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide may need to be provided within the island to deal with large trucks overrunning the island (Gattis et al. 2010). 4.5.7 One-Way versus Two-Way Operations If a driveway is to operate with two-way traffic, the designer needs to consider whether the driveway should be designed to accommodate simultaneous entry and exit maneuvers by trucks (and if so, what types of trucks). One-way operation simplifies the design of the driveway because there are no potential conflicts between trucks traveling in opposite directions. More guidance about this issue is presented in Section 3.1.6 on site layout. 4.5.8 Right-in/Right-out Operations Both one-way and two-way driveways can be designed for right-in/right-out operation, where left turns in and out of the driveway are not permitted. Right-in/right-out operation reduces the number of traffic conflict points at the driveway, analogous to the difference between the number of conflict points at three-leg and four-leg intersections. On undivided streets, the left-turn restric- tion is communicated to drivers by signing. On divided streets, the left-turn restriction is commu- nicated to drivers by not providing a median opening at the driveway; the lack of a median opening is typically reinforced by the provision of one-way signs in the median opposite a drive- way restricted to right-out operation. Part of the decision to use right-in/right-out operations at a driveway should be an assessment of the appropriateness of alternative routes that trucks and other vehicles will use to reach or leave the driveway in question. Figure 50. Design of a driveway median end with a half-bullet nose shape (Gattis et al. 2010). Figure 49. Geometry of a symmetrical three-centered curve (Gattis et al. 2010). Offset R1 R 2 R2R 1
Geometric Design and Access Management to Accommodate Trucks 77 4.5.9 Combining Driveways A common access management solution is combining multiple driveways to reduce the number of driveways and the driveway density along a corridor and to minimize the conflicts between turning vehicles and through traffic. Combining driveways to a single commercial business establishment makes sense when three or more driveways or a continuous driveway frontage are provided. There are locations, however, where at least two driveways are needed to accommodate delivery truck circulation within the site. Combining driveways can also be accomplished by using a single driveway or a pair of driveways to provide access to two or more adjacent commercial business establishments. One solution used by a transportation agency to reduce the apparent number of driveways during normal business hours (including peak traffic periods) is to provide one conventional driveway plus a textured concrete mountable curb/apron that can be used as a driveway by trucks during nonbusiness hours (see Figure 51). During business hours, parking is permitted along the apron, preventing its use as a driveway by customers. The textured and colored surface also discourages passenger-car use, while drivers arriving during nonbusiness hours become familiar with the textured concrete entrance. Pervious pavement may also be used for this application. 4.5.10 Parking Adjacent to Driveways On-street parking should be restricted immediately adjacent to driveways. When vehicles are parked adjacent to a driveway, truck drivers indicate that it is nearly impossible to turn into and out of the driveway. 4.5.11 Location of Gates for Driveways Some industrial facilities that are origins or destinations for truck shipments have gates to control access to the site and the loading dock. When the gate is closed, queues of trucks may form, waiting to enter the facility. Therefore, such gates should be located so that any queue of trucks will not extend into the traveled way of the adjacent street and delay through traffic or create traffic conflicts on the street. Gates can either be located on the driveway away from the street or a shoulder, parking, or waiting area for trucks can be created along the street outside of the traveled way. The location of any gates should be considered part of the site layout and internal circulation review (see Section 3.1.6). The storage needs for trucks waiting for the gate to open depend on the arrival rates of trucks and hours of service for the loading and unloading facility. Figure 51. Mountable truck apron for use by delivery trucks. (Photo source: Strand Associates, Inc.)
78 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide 4.5.12 Pedestrian and Bicycle Considerations Every driveway creates a potential for conflicts between motor vehicles, pedestrians, and bicyclists. Driveways are crossed by pedestrians on sidewalks, and bicyclists cross the paths of vehicles entering and leaving driveways. A challenge in driveway design is to identify an appropriate balance between the sometimes conflicting needs of the anticipated user groups. Driveways should be designed so that the driveway is clearly visible to pedestrians (e.g., use of a paving material that contrasts with the sidewalk) and with sufficient sight distance that the sidewalk is visible to drivers of turning vehicles, including trucks. Bicycle lanes should be marked so that they are clearly visible to turning drivers. 4.6 Roadways 4.6.1 Lane Width The Green Book provides substantial flexibility in the selection of lane widths, particularly for urban arterial streets. The Green Book permits the use of lane widths from 10 ft to 12 ft. Transportation agencies are increasingly converting existing roadways with 12-ft lanes to 10- or 11-ft lanes to provide space for other desirable roadway features, such as medians, turn lanes, bicycle lanes, and shorter pedestrian crossing distances. However, given widths of large trucks up to 8.5 ft, and up to 10.5 ft including mirrors, wider lanes are preferable on truck routes. It is desirable to provide 12-ft lanes on truck routes or to use differential lane widths with a 12-ft outside or curb lane and narrower center or left lanes. 4.6.2 Pavement Widening on Horizontal Curves The traveled way may need to be widened at some locations on horizontal curves to fully accommodate trucks. The Green Book indicates that widening is needed on certain curves for one of the following reasons: (1) the design vehicle occupies a greater width because the rear wheels generally track inside front wheels (offtracking) in negotiating curves, or (2) drivers experience difficulty in steering their vehicles in the center of the lane. The added width occupied by the vehicle as it traverses the curve as compared with the width of the traveled way on a tangent can be computed by geometry for any combination of radius and wheelbase. The effect of variation in lateral placement of the rear wheels with respect to the front wheels and the resultant difficulty of steering should be accommodated by widening on curves, but the appro- priate amount of widening cannot be determined as precisely as that for simple offtracking. The traveled-way widening values for the assumed design condition for a WB-62 vehicle on a two-lane highway are presented in Table 8. Widening is costly and little is actually gained from a small amount of widening. It is suggested that a minimum widening of 2.0 ft be used and that values less than 2.0 ft in the shaded area of Table 8 can be disregarded. For design vehicles other than the WB-62 truck, an adjustment from Table 9 should be applied. The widening values in Tables 8 and 9 apply to both two-lane undivided highways and to the individual two-lane roadways of a four-lane divided highway. Traveled-way widening, where needed, is best provided in the original construction of a facility, because it is difficult to incorporate in projects on existing roads. Widening should transition gradually on the approaches to the curve to provide a reason- ably smooth alignment of the edge of the traveled way and to fit the paths of vehicles entering or leaving the curve. The principal points of concern in the design of curve widening, which apply to both ends of highway curves, as presented in the Green Book, are as follows: â¢ On simple (unspiraled) curves, widening should be applied on the inside edge of the traveled way only. On curves designed with spirals, widening may be applied on the inside edge or
U.S. Customary Radius of Curve (ft) Traveled-way width = 24 ft Traveled-way width = 22 ft Traveled-way width = 20 ft Design speed (mph) Design speed (mph) Design speed (mph) 30 35 40 45 50 55 60 30 35 40 45 50 55 60 30 35 40 45 50 55 60 7,000 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.7 0.7 0.8 0.8 0.9 1.0 1.0 1.7 1.7 1.8 1.8 1.9 2.0 2.0 6,500 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.7 0.8 0.8 0.9 1.0 1.0 1.1 1.7 1.8 1.8 1.9 2.0 2.0 2.1 6,000 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.7 0.8 0.9 0.9 1.0 1.1 1.1 1.7 1.8 1.9 1.9 2.0 2.1 2.1 5,500 0.0 0.0 0.0 0.0 0.1 0.1 0.2 0.8 0.9 0.9 1.0 1.1 1.1 1.2 1.8 1.9 1.9 2.0 2.1 2.1 2.2 5,000 0.0 0.0 0.0 0.1 0.1 0.2 0.3 0.9 0.9 1.0 1.1 1.1 1.2 1.3 1.9 1.9 2.0 2.1 2.1 2.2 2.3 4,500 0.0 0.0 0.1 0.1 0.2 0.3 0.4 0.9 1.0 1.1 1.1 1.2 1.3 1.4 1.9 2.0 2.1 2.1 2.2 2.3 2.4 4,000 0.0 0.1 0.2 0.2 0.3 0.4 0.5 1.0 1.1 1.2 1.2 1.3 1.4 1.5 2.0 2.1 2.2 2.2 2.3 2.4 2.5 3,500 0.1 0.2 0.3 0.4 0.5 0.5 0.6 1.1 1.2 1.3 1.4 1.5 1.5 1.6 2.1 2.2 2.3 2.4 2.5 2.5 2.6 3,000 0.3 0.4 0.4 0.5 0.6 0.7 0.8 1.3 1.4 1.4 1.5 1.6 1.7 1.8 2.3 2.4 2.4 2.5 2.6 2.7 2.8 2,500 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.5 2.6 2.7 2.8 2.9 3.0 3.1 2,000 0.7 0.9 1.0 1.1 1.2 1.3 1.4 1.7 1.9 2.0 2.1 2.2 2.3 2.4 2.7 2.9 3.0 3.1 3.2 3.3 3.4 1,800 0.9 1.0 1.1 1.3 1.4 1.5 1.6 1.9 2.0 2.1 2.3 2.4 2.5 2.6 2.9 3.0 3.1 3.3 3.4 3.5 3.6 1,600 1.1 1.2 1.3 1.5 1.6 1.7 1.8 2.1 2.2 2.3 2.5 2.6 2.7 2.8 3.1 3.2 3.3 3.5 3.6 3.7 3.8 1,400 1.3 1.5 1.6 1.7 1.9 2.0 2.1 2.3 2.5 2.6 2.7 2.9 3.0 3.1 3.3 3.5 3.6 3.7 3.9 4.0 4.1 1,200 1.7 1.8 1.9 2.1 2.2 2.4 2.5 2.7 2.8 2.9 3.1 3.2 3.4 3.5 3.7 3.8 3.9 4.1 4.2 4.4 4.5 1,000 2.1 2.3 2.4 2.6 2.7 2.9 3.0 3.1 3.3 3.4 3.6 3.7 3.9 4.0 4.1 4.3 4.4 4.6 4.7 4.9 5.0 900 2.4 2.6 2.7 2.9 3.1 3.2 3.4 3.6 3.7 3.9 4.1 4.2 4.4 4.6 4.7 4.9 5.1 5.2 800 2.7 2.9 3.1 3.3 3.5 3.6 3.7 3.9 4.1 4.3 4.5 4.6 4.7 4.9 5.1 5.3 5.5 5.6 700 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 600 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 500 4.6 4.9 5.1 5.3 5.6 5.9 6.1 6.3 6.6 6.9 7.1 7.3 450 5.2 5.4 5.7 6.2 6.4 6.7 7.2 7.4 7.7 400 5.9 6.1 6.4 6.9 7.1 7.4 7.9 8.1 8.4 350 6.8 7.0 7.3 7.8 8.0 8.3 8.8 9.0 9.3 300 7.9 8.2 8.9 9.2 9.9 10.2 250 9.6 10.6 11.6 200 12.0 13.0 14.0 NOTES: Values shown are for WB-62 design vehicle and represent widening in feet. For other design vehicles, use adjustments in Table 3-25 in the Green Book. Values less than 2.0 ft may be disregarded. For 3-lane roadways, multiply above values by 1.5. For 4-lane roadways, multiply above values by 2. Table 8. Design values for traveled-way widening on open highway curves for two-lane undivided and four-lane divided highways (AASHTO 2018).
U.S. Customary Radius of Curve (ft) Design Vehicle SU-30 SU-40 WB-40 WB-67 WB-67D WB-92D WB-100T WB-109D 7,000 â1.2 â1.2 â1.2 0.1 â0.1 0.1 â0.1 0.2 6,500 â1.3 â1.2 â1.2 0.1 â0.1 0.1 â0.1 0.2 6,000 â1.3 â1.2 â1.2 0.1 â0.1 0.1 â0.1 0.2 5,500 â1.3 â1.3 â1.2 0.1 â0.2 0.1 â0.1 0.2 5,000 â1.3 â1.3 â1.3 0.1 â0.2 0.1 â0.1 0.3 4,500 â1.4 â1.3 â1.3 0.1 â0.2 0.1 â0.1 0.3 4,000 â1.4 â1.4 â1.3 0.1 â0.2 0.1 â0.1 0.3 3,500 â1.5 â1.4 â1.4 0.1 â0.3 0.1 â0.1 0.4 3,000 â1.6 â1.5 â1.4 0.1 â0.3 0.1 â0.1 0.5 2,500 â1.7 â1.6 â1.5 0.2 â0.4 0.2 â0.1 0.5 2,000 â1.8 â1.7 â1.6 0.2 â0.5 0.2 â0.2 0.7 1,800 â1.9 â1.8 â1.7 0.2 â0.5 0.2 â0.2 0.8 1,600 â2.0 â1.9 â1.8 0.2 â0.6 0.3 â0.2 0.8 1,400 â2.2 â2.0 â1.9 0.3 â0.6 0.3 â0.3 1.0 1,200 â2.4 â2.2 â2.1 0.3 â0.8 0.3 â0.3 1.1 1,000 â2.7 â2.4 â2.3 0.4 â0.9 0.4 â0.4 1.4 900 â2.8 â2.6 â2.4 0.4 â1.0 0.5 â0.4 1.5 800 â3.1 â2.8 â2.6 0.5 â1.1 0.5 â0.4 1.7 700 â3.4 â3.0 â2.9 0.6 â1.3 0.6 â0.5 1.9 600 â3.8 â3.4 â3.2 0.7 â1.5 0.7 â0.6 2.3 500 â4.3 â3.8 â3.6 0.8 â1.8 0.8 â0.7 2.7 450 â4.7 â4.2 â3.9 0.9 â2.0 0.9 â0.8 3.0 400 â5.2 â4.6 â4.3 1.0 â2.3 1.0 â0.9 3.4 350 â5.8 â5.1 â4.7 1.1 â2.6 1.2 â1.0 3.9 300 â6.6 â5.8 â5.4 1.3 â3.0 1.4 â1.2 4.6 250 â7.7 â6.7 â6.3 1.6 â3.6 1.7 â1.4 5.5 200 â9.4 â8.2 â7.6 2.0 â4.6 2.1 â1.8 7.0 NOTES: Adjustments are applied by adding to or subtracting from the values in Table 3-24 in the Green Book. Adjustments depend only on radius and design vehicle; they are independent of roadway width and design speed. For 3-lane roadways, multiply above values by 1.5. For 4-lane roadways, multiply above values by 2.0. Table 9. Adjustments for traveled-way widening values on open highway curves for two-lane undivided and four-lane divided highways (AASHTO 2018).
Geometric Design and Access Management to Accommodate Trucks 81 divided equally on either side of the centerline. In the latter method, extension of the outer edge tangent avoids a slight reverse curve on the outer edge. In either case, the final marked centerline, and preferably any central longitudinal joint, should be placed midway between the edges of the widened traveled way. â¢ Curve widening should transition gradually over a length sufficient to make the whole trav- eled way fully usable. Although a long transition is desirable for traffic operation, it may result in narrow pavement slivers that are difficult and expensive to construct. Preferably, widening should transition over the superelevation runoff length, but shorter lengths are sometimes used. Changes in width normally should be affected over a distance of 100 ft to 200 ft. 4.6.3 Lateral Offset The Green Book and most transportation agency geometric design criteria call for a 1.5-ft area outside the edge of the traveled way or shoulder to be clear of roadside obstacles, such as poles, signs, and other roadside hardware. This 1.5-ft lateral offset provides an allowance for truck and bus mirrors and also provides space for opening the doors of parked vehicles. This lateral offset is important to accommodate trucks on the roadway at all locations, but especially where lanes are narrower than 12 ft or where trucks frequently make right turns. The AASHTO Roadside Design Guide (AASHTO 2011) recommends that the 1.5-ft lateral offset be increased to 3 ft at intersections. Additional lateral offset to roadside safety hardware may be needed at the crossroad ramp terminals within interchanges. 4.6.4 Shoulders Paved shoulders at least 10 ft wide are desirable on truck routes in rural areas to provide a recovery area for vehicles that leave the roadway and to accommodate large trucks that need to stop because of breakdowns or law enforcement activities. Unpaved shoulders may be provided, if well stabilized, to support the weight of a truck without settlement. Large trucks are allowed to have widths up to 8.5 ft, so a wide truck that stops on a shoulder with a width of 8 ft or less is likely to encroach on the traveled way or an unpaved roadside area not suitable to support the weight of a large truck. Shoulders are desirable to accommodate trucks on urban roadways with design speeds of 50 mph or higher. However, many urban streets with design speeds of 45 mph or less have curb-and-gutter cross sections, rather than shoulders. This cross section can accommodate trucks, because speeds are lower in urban than in rural areas, and alternative stopping places for trucks are more frequent. Where shoulders are not provided, the provision of a 1.5-ft lateral offset outside the curb that is clear of signs, poles, or other roadside objects is especially important to prevent the mirrors of trucks from striking roadside objects. The Green Book specifies a maximum 8% algebraic difference in cross slope between a shoulder and the adjacent traveled-way pavement surface. This maximum cross-slope break is most critical on superelevated cross sections on the outside edge of a horizontal curve, where the shoulder typically slopes in the opposite direction to the adjacent traveled-way pavement surface. The maximum cross-slope break is particularly important for large trucks, which may lose control when traversing a large change in cross slopes (Torbic et al. 2017). 4.6.5 Horizontal and Vertical Clearance at Structures The minimum clear width for new bridges on arterials, including truck routes, should be the same as the curb-to-curb width on the approach roadway (AASHTO 2018). New or reconstructed structures on arterials, including truck routes, should provide at least 16 ft of vertical clearance. The Green Book allows existing structures with 14-ft vertical clearance
82 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide to be retained, where permitted by local statute, although such reduced clearances are undesir- able on truck routes. 4.6.6 Turnouts for Law Enforcement Activities Where paved shoulders wider than 8.5 ft are not provided on truck routes, it may be desir- able for transportation agencies to provide paved turnouts, at intervals, so that trucks can stop outside the traveled way for law enforcement activities. 4.7 Intersection Sight Distance Chapter 9 of the Green Book presents design criteria for intersection sight distance (ISD). Seven cases for ISD design are addressed in the Green Book based on the type of traffic control present: â¢ Case AâIntersections with no control. â¢ Case BâIntersections with Stop-control on the minor road. â¢ Case CâIntersections with yield control on the minor road. â¢ Case DâIntersections with traffic-signal control. â¢ Case EâIntersections with all-way Stop-control. â¢ Case FâLeft turns from the major road. â¢ Case GâRoundabouts. Since trucks accelerate from a stop more slowly than passenger cars, the models for three of the seven cases (Cases B, C, and F) include alternative parameter values suitable for accom- modating trucks. Sight distances for these three cases are based on a gap-acceptance model, and the value for the accepted gap is increased by 2 s above the passenger-car value for single-unit trucks and 4 s above the passenger-car value for combination trucks. 4.8 Highway-Railroad Grade Crossings Trucks are an important consideration in the design of highway-railroad grade crossings on truck routes. Geometric design policies for highway-railroad grade crossings are established in the Green Book. The geometric design criteria in the Green Book that can incorporate truck considerations include the following: â¢ Sight distance for motor vehicles stopped at a grade crossing. Accommodation of trucks is addressed in Case B in Green Book Section 9.12.4, which addresses the sight distance needed for a 73.5-ft truck to cross a single set of tracks from a stopped position. â¢ Distance between grade crossing and adjacent intersections. This distance should be long enough to accommodate a truck or so short that truck drivers will pass through both the grade crossing and the intersection without stopping. â¢ Vertical profile of crossing. Trucks, even those with low ground clearance, should be able to pass through the crossing without contacting the roadway surface. 4.9 Sidewalks Curb-attached sidewalks are undesirable on truck routes where the traveled way is imme- diately adjacent to the curb. Based on Green Book criteria, separation of at least 2 ft between the traveled way and sidewalk is desirable for sidewalks. This criterion appears appropriate for sidewalks along truck routes, just as it is for any roadway without shoulders.
Geometric Design and Access Management to Accommodate Trucks 83 4.10 Bicycle Facilities In the multimodal planning process, it may be desirable, where practical, to locate truck routes and bicycle facilities on different roads or streets within a corridor. Where bicycle facilities are provided along a truck route, the bicycle facilities should be designed to mini- mize conflicts between bicycles and trucks or other vehicles. This may be accomplished by the following: â¢ Providing an off-road bicycle path or shared-use path. â¢ Providing a buffer to separate on-road bicycle lanes from adjacent travel lanes to accommodate greater separation between bicycles and trucks. â¢ Providing wider lanes, especially wider curb lanes, to accommodate greater separation between bicycles and trucks. Where buffered bicycle lanes are provided, a minimum buffer width of 1.5 ft to 2.0 ft is appropriate (Torbic et al. 2014), but buffer widths of 3 ft are increasingly being used even along roadways that are not truck routes. Where neither on-road nor roadside bicycle facilities are provided, bicyclists can ride on paved shoulders to keep away from motor vehicle traffic. It is undesirable to place a bicycle lane on a route where frequent pickup and delivery operations occur, because the presence of parked vehicles in a bicycle lane would create a challenge for bicyclists. 4.11 Work Zones Work zones on truck routes should be designed to accommodate the appropriate truck design vehicle, selected based on the vehicle mix expected to travel through the work zone. Where turning trucks are expected to be present in work zones, CADD software should be used to verify that the turning path of the truck can be accommodated within the work zone. Lane widths equivalent to those on the upstream roadway should be used in work zones. Where lane width restrictions are present in a work zone, signs indicating the lane width restric- tions in the work zone should be placed far enough upstream of the work zone to allow the truck driver the opportunity to seek alternate routes if needed. Lane widening on horizontal curves within the work zone should be provided where needed (see Section 4.6.2). Some construction projects may temporarily close a road to allow the work to be completed more quickly than if traffic were maintained through the work zone. In other cases, perhaps because of vertical clearance restrictions, a decision may be made to prohibit trucks through a work zone, while keeping the work zone open to other traffic. In such cases, a suitable detour route that can accommodate the appropriate design vehicle should be established and signed. Some agencies have reported problems near road closures in which truck drivers chose to use roadways not designed for trucks or where trucks are prohibited. Enforcement in the vicinity of detours may be needed to make sure trucks are following the signed detour route.