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Geometric Design Elements 65 Exhibit 5-61. Minimum desirable circular turnaround dimensions. Vehicles Accommodated Radius to Face of Outer Curb Comments (in feet) Passenger cars only 30 Passenger cars, school 42 A 45-ft radius would allow a central buses, delivery trucks, landscaped island with an inside emergency vehicles curb radius of 25 feet and a 20-ft- wide turning roadway. However, if a passenger vehicle is parked on the turning roadway, this geometry may not accommodate ambulance, fire truck, or solid waste vehicles. Curb parking is permitted 50 Where a landing is provided at the base of the circle, a tangent section of 22 feet for each car should be provided. Source: Kulash, Residential Streets, 3rd ed., Institute of Transportation Engineers, Washington, DC © 2001. Used by permission Specific Turnaround Facilities When a site does not include features such as an internal roadway system or parking lot circu- lation that allow a driver to re-orient a vehicle and proceed from the site in a forward direction onto the public roadway, then specific turnaround facilities may be needed. One common form is the cul-de-sac or circular turnaround. A circular turnaround can be cen- tered on the driveway or offset to one side. Circular turnarounds are generally preferable, although T-shaped and Y-shaped (hammerhead) turnarounds may be used. Exhibit 5-61 offers minimum desirable circular turnaround dimensions (5-23). With the T- and Y-shapes, vehicles turn left into the special roadway at the end of the driveway, back across the drive, and then proceed forward to turn left into the driveway. For passenger cars, a 60- by 20-ft area is needed. Advantages of T- or Y-shaped turnarounds are that they have lower construction and maintenance costs and require less land than circular turnarounds. Because T- or Y-shaped turnarounds require all vehicles to make a back-up movement, their application is limited to very low-volume driveways. Driveway Vertical Alignment Elements This section provides guidelines for designing the vertical alignment (or profile), which con- sists of grades and vertical curves. Designers should establish a vertical alignment that allows vehicles to conveniently and expeditiously enter and exit the driveway. Designers should avoid profiles that allow the underside of a vehicle to drag or hang-up. When establishing the vertical alignment of the driveway, the designer must consider limitations on the sidewalk cross slope to accommodate pedestrians and pedestrians with disabilities. Also, designers should check the profile to make sure it is not creating drainage problems. Vertical Clearance For many driveways, vertical clearance is not an issue. But where there are overhead struc- tures, utility lines, or vegetation, the designer should check that the vertical clearance is adequate for the design vehicles.
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66 Guide for the Geometric Design of Driveways Sidewalk Cross Slope (Driveway Grade) When this guide was prepared, the ADA design requirements for accessibility related to pedes- trian facilities in the public right-of-way were still being developed. Although only draft accessi- bility guidelines and other technical assistance advisory documents from federal sources were available, the ADA does and will continue to apply to sidewalks, curb ramps, and pedestrian crossings at driveways that are newly constructed or altered since January 26, 1992. The scop- ing provisions of these draft guidelines define where and to what degree accessibility within the public right-of-way is required and state the following: R201.1 Scope. All newly designed and newly constructed facilities located in the public right-of-way shall comply with these requirements. All altered portions of existing facilities located in the public right-of-way shall comply with these requirements to the maximum extent feasible. The ADA defines "facilities" very broadly and the US Department of Justice states in its ADA Title III regulations that this definition " . . . includes both indoor and outdoor areas where human-constructed improvements, structures, equipment, or property have been added to the natural environment" (see 56 Fed. Register page 35550). The pedestrian crossing at a driveway is a facility covered by the ADA and thus must be made accessible in new construction projects and must be made accessible to the maximum extent feasible in alteration projects. The draft ADA guidelines for public rights-of-way basically require that there be a continuous accessible pedestrian route (i.e., PAR) leading up to and crossing each driveway. This will typically include the following: · The transition between the public sidewalk and the pedestrian crossing (marked or unmarked) at the driveway, which is usually achieved by means of an accessible curb ramp; · The pedestrian cross walk pavement surface; and · Any island improvements within the driveway that pedestrians must traverse. The pedestrian crossing at newly constructed driveways must offer a minimum 48-in.-wide route with a cross slope no greater than 2 percent. Where the driveway is an alteration to existing improve- ments within the public right-of-way, the pedestrian crossing portion must offer a cross slope no steeper than 2 percent to the maximum extent feasible, given existing site-related constraints. Site-related constraints that may prohibit strict compliance with the ADA maximum 2 percent cross slope specifications include severely limited right-of-way or sidewalk width in which to nego- tiate the vertical rise between the roadway elevation and the parking area, or steep existing grades on an adjoining, densely developed property that the driveway serves. Engineering judgment plays a key role during the design of driveway alteration projects where full accessibility is not being offered and that judgment may be challenged under ADA by experts analyzing every detail of the design and site factors that may or may not be found to justify any alleged access barriers created by the design. Exhibit 5-62 shows a driveway grade rising quickly from the gutter line, creating an excessive and unacceptable cross slope for the pedestrian path. The five illustrations constituting Exhibit 5-63 (taken from the recommendations of the US Access Board's Public Rights-of-Way Accessibility Advisory Committee published in "Building a True Community") demonstrate driveway design options that comply with the accessibility specifications in the draft ADA guidelines. · Option A, Ramp Sidewalk, shows the sidewalk simply ramping down at each side of the drive- way with a maximum 2 percent or 1:48 cross slope along the pedestrian crossing. · Option B, Apron Offset Sidewalk, shows a directional offset in the sidewalk to avoid the steep cross slope that would otherwise be created by crossing the driveway apron on the steeply slop- ing portion.
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Geometric Design Elements 67 Exhibit 5-62. Example of driveway grade creating Exhibit 5-63. Examples of driveways that unacceptable sidewalk cross slope. comply with accessibility specifications. · Option C, Gutter Bridgeplate, shows the whole width of the sidewalk having a limited cross slope and employs a bridgeplate over the gutter at a rolled curb condition to limit the likeli- hood of vehicles bottoming out. · Option D, Wide Sidewalk, uses the rear most 48 inches of the driveway apron to cross the drive without a cross slope steeper than 2 percent. · Option E, Setback Sidewalk, shows how a more traditional returned curb style driveway apron can be installed between the gutter and the street side of the sidewalk which adjoins a land- scaped green space. At pedestrian crossings in driveways of more developed commercial sites where the drive- way more closely resembles a street, the design and construction of the pedestrian crossing area between the curb ramps is subject to the same 2% maximum cross slope that other drive- ways are subject to. In Exhibit 5-64, the driveway leading into a regional shopping mall looks similar to a roadway intersection; the curb ramps and accessible pedestrian crossings should be constructed with a maximum 2% cross slope. Exhibit 5-64. Driveway entrance at regional mall.
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68 Guide for the Geometric Design of Driveways Driveway-sidewalk crossing transitions call for special attention. Of particular concern are the multi-dimensional tapers that arise from dust-pan and similar flared treatments. The 2004 AASHTO pedestrian design guide points out that "side flares and cross slopes at driveway aprons may cause a drive wheel, caster, or leg tip to lose contact with the surface" (5-2, p. 6162). Therefore, such flares should not be used unless there is another suitable PAR, such as might be provided by a wide sidewalk. Driveway Grade (Sidewalk Cross Slope), Change of Grade, and Vertical Alignment Three types of control for the design of the driveway profile are physical, operational, and drainage: · Physical controls call for a design that maintains enough clearance so the underside of a vehi- cle does not drag on the roadway or driveway surface. This control is necessary for all drive- ways, even one connecting to an alley. Because of the changes in vertical profile grade often found at driveway entrances, these locations are among the more vulnerable to hang ups when the undercarriage of the vehicle comes into contact with or drags the pavement surface. · Operational controls dictate a vertical alignment for the driveway that allows a convenient and safe entry with minimal conflicts. To achieve this, the changes of gradient must not be too abrupt. This is especially important on driveways that intersect higher volume or higher speed roadways. Operational problems may arise from certain combinations of vertical profiles and vehicles. One problem is vehicle-occupant discomfort due to poor vertical alignment such as bumps, steep grades, and abrupt changes in grade. In extreme cases, there may be restricted sight distance, which affect safety adversely. In addition, excessive differences in speed between through vehicles and vehicles turning into or out of the driveway, because of the vertical pro- file, can also increase vehicles' exposure to crashes. · Drainage, requires a profile that does not create undesirable drainage patterns. It may be unac- ceptable for surface runoff in the gutter to flow into the driveway opening and onto private property. Physical Vehicle Ground Clearance Control As Exhibit 5-65 shows, the underside of a vehicle entering or exiting a driveway can drag on either a crest or a sag alignment with an abrupt change of grade. Any excessive grade change between the cross slope of the roadway and the driveway grade, between the driveway grade and an intersecting sidewalk, or between successive driveway grades can cause a vehicle to drag (see Exhibit 5-66). Vehicles with low ground clearance and a long wheelbase or overhang can even become lodged (also referred to as "hung up" or "high-centered") on alignments with sharp grade changes. At best, hang-ups result in some vehicular delay and minor damage to the undercarriage of the vehicle and to the pavement surface. At worst, a crash can occur. Exhibit 5-65. Geometry of ground clearance dragging. CREST: Have problem if axle-to-axle SAG: Have problem if axle-to-bumper underclearance is inadequate. For symmetrical underclearance is inadequate. For symmetrical crest, is critical when axles are a distance WB/2 sag, is critical when one axle is at distance WB from the high point. from the low point. driveway curb roadway curb driveway WB/2 WB/2 roadway WB=wheelbase OHF= front overhang OHR= rear overhang OHR WB OHF WB=wheelbase OHF= front overhang OHR= rear overhang (a) (b)
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Geometric Design Elements 69 Exhibit 5-66. Driveway with multiple scrapes from underside dragging. To design the vertical alignment elements, the designer needs to determine an appropriate design vehicle. As previously discussed, several types of long-wheelbase, low-ground-clearance vehicles can be expected to use some driveways, including articulated beverage trucks, car carri- ers, and passenger car-trailer combinations. The design vehicle for vertical alignment may be dif- ferent from the design vehicle used to design the horizontal alignment (e.g., turning radii). The designer also needs to have a general understanding of the shape of the vertical profile to be nego- tiated by the design vehicle. This includes, for example, the roadway cross slope, the driveway grade line, and other controls (e.g., locations and elevations of intersecting sidewalks). Using reasonable care in selecting a design vehicle and designing the vertical elements to accommodate that vehicle will not completely preclude hang-ups, dragging, or other operational problems from occurring. A vehicle longer and/or lower than the design vehicle may enter a driveway and encounter problems. To meet the needs of shippers, commercial vehicle manufac- turers continue to introduce longer and/or lower vehicles and new vehicle configurations that will require periodic updating of the list of design vehicles. Similarly, as property changes hands or as redevelopment occurs, the nature of the land use served by the driveway may change over time. A different class of vehicle than originally intended may use the driveway. Although this is beyond the control of the designer, it offers an explanation of why hang-ups may happen at locations where they formerly did not occur and represents an issue to be addressed in the permitting process. Also, a vehicle for which the vertical elements have been appropriately designed may encounter problems at a particular driveway. This could be due to vehicle loading condition (e.g., an over- loaded vehicle) that reduces actual ground clearance to something below the design value. The vertical profile is subject to changes over time. For example, the roadway may be milled or resur- faced such that its elevations and cross slopes change. In addition, the roadway and/or driveway (and associated features such as sidewalks) may deform over time due to applied loads, the effects of weather, or construction deficiencies. As mentioned above, the vertical profile(s) used in design should be that reasonably expected to be used by the design vehicle. The possibility always exists that a design vehicle will follow an unusual or out-of-the-ordinary path in negotiating the drive- way such that hang-up or dragging could result. Exhibit 5-67 shows maximum uphill and downhill grades, as reported by transportation agencies in a survey.
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70 Guide for the Geometric Design of Driveways Exhibit 5-67. Reported steepest allowed driveway grades. Normally, Use This in Most Commercial Residential Situations Smallest Smallest Smallest reported reported reported reported reported reported Average Average Average Largest Largest Largest Grade: maximum (+) ill 2.6 9.7 15 5 7.5 10 6 11 15 uphill from road allowed roadway + uph y wa drive Grade: maximum (-) roadway downhill from road - dow n -5 -9.4 -15 -5 -7.8 -10 -6 -11.0 -15 allowed drive hill way NOTE: These values reflect survey responses from 1 local and 16 state transportation agencies. Maximum allowable grade, by itself, is not a sufficient control. What matters is the difference between successive grades, or the change of grade. The change of grade is what creates the crests and sags that cause the underside of a vehicle to drag. Although perhaps not widely recognized, guidance on vertical geometry applicable to driveways has been available for some time. The sec- tion on railroad-highway grade crossing design in AASHTO's policy on geometric design (5-1) provides recommendations on designing the vertical profile at grade crossings. AASHTO rec- ommends that the crossing surface be in the same plane as the top of rails for a distance of 2 feet outside of the rails, and that the surface of the roadway be not more than 3 inches higher or lower than the top of the nearest rail at a point 30 feet from the rail, unless track superelevation dic- tates otherwise. Similarly, a 1987 ITE guideline for driveway design discussed vertical alignment. Eck and Kang (5-24) used a vehicle with a 36-ft wheelbase and 5 inches of ground clearance to analyze a maxi- mum grade change of 3 percent (for low-volume driveways on major or collector streets). This "design vehicle" had problems with the aforementioned geometry, suggesting that the ITE drive- way design recommendations did not accommodate low-clearance vehicles. A similar statement can be made about the AASHTO standard railroad-highway grade crossing since French, Clawson, and Eck (5-25) found that car carrier trailers would hang-up on this crossing. Thus, additional research was conducted to develop driveway vertical alignment guidelines to accom- modate selected design vehicles. Operational Control A research team made measurements at 31 driveways with visible scrapes from the undersides of vehicles, and then measured speeds and elapsed travel times for over 1500 vehicles observed turning right or left into a number of driveways. The speed and elapsed time studies were con- ducted at commercial driveways on built-up suburban (but not CBD) arterial multilane roadways with posted speeds of 40 and 45 mph. All of the roadways had either a raised median or a TWLTL. These data were collected at driveways with right-turn entry radii ranging from 13 to 19.5 feet, and an entry lane width of about 13 feet. Very few vehicles about to enter a driveway exceeded 20 mph at the locations at which speeds were measured. After crossing the driveway threshold, average speeds for vehicles turning left into the driveway were around 10 mph. Vehicles that had turned right into the driveways were slightly slower, with average speeds around 7 mph. The speeds of vehicles entering driveways with breakover sag grades up to 10.5% were close to the speeds of vehicles entering flatter driveways. Scrapes on the pavement surface, presumably from the undersides of vehicles, began to be com- mon with a sag breakover of around 10 percent, and a crest breakover of about 11 percent.
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Geometric Design Elements 71 The study led to the suggestions following in Exhibit 5-68. Except where noted, these guide- lines are based on observations of passenger vehicles (P-vehicle). Where low-clearance vehicles are expected to traverse crest curves, refer to Exhibit 5-69 devel- oped by Eck and Kang (5-26) that suggests vertical curve lengths for various breakover angles (i.e., algebraic difference in grades). Drainage Control Surface runoff from the roadway should not inundate the sidewalk or spill over onto private property. It is also undesirable for the depth of flow to cover the driveway, making it difficult for motorists to determine were the edges of the driveway are. Exhibit 5-68. Driveway vertical profile guidelines. Category Description of Common Vertical Profile Guidelines Applications* Suggestion Rationale STANDARD DRIVEWAYS Very high Urban activity center, Refer to roadway design These driveways are often built to intensity with almost constant guidelines. the standards of and resemble driveway use during public roads and streets. hours of operation. FOR Medium-size office or Limit the maximum From observations of vehicles BOTH retail, such as driveway grade to +8% entering driveways with radii up community shopping (except where a lesser grade to 20 ft and comparisons of Flatter Higher center, with frequent is required, such as when (1.5-5%) and Moderate (6-9%) intensity driveway use during crossing a sidewalk), and grades revealed (1) little difference hours of operation. the maximum sag breakover between speeds and travel times without a vertical curve of vehicles turning right; and AND between the roadway cross (2) only slight differences between slope and an uphill speeds and travel times of vehicles Medium Smaller office, retail, or driveway grade to 9%. turning left. intensity other sites with Limit the driveway profile From measurements of 31 occasional driveway use maximum grade change driveways with scrape marks, during hours of without a vertical curve for: underside dragging became a operation. a crest to 10% and a sag problem at a crest of about 11%, to 9%. and at a sag of about 10%. Apartment complexes May limit the sag to 7%. Due to trailers. Lower Single family or duplex Limit the driveway profile From measurements of 31 intensityresidential, other types maximum grade change driveways with scrape marks, with very low use. May without a vertical curve for: underside dragging became a not apply to rural a crest to 10% and a sag problem at a crest of about 11%, residential. to 9%. and at a sag of about 10%. SPECIAL SITUATION DRIVEWAYS CBD Building faces are Refer to the guidelines above close to the street. for "Higher intensity" and "Medium intensity." Farm or A mix of design Limit the driveway profile These driveways should ranch; vehicles; some may maximum grade change accommodate trailers. Field be very low volume. without a vertical curve for: a crest to 10% and a sag to 7%. Industrial Driveways are often Varies, depending on types used by large vehicles. of vehicles. If low-boy trailers are expected, then limit crest breakover grades without a vertical curve to 3.5%. Other Motels Limit the driveway profile Travelers pulling a trailer maximum grade change may stay at a motel; without a vertical curve for: therefore, motel driveways a crest to 10% and a sag to 7%. should accommodate trailers. NOTES: Additional information on which to assess ground clearance is in Chp 3. The sag clearance for trailers is based on Eck's evaluation; truck+trailer clearances will vary. * These descriptions are intended to help the designer form a mental image of some of the more common examples of the category.
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72 Guide for the Geometric Design of Driveways Exhibit 5-69. Minimum length of Type-II crest vertical curve to accommodate low-clearance vehicle. Algebraic Difference (%) Curve Length ft(m) 1 4 (1.2) 2 8 (2.4) 3 12 (3.7) 4 16 (4.9) 5 20 (6.1) 6 24 (7.3) 7 28 (8.5) 8 32 (9.8) 9 35 (10.7) 10 39 (11.9) There are a number of possible design scenarios, based on combinations of curbed or uncurbed roadways with driveway profiles that extend uphill or downhill from the connection with the roadway. Among the tools to combat surface runoff are driveway profile, driveway cross slope, drainage inlets near the driveway area, and drainage grates in the driveway. Exhibit 5-70 shows how profile design can be used to prevent water in the gutter from flowing onto private property. Roadway-Driveway Threshold Treatment The threshold is the edge or line where the roadway and the driveway join or touch. This line is often at the curb edge. Design concerns in this area include ease of travel for users (e.g., bicycles and motor vehicles), ease of construction, and, in cases where the roadway has a curb and gut- ter, confining drainage to the gutter line. Exhibit 5-71 shows four common driveway threshold treatments where the roadway has curbs: rolled curb, vertical lip, counterslope, and continuous. Exhibit 5-72 suggests design guidelines for driveway threshold treatments. The Continuous design is the preferred method. Except for single-family or duplex access on lower volume, lower speed residential streets, designers should avoid designs that create a bump at the threshold. Even in the single-family context, consider that a vertical discontinuity can be an impediment for bicyclists as well as pedestrians with disabilities (especially using wheelchairs). Vertical lip design is another topic needing additional research to assess the ability of other treat- ments to address drainage, to assess the detrimental effects of a pronounced lip, and to determine whether a low lip, perhaps on the magnitude of 1/2 inch, has any detrimental effects on users. For any type of treatment, the curb and gutter should not be broken off to leave a ragged edge, but should be cut with a saw and cleanly removed. Exhibit 5-73 shows a gutter treatment used in some jurisdictions. The gutter cross slope is sig- nificantly greater than that of the adjacent traveled lanes. This treatment is believed to improve drainage; however, it also increases the profile breakover angle which motorists entering and Exhibit 5-70. Confining surface runoff flow. Roadway with curb: Setting the driveway Roadway without curb: The driveway will profile with a crest vertical curve to slope need a crown, cross slope, or a grate in down to the gutter, to confine the flow. the sag to provide surface drainage. ground should driveway er driveway roadway roadway ground (a) (b)
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Geometric Design Elements 73 Exhibit 5-71. Driveway threshold treatment types. ROLLED CURB VERTICAL LIP STREET CROSS SECTION - STREET CROSS SECTION - DRIVEWAY PROFILE VIEW DRIVEWAY PROFILE VIEW curb shape does not change near-vertical lip at the gutter at a driveway line curb street curb driveway street (may slope driveway up or down) (may slope up or down) (a) (b) COUNTERSLOPE CONTINUOUS STREET CROSS SECTION - STREET CROSS SECTION - DRIVEWAY PROFILE VIEW DRIVEWAY PROFILE VIEW incline (steeper than driveway no abrupt vertical component; grade) behind the gutter line driveway grade connects at gutter line street street driveway curb driveway curb (may slope (may slope up or down) up or down) (c) (d) Exhibit 5-72. Driveway threshold treatment guidelines. Method Advantages and Disadvantages Comments Rolled curb Easiest threshold to construct, because the This method is generally existing curb is not modified or removed. unsuitable. Confines the gutter flow, since the existing It may be acceptable for curb remains intact. single-family or duplex Vehicles entering or exiting the driveway access on lower volume, experience a jolt while crossing a curb of lower speed residential typical height. streets. Vertical lip Construction requires curb modification or Is often constructed by removal. forming the threshold with Can confine very low flows in the gutter and lumber that leaves a vertical reduce the spread of the gutter flow. face or lip of 1 to 2 inches at Bump created by the vertical lip is a minor the threshold. impediment to automobile movements and a more significant problem for turning bicyclists (i.e., bicycle tire strikes the face at a skew angle). Counterslope Construction requires curb modification or The proportion and amount of removal. rise and run affect the degree Can confine very low flows in the gutter and of disruption to automobiles reduce the spread of the gutter flow. and bicycles. Less abrupt to cross, but can still be disruptive to automobiles and bicycles. Continuous Construction requires curb modification or With this method, the profile OR Smooth removal. slopes continuously but not Is more bicycle- and automobile-friendly. abruptly upward from the If the driveway immediately slopes downward gutter line. Thus the drainage from the gutter line, this does not confine the objective can be suitably drainage as well. achieved by means that do not create problematic bumps for bicyclists or drivers.
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74 Guide for the Geometric Design of Driveways Exhibit 5-73. Increased gutter cross slope. exiting the driveway have to negotiate. Many scrape marks on the driveway surface from the dragging of vehicles bumpers are clearly visible. More study should be done on this type of design to weigh any drainage benefits against impediments to traffic flow. Vertical Alignment Examples The following examples apply some of the guidelines for designing the vertical alignment of driveways. Exhibit 5-74 shows the driveway profile rising from the gutter line up to the sidewalk, then flattening at the sidewalk before falling as the driveway continues onto the private property. This type of design will confine normal depths of water in the gutter and not allow water to flow on to private property and down the driveway. Exhibit 5-75 shows the suggested values for driveways at which the P-vehicle is the design con- trol. If the near edge of the sidewalk is 5.5 feet from the face-of-curb or gutter line, and the drive- way is on a +7.0% grade, then the near edge of the sidewalk is 0.39 feet above the elevation of Exhibit 5-74. Schematic showing driveway vertical alignment concepts. Normal curb maximum location 2.0% sidewalk roadway Driveway Exhibit 5-75. Example of a driveway vertical profile design. um maxim maximum 8.0% 2.0% sidewalk Maximum - ELSE - Vertical curve Driveway breakover roadway crest = 10% Maximum * Maximum breakover is breakover the maximum without a sag = 9% vertical curve.