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10 Guide for the Geometric Design of Driveways · Accessibility for pedestrians with disabilities and incorporating requirements of the ADA Accessibility Guidelines); · Interactions where bicycle lanes or paths are present; and · Interactions where public transportation stops are in the vicinity of the driveway. These considerations affect design details such as sidewalk alignment and cross slope across the driveway, driveway entry shape (curved or straight) and dimension, and driveway width. Attributes of Bicyclists, Drivers, and Pedestrians The capabilities and limitations of the people using the driveway, whether as bicyclists, drivers, or pedestrians, affect design choices. An appreciation of the concept of driver work load leads to the objective of trying to limit the number of (1) decisions a driver has to make and (2) potential conflicts with different streams of traffic. Acknowledging that rain, fog, and nighttime conditions can make physical objects more difficult to detect, a designer tries to create well-defined edges and increase the contrast between different surfaces, such as between the driveway opening and the border area. Refer to the AASHTO guides for the design of bicycle facilities (3-1), highways and streets (3-2), and pedestrian facilities (3-3) for a discussion of user characteristics. Characteristics of Emerging Road and Trail Users and Their Safety (3-4) provides data for a wide range of users, including bicyclists and pedestrians. Driveways are crossed by pedestrians on sidewalks. Exhibit 3-4 shows a distribution of the walking speeds of pedestrians under 60 and over 60. In both age groups, most pedestrians walk at speeds between 3 and 6 ft/s. A synthesis of default values (3-5) cited one study listing 15th percentile walking speeds for those less than 60 years old as 3.8 ft/s, and 3.5 ft/s for those over 60. Another study listed 15th percentile walking speeds for those less than 65 years old as 4.0 ft/s, and 3.1 ft/s for those over 65. When estimating the time required for a pedestrian to cross the driveway, make an allowance for the pedestrian not starting from the exact edge of the driveway. A pedestrian may be stand- ing 2 or more feet back from the driveway edge when the pedestrian begins to walk across the driveway. Bicyclists also cross the paths of vehicles entering and leaving driveways. On shared use paths ("a bikeway physically separated from motorized vehicular traffic," the 1999 AASHTO bicycle guide (3-1, p.3) suggested a design speed of at least 20 mph on shared use paths, noting that grade and wind can affect the speeds of bicyclists. With a downgrade greater than 4%, a design speed of 30 mph or more was offered (3-1, p.36). Discussing urban street design criteria, the Urban Street Geometric Design Handbook by ITE (3-6, p.41) stated: Studies show that nearly all bicyclists travel within a range of 7 to 15 mph, with an average of 10 to 11 mph. A study that examined characteristics of a wide range of users found that the 85th percentile speed for bicycles was 14 mph, and for recumbent bicycles was 18 mph (3-4, p.74). Motor Vehicle Traffic Attributes The designer should consider the attributes of the motor vehicles used by the drivers. Attributes that affect driveway design include vehicle width, vehicle length, vehicle height, vehicle turning radius, vehicle off tracking, and vehicle ground clearance dimensions.
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Design Controls 11 Exhibit 3-4. Distribution of pedestrian walking speeds. Design Vehicles In its design policy, AASHTO indicates that key controls in roadway geometric design are the physical characteristics and the percentages of vehicles of various sizes using the roadways. According to AASHTO, it is appropriate to examine all vehicle types, establish general class groupings, and select vehicles of representative size within each class for design use: These selected vehicles, with representative weight, dimensions, and operating characteristics, used to establish highway design controls for accommodating vehicles of designated classes, are known as design vehicles (3-2, p.15). AASHTO identifies general classes of design vehicles and dimensions for design vehicles within these general classes. The design policy advises that "the designer should consider the largest design vehicle likely to use the facility with considerable frequency or a design vehicle with special characteristics appropriate to a particular intersection in determining the design of such critical features as radii at intersections and radii of turning roadways." General guidance is given for selecting a design vehicle. With one exception (i.e., a passenger car may be selected when the main traffic generator is a parking lot), the guidelines deal with road and street intersections as opposed to driveway-roadway intersections. Design Vehicle Dimensions Widths and turning paths of design vehicles can be found in the latest edition of the AASHTO design policy. There is some indication that slow-turning vehicles may follow a path with a
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12 Guide for the Geometric Design of Driveways smaller radius than indicated in the turning dimensions and the turning templates provided in the current AASHTO Green Book (3-2, pp.1643). Underclearance or ground clearance is the distance from the bottom of the vehicle body to the ground (3-7). Ground clearance and wheelbase are critical dimensions at a crest situation. The ground clearance, in combination with either the front or rear overhang, is critical at sag situations. For example, rear-load garbage trucks may drag in the rear; therefore, rear overhang is the crit- ical parameter. Car carrier trailers can drag in the rear or hang up between the wheels; therefore, either wheelbase or rear overhang may be critical. When the designer does not take these dimen- sions into account, the result can be vehicles dragging, scraping, and even becoming lodged on the vertical profile grade changes. Although a designer can consult the AASHTO design policy for lengths, widths, overall heights, turning radii, and swept path templates for a menu of vehicle types, the policy does not include vehicle ground clearance or underclearance data. Exhibit 3-5 presents vehicle ground clearance dimensions. Note that dashes (--) in cells in the table indicate that hang-up problems are not expected on this part of the vehicle. Exhibit 3-6 shows the findings from a recent study in which the underside dimensions of a select group of vehicles were measured. From this, the crest and sag angles at which underside dragging would occur were calculated. These values reflect the physical limits of the vehicles. Exhibit 3-5. Vehicle ground clearance dimensions. Design Vehicle Rear Wheelbase Front Ground Ground Ground Overhang (ft) Overhang Clearance Clearance Clearance (ft) (ft) for Rear for for Front Overhang Wheelbase Overhang (in) (in) (in) Rear-Load Garbage Truck 10.5 20 -- 14 12 -- Aerial Fire Truck 12 20 7 10 9 11 Pumper Fire Truck 10 22 8 10 7 8 Single-Unit Beverage Truck 10 24 -- 8 6 -- Articulated Beverage Truck -- 30 -- -- 10 -- Low-Boy Trailer <53 feet -- 38 -- -- 5 -- Double-Drop Trailer -- 40 -- -- 6 -- Car Carrier Trailer 14 40 -- 6 4 -- Belly Dump Trailer -- 40 -- -- 11 -- Mini-Bus 16 15 -- 8 10 -- School Bus 13 23 -- 11 7 -- Single-Unit Transit Bus -- 25 18 -- 8 6 Motorcoach 10 27 7.6 8 7 10 Articulated Transit Bus 10 -- -- 9 -- -- Passenger Vehicle and Trailer 13 20* -- 5 5 -- - Private Use Passenger Vehicle and Trailer 13 24* -- 7 7 -- - Commercial Use Recreational Vehicle (RV) 16 27 7.8 8 7 6 NOTES: * indicates distance from rear wheels to hitch -- indicates hang-up problems not expected on this part of the vehicle These dimensions reflect only the physical limits of vehicles. They do not account for the effects on vehicles in operation (e.g., dynamic load--vehicle bounce). The desirable maximum grade changes will be less than those reflected in these values.
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Design Controls 13 Exhibit 3-6. Ground clearance geometry for specific models. These calculations do not account for effects of static load (weight of passengers or cargo) or dynamic load (vehicle bounce). Maximum desirable grade change will be less than these values. P-CAR: based on PICKUP TRUCK WITH TRAILER: based on Chevrolet Camaro 1998 Ford F-150 with Wells Cargo 32 ft two-axle Chevrolet Corvette Z06 2008 ball-hitch trailer GCREST = 18.9% GCREST = 13.0% GSAG = 13.9% GSAG = 7.0% CLASS A DIESEL MOTOR HOME TRACTOR WITH 10-BAY BEVERAGE (DIESEL PUSHER): based on TRAILER: based on Alfa See Ya'! Gold ® International tractor, Centennial Body trailer, about 5/8 loaded GCREST = 18.9% GCREST = 13.5% GSAG = 13.9% GSAG = 15.0% Pick-up with trailer and beverage truck calculations by R. Eck. Passenger car and motor home calculations by J. Gattis. Angles used for design, reflecting attributes of vehicles under actual operation conditions, should be less than these. Selecting a Design Vehicle The activities served and the location of a driveway will affect the types of vehicles using the driveway. Typical vehicles include passenger cars, service vehicles, and bicycles. Large trucks, with their wide offtracking, use many commercial driveways--although usually few in number, larger trucks must be able to negotiate curves and grades. They should be the design vehicle for driveways serving industrial areas. Design vehicle selection involves two conflicting mandates: (1) select a vehicle with sufficiently large dimensions so that all users can negotiate the driveway in the future and (2) confine the dimensions so that the driveway is not overdesigned. Designers can easily believe that they lack information needed to select a design vehicle. Designers may not know how frequently certain larger vehicles will use a site; regardless, the word "considerable" in the phrase "use . . . with con- siderable frequency" is undefined. Designers are left to their judgment to assess to what extent it is acceptable for offtracking turning vehicles to encroach into other lanes. Not only is the frequency of vehicle use a consideration, but the volume and speeds on the main roadway are also factors. Exhibit 3-7 lists suggested design vehicles for various types of driveways. Exhibit 3-8 shows an example from a state transportation agency. Vehicles for Farm/Ranch and Field Entrance Design Design vehicle information for farm vehicles is not generally available. The County Engineer for Delaware County, Iowa, Mark J. Nahra, P.E., observed that large equipment will be found using both the field entrances and driveways to farm residences. Also, P-vehicles use field entrances, so the designer should use both the standard driver eye height for a P-vehicle and the eye height for a heavy vehicle. Despite their size, large combines and other pieces of farm equipment are very maneuver- able. Large combines are usually less than 16 feet wide. Based on this, farm driveways and
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14 Guide for the Geometric Design of Driveways Exhibit 3-7. Suggested design vehicles for common driveway types. Category Description of Common Applications Design Vehicles STANDARD DRIVEWAYS Very high Urban activity center, with almost Large truck, buses (May be P-vehicle intensity constant driveway use during hours of if have separate truck entrances.) operation. Higher Medium-size office or retail (e.g., a Large truck, buses intensity community shopping center) with frequent (May be P-vehicle if have separate driveway use during hours of operation. truck entrances.) Medium Smaller office or retail, some apartment P-vehicle, single-unit truck intensity complexes, with occasional driveway use during hours of operation. Lower Single-family or duplex residential, other P-vehicle intensity types with low use. May not apply to rural residential. SPECIAL SITUATION DRIVEWAYS Central Parking lot or garage for automobiles only P-vehicle business district Other than exclusive automobile facility Single-unit truck Farm or Mix of residential and industrial Single-unit truck, farm equipment ranch characteristics Field Seldom used, very low volume Single-unit truck, farm equipment Industrial Driveways are often used by large vehicles Large truck Other Bus terminal Bus Fire or Ambulance station Emergency vehicle Notes: P-vehicle is the AASHTO passenger car design vehicle. Large truck may be WB-50, WB-62, or WB-65. These descriptions are intended to help the designer from a mental image of some of the more common examples of the category. field entrances should be at least 16 feet wide, although 20 feet is recommended. A 30-foot top-width over the driveway culvert is recommended to allow large combines and tractor- semitrailer combinations to pull into farm driveways. A radius of at least 20 feet is recom- mended to allow service vehicles (e.g., propane or fuel oil trucks) (single-unit vehicles) to be able to turn safely into a rural residential driveway. A site review is recommended to assess ground clearance issues. Exhibit 3-8. Example design vehicles for driveway types.
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Design Controls 15 Design Volumes Estimating the expected driveway volume can help identify how many driveway lanes are needed. For more information, refer to publications that discuss methodology for site impact studies. The basic steps are as follows: 1. Establish the extent to which access is allowed, and estimate the number of driveways. Review the local access policy and spacing standards of the governing agency to establish whether the desired access will be allowed and, if so, to identify the number of driveways. 2. Identify the type and size of land use activity to be served. 3. Determine the daily and peak-hour vehicle trip rates. If the site currently exists and traffic vol- umes are expected to remain the same, then counts of existing traffic can be made. For proposed development, the designer may use ITE Trip Generation or locally developed trip generation rates. By definition, median rates are exceeded 50% of the time, so it may be desirable to calcu- late and use the 80th to 90th percentile trip rates rather than using median or average rates. If driveways are or will be in the CBD or outlying urban business districts, some person trips to or from activities may be made as pedestrians or via public transit. In these cases, some down- ward adjustment of published ITE vehicle trip rates may be warranted. 4. Estimate the daily and peak-hour trips ends for the activity. Multiply the trip generation rate by the appropriate independent variable to arrive at the total number of expected trip ends. 5. Estimate the driveway volumes. Based on the preceding steps, estimate how much site traffic will use each driveway. Exhibit 3-9 lists examples of land uses and their expected number of driveway trips. Design Speeds Various factors, including the setting and the functional classification, will affect the design speed of a given roadway. After a roadway has been constructed and is in operation, actual speeds on the roadway can be observed. The speeds on the through roadway will normally govern geo- metric features such as sight distance and the length of acceleration or deceleration lanes. A few studies have measured the speeds at which drivers turn into driveways or side streets. Studies of turning behavior have reported speeds of less than 15 mph for a radius of 30 ft or less. Different studies may measure speeds at different locations or over different lengths during a turn. Exhibit 3-10 shows findings from an older study. In 2007 and 2008, the speeds of over 1500 vehicles entering 12 driveways were measured near the roadway-driveway intersection and in the driveway throat (see Exhibit 3-11). All of the sites were lower-intensity commercial (e.g., retail and professional offices) developments in built-up suburban environments along multilane arterial roadways with either 40-mph or 45-mph posted speed limits. The right-turn entry radii ranged from 13 to 19.5 ft. Almost all of the vehicles in the study were passenger cars. Sidewalks were present at all sites, but pedestrian volumes were Exhibit 3-9. Estimated number of trips from given sites. Example Land Uses Expected Number of Site Trips 150,000 sq. ft. shopping center Over 4,000 trips/day or over 400 Grocery/drugstore with 10-15 smaller stores trips/hour (9,000 daily trips split w/2 driveways) Small "strip" shopping center (20,000-75,000 sq. ft.) 601-4,000 trips/day or 61-400 trips/hour Gas station/convenience market 3 to 60 housing or apartment units 21-600 trips/day or 6-60 trips/hour Small office in converted home "Mom and pop" business 1 or 2 single-family homes 1-20 trips/day or 1-5 trips/hour Source: Driveway Handbook, Florida Dept. of Transportation, March 2005
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16 Guide for the Geometric Design of Driveways Exhibit 3-10. Driveway entry speed related to driveway radius and width. Exhibit 3-11. Measured speeds of vehicles entering driveways. Land Use Entry Entry Location Rt. Turn Lt. Turn Type Lane Radius Where Entry 90th% Entry 90th% Width Measured Speed Speed (ft) (ft) (mph) (mph) Commercial 13 ft 13.0-19.5 2Rt 15.5 to 18.0 na Commercial 13 ft 13.0-19.5 2Lt na 10.0 to 13.0 Commercial 13 ft 13.0-19.5 4 7.0 to 10.4 7.8 to 13.9 LOCATION NOTES -- Speeds measured at: 2Rt-right turn, 25 ft before the near perpendicular edge of the driveway 2Lt-left turn, one lane width in advance of the driveway threshold (curb line) 4-in the driveway throat, 15 ft. past the driveway threshold (curb line) NOTE: "na" = not applicable ROADWAY 25' 2 Lt 11 ' 2 Rt 15' 4