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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
