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OCR for page 65
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.
OCR for page 71
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.
