CEE 4651

Geometric Design of Roads

Moinul Hossain

Week 4
Road Classifications
And
Horizontal Curve
 Road Classification (1)
• Trip Functions [AASHTO]

 Main movement

 Transition

 Distribution

 Collection

 Access

 Termination

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 Road Classification (2)
• Trip Functions [AASHTO]

 Main movement – through portion of the trip; primary

connection between origin and destination.

 Transition - vehicle transfers from the through portion of trip to

the remaining functions that lead to access and termination. (e.g.
using ramp to exit from freeway to surface arterial.)

 Distribution - providing drivers and vehicles with the ability to

leave a major through facility and get to the general area of their
destinations.

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 Road Classification (3)
• Trip Functions [AASHTO]

 Collection - brings the drivers and vehicles closer to the final

destination.

 Access and Termination - providing the driver with a place

to leave his or her vehicle and enter the land use sought.

• Not all trips will involve all of these components.

• The hierarchy of the trip functions should be matched by the
design of the roadways provided to accomplish them.
• A typical trip has two terminals - one at origin and the other at
destination. 4
Road Classification (4)

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 Road Classification (5)

There are sub-categories of highway classification

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Road Classification (6)

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Road Classification (7)

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 Road Classification (8)
• Preserving the Function of a Facility

 Local street - incorporate sharp curves,

cul-de sacs; prohibit direct access to arterial; no
residence front on the collector.

 Arterial - i) limiting entry and exit; ii) parking

prohibition; iii) coordinated signals; iv) median
dividers to limit midblock right turns; v)
appropriate speed limit.
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 Road Classification (9)
• What about the Old Cities?

 Difficult to separate functions served by various

facilities due to basic design and control
problems.

 Open grid systems - the only thing that

distinguishes an arterial in such a system is its
width and provision of progressive signal timing
to encourage through movement.
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 Road Classification (10)

Open Grid System

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Road Classification (11)

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Road Classification (12)

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 Highway Design Elements (1)
• Highway structure involves

 Compacted soil

 sub-base layers of aggregate

 pavements

 drainage structures

 bridge structures

 others.

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 Highway Design Elements (2)
• Geometric characteristics of the roadway primarily
influence traffic flow and operations. Three main
elements of geometry of a highway section are:

 Horizontal alignment

 Vertical alignment

 Cross-sectional elements

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 Horizontal Alignment (1)

• Plan view of the highway which includes

tangent sections, horizontal curves and
transition elements joining them.

• Generally initiated by laying out a set of

tangents on topographical and development
maps of the service area.

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 Highway Design Elements (3)
• Highway design depends on:

 Demand (from O-D)

 Pattern of development

 Topography

 Natural barriers  Economy

 Surface conditions  Environment

 Drainage patterns  Social considerations

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 Surveying and Stationing

• Marking of cross-section at every 100’ interval

• Or, whenever there is a change in alignment

within 100’

• Stationing convention: xx + yy.

• How long is 1200+52?

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 Horizontal Curve
• All horizontal curves are circular

• Low radius means high degree of curvature, i.e., sharper/more

severe curves

• Up to 4 degree there is little difference between arc and chord

definition. In general Arc Definition is popularly used.

• D (degree of
curvature) = 5729.58/R

D (degrees), R (ft)

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Horizontal Curve
P.I. = point of intersection
P. C. = point of curvature
P. T. = point of tangency
T = length of tangent
E = external distance
M = middle ordinate dist.
L. C. = long chord
Δ = external angle of curve (angle of
deflection)

Others:
R = radius of curvature
D = degree of curvature

• Self Study: pg. 44-48 [review

from CEE 4101 (Surveying)]

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Superelevation (e) of Horizontal Curves
• Fundamentals:
𝑡𝑜𝑡𝑎𝑙 𝑟𝑖𝑠𝑒 𝑖𝑛 𝑝𝑎𝑣𝑒𝑚𝑒𝑛𝑡 𝑓𝑟𝑜𝑚 𝑒𝑑𝑔𝑒 𝑡𝑜 𝑒𝑑𝑔𝑒
 𝑒= 𝑋 100
𝑤𝑖𝑑𝑡ℎ 𝑜𝑓 𝑝𝑎𝑣𝑒𝑚𝑒𝑛𝑡
𝑆2
𝑅 = (Here, R-ft, S-mi/h, e-%)
15 0.01𝑒+𝑓
 ‘e’ is the maximum rate of superelevation

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 Superelevation of Horizontal Curves
• More rules revisited:
 Maximum between 4% - 12%.
 Only increments of 2% are used.
 Vary on: climate, terrain, development density,
frequency of slow moving vehicles.
 Urban area (congested): 4%-6%
 Low-speed urban streets/intersections: no
superelevation required.
 Minimum of 1.5% (high-type) – 2.0% (low-type)
superelevation is maintained for highways for
drainage.
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 Superelevation of Horizontal Curves
• Steps in determining design values of superelevation:
 Set the max. superelevation rate and design
speed to calculate minimum radius of curvature
as:

 Calculate max. degree of curvature as:

 Lower round the value of Dmax or upper round

the value of Rmin

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 Superelevation of Horizontal Curves
• Steps in determining design values of superelevation:
 Recalculate superelevation using the following
equation using the revised R:

 Or, you can use the same equation but for D.

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 Achieving Superelevation
Transition from a tangent section with a normal superelevation (SE)
for drainage to a superelevated horizontal curve occurs in two stages:

• Tangent runoff (Lt): The outside lane of the

curve must have a transition from the normal
drainage SE to a flat condition prior to being
rotated to the full SE.

• Superelevation runoff(Lr): Once a flat cross-

section is achieved for the outside lane of the
curve, it must be rotated (with other lanes) to the
full SE rate of the horizontal curve.
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 Achieving Superelevation

For divided highways, each direction is separately elevated 26

 Achieving Superelevation
• Transition from a normal cross-slope to a fully superelevated section is
accomplished by creating a grade differential between the rotation axis and
the pavement edge lines.

• Recommended minimum Lr is:

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 Achieving Superelevation
• For divided highways, each direction is separately elevated

• Adjustment factor bw is 1.0 if 1 lane is rotated, 0.75 of 2 lanes

are rotated and 0.67 for 3 lanes to be rotated.
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 Achieving Superelevation
• Recommended minimum Lr is:

• The total transition length between the normal cross-section to

the fully superelevated cross-section is the sum of the
superelevation and tangent runoff 29
 Minimum Length of Superelevation Runoff

• Tangent runoff (Lt) and Superelevation runoff(Lr) are related

• For comfortable operation, 60%-90% of total runoff is achieved on

the tangent section

• Depends on: Design speed, number of lanes rotated

• Design Speed, 15-45 mph: 80% (1 lane rotated), 90% (2+ lanes
rotated)

• High design speed: 70% (1 lane rotated), 80-85% (2+ lanes rotated)

• If spiral curve is available then full superelevation runoff is

accomplished on the spiral. 30
 Spiral Transition Curves
• Benefits:
 Provides easy path: centrifugal and centripetal
forces are increased gradually
 Provides desirable arrangements for
superelevation runoff
 Facilitates pavement widening at curves
 Enhances appearance
• Issues:
 Difficult to construct
 Expensive
Recommended for high volume situations where D>30 31
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Transition Spiral Curve
P.I. = point of intersection
P. C. = point of curvature
P. T.. = point of tangency
T = length of tangent
E = external distance
M = middle ordinate dist.
L. C. = long chord
Δ = external angle of curve (angle of
deflection)

Others:
R = radius of curvature
D = degree of curvature

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Transition Spiral Curve
Ls = length of spiral
Δ = angle of deflection for original
circular curve
Δs = angle of deflection for circular
portion of curve with spiral
δ = angle of deflection for spiral
portion of the curve

• Self Study: pg. 52-55 [review

from CEE 4101 (Surveying)]

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 Length of Spiral Curve
• Length of spiral can be set in two ways:
 set equal to the superelevation runoff, or,
 calculated based on equation

• C ranges between 1 and 3 ft/s3. The most commonly adopted

values is 1.97.
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 Sight distance on horizontal curves
• A minimum sight distance
equal to the safe stopping
distance must be provided at
every point along the roadway,
i.e., On horizontal curves sight
distance is limited by road side
objects blocking the drivers'
line of vision.
• Sight distance is measured
along the arc of the roadway,
using the centerline of the • length of the curve is set equal to the

inside travel lane. required stopping sight distance

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 Sight distance on horizontal curves
• Required equations:

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 Compound Horizontal Curves
Consists of two or more consecutive horizontal curves
in a single direction with different radii.

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 Compound Horizontal Curves
• General criteria:
 Use only when physical conditions require it.
 Whenever two consecutive curves are connected on a
highway segment, the larger radii should not be more than
1.5 times the smaller, and, degree of curvature should not
differ by more than 50.
 Whenever two consecutive curves in the same direction are
separated by a short tangent (<200 ft), they should be
combined in a compound curve.
 A compound curve is merely a series of simple horizontal
curves subjected to the same criteria as isolated horizontal
curves.
 AASHTO relaxes some of these criteria for compound curves
for ramp design. 39
 Reverse Horizontal Curves
• General criteria:
 Consists of two consecuitive horizontal curves in opposite
directions.
 Should always be separated by a tangent at least 200ft.
 Use of spiral transition curves is a significant assist to drivers
negotiating reverse curves.

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