lec 4_geometric design

7/28/2019 Lec 4_Geometric Design

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Geometric design of highway

Transportation Engineering


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Static Characteristics Size:

Design of lane widths, parking bays, etc

Vertical clearance Weight:

Pavement design, bridge design, axle loads

Maximum grade

Radius of Curvature: Intersection design

Interchange ramps


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Kinematic (Dynamic)

CharacteristicsAcceleration (Constant or variable):

Passing maneuvers

Gap acceptance

Dimensions of freeway ramps and passinglanes

Motion elements (Distance and Velocity)


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Sight distance

Distance a driver can see ahead at any specific time

Must allow sufficient distance for a driver toperceive/react and stop, swerve etc when necessary


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Stopping Sight Distance

where:

Db = braking distance

u = initial velocity when brakes are applied

f = coefficient of friction

G = grade (decimal)t = time to perceive/react

a = vehicle acceleration

g = acceleration due to gravity (32.2 ft/sec2)

Distance to stop vehicle, includes P/R and braking distance

S = 1.47ut + _____u2_____

30({a/g} G)


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Stopping Sight Distance

where:

Db = braking distance

u = initial velocity when brakes are applied

f = coefficient of friction

G = grade (decimal)

t = time to perceive/react

With assumed acceleration, using friction

S = 1.47ut + _____u

2

_____30(f G)


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SSD ExampleSSD = 1.47ut + _____u2_____30({a/g} G)

Stopping distance = 405 feet

405 feet = 1.47u(0.8 sec) + ________u2________30({11.2/32.2}+ 0.01)

405 feet = 1.17u + ________u2________30(0.358)

405 feet = 1.17u + ________u2________

10.73Solving for u, u = 59.9 mph


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Decision Sight DistanceWhen situation is unexpected or driver makes unusual

maneuvers or under difficult to perceive situations

Requires higher PIEV timeDepends on type of maneuver made and roadway

setting (urban vs. rural)


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Passing Sight Distance


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Passing Sight DistanceDpassing = d1 + d2 + d3 + d4d1 = distance traveled during P/R time to point where

vehicle just enters the left lane

d1 = 1.47t1(um + at1)

2

where

t1 = time for initial maneuver (sec)

u = average speed of passing vehicle (mph)a = acceleration (mph/s)

m = difference between speeds of passing and passed

vehicle


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Passing Sight Distance

Dpassing = d1 + d2 + d3 + d4

d2 = distance traveled by vehicle while in left lane

d2 = 1.47ut2

where:u = speed of passing vehicle (mph)

t2 = time spent passing in left lane (sec)


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Passing Sight DistanceDpassing = d1 + d2 + d3 + d4

d3 = clearance distance varies from 110 to 300 feet

d4 = distance traveled by opposing vehicle during passing

maneuver

d4 usually taken as 2/3 d2


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Lateral displacement

a

l


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Lateral displacement Experiments showed that the narrower the pavement and the

closer the object to the pavement edge, the greater themagnitude of lateral displacement.

The relationship between the longitudinal distance l, the lateralplacement of a given object a, and the visual angle is l= a cot

By differentiating both side of equation with respect to time and

22

2222

2

/)(csc/

csc

la

va

dt

d

alaandvdtdl

dt

da

dt

dl

V= vehicles speed

d/dt= rate of change ofthe visual angle


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Functional classification of

highways Highways are classified either by

The function they serve (functional classification) The entity responsible for their construction and

maintenance (jurisdictional classification) Two concepts are used when classifying

highway by their function Mobility: continuous travel

Accessibility: direct access to abutting (next to oradjacent) property

Some roads serve one of these two functionwhile few can satisfy both


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Mobility

Accessibility

Function

classification


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Mobility versus accessibility


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Three Major Roadway Classesand their functions:

Arterial (high mobility, low access,long trips, fast speeds)

Collector (moderate, moderate,moderate, moderate)

Local (low, high, short, slow)


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Roadway Functional Classes

Determined by characteristics:

Function

access density

traffic demands

trip length

expected speed


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Freeway versus expressway

The distinction between freeways andexpressway is based on the accessibility

control Freeways have a full control of access [access or

exit are permitted only at controlled locations (exitramp)]

Expressways have a partial access control. (accessor exit are maybe permitted directly from oradjacent land or via a limited number of at gradeintersections)


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Rural road classification

Principal Arterials

Function: Long disance Travel andConnect urbanized areas

Characteristics: Long trips, no stubs, highspeed, widely spaced, full/partial access

control Subclasses: Freeways, Other non-

Freeways with access control


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Minor Arterials

Function: connect small urban areaswith major trip generators

Characteristics: Moderate everything(routes, speed, spacing, mostlymobility)


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Rural Collectors

Major Collectors: Connect small twon to large townsnot served by arterials, link entities with nearbyarterials, urban areas

Minor Collectors: Serve remaining small towns, linklocal traffic generators with rural areas

Characteristics: Shorter, Moderate Speeds (in ruralareas), Intermediate spacing


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Rural Local Roads

Function: Provide access to land on collectornetwork, serve short distance travel

Characteristics: Short routes that terminate at

higher classes, relatively low speeds,intermediate spacing


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Urban Functional SystemPrincipal Arterials

Functions: carry most trips entering/leavingurban area, serve intra-urban trips, carryintra-urban and intercity bus routes, providecontinuity to rural arterials

Characteristics: longest distance routes andhighest volume, high speed, spacing is lessthan 1 km in CBD and greater than 8 km inurban fringe, full/partial access control

Subclasses: interstates, other freeways andexpressways, other principal arterials withpartial access control


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Urban Minor Arterials Functions: interconnect with and augment

principle arterials, accommodate moderatelength trips, distribute traffic, carry localbuses, provide intra-community continuity,connect to urban collectors

Characteristics: moderate trip length high

volumes, moderate speeds, spacing 0.2to1.0 km in CBD and 3 to 5 km in suburbs


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Urban Collectors

Functions: Provide traffic circulation withinresidential neighborhoods and

industrial/commercial areas, Collect trafficfrom locals and channel to arterials, Carrylocal buses, Provide some direct land access

Characteristics: Shorter than minor arterials,

Moderate speeds, Spacing intermediate toarterials, Penetrates residentialneighborhoods

Subclasses: major and minor


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Urban Locals

Functions: Provide direct access to land,Connect properties to higher class

streets (i.e., collectors) Characteristics: Short in length, Dead

end terminate at collections, Low

speeds, No bus routes


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Highway

ComponentsCross-section


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Cross section components

Highways are categorized into Divided highways Undivided highways

The distinction is based on the viability of median (Dividedhighways).

The components of divided highways within the right of way are Carriageway or travelling lanes Median Shoulder Ditches Side-slopes Barrier Marking Cross section slope

lec 4_geometric design

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