Lec 14 Demand Forecasting 2008 ( Transportation Engineering Dr.Lina Shbeeb )
Lec 04 Geometric Design ( Transportation Engineering )
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Transcript of Lec 04 Geometric Design ( Transportation Engineering )
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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) Characteristics
Acceleration (Constant or variable):
Passing maneuvers
Gap acceptance
Dimensions of freeway ramps and passing lanes
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 to
perceive/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 + _____u2_____
30(f ± G)
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SSD Example SSD = 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________ 10.73 Solving for u, u = 59.9 mph
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Decision Sight Distance
When situation is unexpected or driver makes unusual
maneuvers or under difficult to perceive situations
Requires higher PIEV time
Depends on type of maneuver made and roadway
setting (urban vs. rural)
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Passing Sight Distance
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Passing Sight Distance Dpassing = d1 + d2 + d3 + d4
d1 = distance traveled during P/R time to point where
vehicle just enters the left lane
d1 = 1.47t1(u – m + 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 Distance
Dpassing = 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 the magnitude of lateral displacement.
The relationship between the longitudinal distance l, the lateral placement 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= vehicle’s speed
d/dt= rate of change of the 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 or
adjacent) property
Some roads serve one of these two function while 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 Classes and 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 and expressway is based on the accessibility control
Freeways have a full control of access [access or exit are permitted only at controlled locations (exit ramp)]
Expressways have a partial access control. (access or exit are maybe permitted directly from or adjacent land or via a limited number of at grade intersections)
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Rural road classification
Principal Arterials
Function: Long disance Travel and Connect urbanized areas
Characteristics: Long trips, no stubs, high speed, widely spaced, full/partial access control
Subclasses: Freeways, Other non-Freeways with access control
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Minor Arterials
Function: connect small urban areas with major trip generators
Characteristics: Moderate everything (routes, speed, spacing, mostly mobility)
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Rural Collectors
Major Collectors: Connect small twon to large towns not served by arterials, link entities with nearby arterials, urban areas
Minor Collectors: Serve remaining small towns, link local traffic generators with rural areas
Characteristics: Shorter, Moderate Speeds (in rural areas), Intermediate spacing
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Rural Local Roads
Function: Provide access to land on collector network, serve short distance travel
Characteristics: Short routes that terminate at higher classes, relatively low speeds, intermediate spacing
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Urban Functional System Principal Arterials
Functions: carry most trips entering/leaving urban area, serve intra-urban trips, carry intra-urban and intercity bus routes, provide continuity to rural arterials
Characteristics: longest distance routes and highest volume, high speed, spacing is less than 1 km in CBD and greater than 8 km in urban fringe, full/partial access control
Subclasses: interstates, other freeways and expressways, other principal arterials with partial access control
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Urban Minor Arterials Functions: interconnect with and augment
principle arterials, accommodate moderate length trips, distribute traffic, carry local buses, provide intra-community continuity, connect to urban collectors
Characteristics: moderate trip length high volumes, moderate speeds, spacing 0.2 to1.0 km in CBD and 3 to 5 km in suburbs
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Urban Collectors
Functions: Provide traffic circulation within residential neighborhoods and industrial/commercial areas, Collect traffic from locals and channel to arterials, Carry local buses, Provide some direct land access
Characteristics: Shorter than minor arterials, Moderate speeds, Spacing intermediate to arterials, Penetrates residential neighborhoods
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 Components Cross-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 (Divided highways).
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