lec 04 geometric design ( transportation engineering )

1

Geometric design of highway

Transportation Engineering

2

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

3

Kinematic (Dynamic) Characteristics

Acceleration (Constant or variable):

Passing maneuvers

Gap acceptance

Dimensions of freeway ramps and passing lanes

Motion elements (Distance and Velocity)

4

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

5

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)

7

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

8

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)

9

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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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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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

13

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

15

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)

22

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