02 design speed and sight distances
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DRIVER PERFORMANCE
.
A transportation‐based human factor theory is critical in guiding designers
’ .
Performance of different age groups are different.
er r vers ave spec a nee s, or examp e, more response t me n
detecting and digesting traffic information.
Control (steering and speed control),
Guidance (lane placement, road following, and car‐following), and
Navigation (trip planning).
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DRIVER PERFORMANCE AND DESIGN
Control Steering Turning Radius
Speed Control Lane Width
Guidance‐
Car‐Following Cross Sections
Passing Manoeuvres Signs and Striping
Navigation Origins and Destinations Message Signs
Traveller Information System
Driver Errors Driver Deficiencies Roadside Design
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3D Drive Through
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PEDESTRIANS CHARACTERISTICS
Pedestrians interactions with traffic are a major consideration in
highway planning and design.
Pedestrian facilities normally include;
,
Crosswalks,
Traffic control measures, Kerb cuts (depressed kerbs and ramped sidewalks),
Ramps (for the elderly walkers and persons with mobility
im airments
Bus and taxi stops or other loading areas,
Sidewalks on grade separation,
GEOMETRIC DESIGN OF ROADSGDR710S TOMEKA C. L. T. G.
,
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PEDESTRIANS CHARACTERISTICS
distance between two points.
Pedestrians have a basic resistance to changes in grade or
elevation when crossing roadways and tend to avoid using
s ecial under ass or over ass edestrian facilities.
Walking speed varies among walking population. . .
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IMPORTANT HUMAN CHARACTERISTICS
VISUAL ACUITYi.
.
PERIPHERAL VISION
COLOUR VISION
ii.
iii.
GLARE VISION AND RECOVERY
DEPTH PERCEPTION
iv.
v.
–
GEOMETRIC DESIGN OF ROADSGDR710S TOMEKA C. L. T. G.
.
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IMPORTANT VEHICLE CHARACTERISTICS
.
WEIGHT OF THE VEHICLEi.
.
ACCELERATION CAPABILITIESi.
DYNAMIC CHARACTERISTICS3.
AIR RESISTANCEi.
GRADE RESISTANCEii.
ROLLING RESISTANCEiii.
GEOMETRIC DESIGN OF ROADSGDR710S TOMEKA C. L. T. G.
v.
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CURVE
RESISTANCE4.
EXTERNAL FORCES ON FRONT WHEELSi.
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CONCEPTS
OF
SIGHT
DISTANCES
GEOMETRIC DESIGN OF ROADSGDR710S TOMEKA C. L. T. G.GEOMETRIC DESIGN OF ROADSGDR710S TOMEKA C. L. T. G.
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SIGHT DISTANCES
Must allow sufficient distance for a driver to perceive, react and
, .
1. Stopping Sight Distance – SSD
2. Overtaking (Passing) Sight Distance – OSD (PSD)
3. Decision Sight Distance – DSD
.
–5. Crossing (Rail‐Road)
GEOMETRIC DESIGN OF ROADSGDR710S TOMEKA C. L. T. G.GEOMETRIC DESIGN OF ROADSGDR710S TOMEKA C. L. T. G.
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SIGHT DISTANCES
A driver’s ability to see ahead is of the utmost importance in
the safe and efficient operation of a vehicle on a highway.
At minimum must rovide si ht distance of sufficient len th
to safely stop when an unexpected object appears on the
travelled way
On Two‐Lane Two‐Way highways, provide sufficient sight distance
to enable drivers to occupy the opposing traffic lane for
overtaking slower vehicles without hazard
On Two‐Lane Two‐Wa hi hwa s NO PASSING zone must be
GEOMETRIC DESIGN OF ROADSGDR710S TOMEKA C. L. T. G.
clearly defined
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FOUR ASPECTS OF SIGHT DISTANCE
g s ances nee e or s opp ng – app ca e on a
highways
Sight distances needed for the passing of overtaking
– ‐ ‐
in design
GEOMETRIC DESIGN OF ROADSGDR710S TOMEKA C. L. T. G.
Sight distance
is
the
length
of
the
roadway
ahead
that
is
visible
to
the
driver.
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STOPPING SIGHT DISTANCE (SSD)
This is required for every point along alignment (horizontal and
vertical) – Design for it, or sign for lower and safe speed
Available SSD = f(roadway alignment, objects off the alignment,
SSD = PRD + BD (with final velocity V2 = 0)
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OVERTAKING (OR PASSING) SIGHT DISTANCE
d2 d223
13
=
PHASE 1 PHASE 2
, 1 2 3 4
d1 – Distance traversed during perception and reaction time AND during the initial
acceleration to the point of encroachment on the right (opposing) lane.
d2 – Distance traveled while the overtaking vehicle occupies the right lane.
d3 – Distance between the overtaking vehicle at the end of its manoeuvre and the
opposing (oncoming) vehicle
GEOMETRIC DESIGN OF ROADSGDR710S TOMEKA C. L. T. G.
d4 – Distance traversed by an opposing vehicle for two-thirds of the time the
overtaking vehicle occupies the left lane (i.e. 2/3 of d2).
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DECISION SIGHT DISTANCE
When situation is unex ected or driver makes unusual manoeuvres or under difficult to perceive situations
‐
Depends on the type of manoeuvre made and roadway setting
(urban or rural)
DSDs are used:
At locations of complex decision making
Where information is difficult to perceive
Where unexpected or unusual manoeuvres are required
GEOMETRIC DESIGN OF ROADSGDR710S TOMEKA C. L. T. G.
Like interchanges, merge areas, so many signs clustered at
one point, etc. (with a lot of visual “noise”)
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FORCES
ON
A
MOVING
VEHICLE
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FORCES ACTING ON A MOVING VEHICLE
V e h i c l W e i g h
t
θ
VEHICLE DYNAMIC CHARACTERISTICS
AIR RESISTANCEi.
ii. FRICTION RESISTANCE
GEOMETRIC DESIGN OF ROADSGDR710S TOMEKA C. L. T. G.
GRADE RESISTANCE
ROLLING RESISTANCE
iii.
iv.
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FORCES ON A VEHICLE BRAKING ON A DOWNGRADE
D b
GV0
Wg
a
.
α
W
V0 = speed when brakes applied
α = angle of incline
G = tanα
KINEMATIC EQUATIONS
W = weight of the vehicle
x = distance traveled by the vehicle along the road during braking
f = coefficient of friction
Db = braking distance
2
i
1d v t at2
2 2f iv v 2ad
v v at i f v v
d t
GEOMETRIC DESIGN OF ROADSGDR710S TOMEKA C. L. T. G.
g = acceleration of gravity
a = vehicle acceleration
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EXAMPLE: A vehicle is travelling at 100 km/h on a road whose pavement
has f = 0.39. Calculate the braking distance and the stopping
s g s ance ;
1. The section that the vehicle is travelling is level.
. .
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CONCEPTS
OF
SPEED
AND
DESIGN
SPEED
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TYPES OF HIGHWAY SPEEDS
, change of distance as time changes, they exist in different types
for engineering and operational purposes
1. POSTED SPEED = SPEED LIMIT
2. OPERATING SPEED = FREE FLOW SPEED
3. RUNNING SPEED = HIGHWAY LENGTH ÷ RUNNING TIME
4. DESIGN SPEED = SELECTED SPEED USED TO DETERMINE
GEOMETRIC DESIGN FEATURES
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WORLD MAXIMUM SPEED LIMIT
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DESIGN SPEED
This is the maximum safe speed that can be maintained over a
specified section of highway when conditions are so favorable
that the design features of the highway govern
It is therefore a speed selected and used to determine the
geometric features of the highway
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OPERATING SPEED
This is defined as the speed at which drivers are observed to
operate their vehicles under free flow conditions
It can therefore be above or below the selected design speedwithin a section of a given geometric feature of the highway
As different drivers may drive their vehicles at different speeds
ey es re w n a sec on a ey ee s sa e an com or a e
operating speed within a highway section may differ significantly
It is a common practice therefore to use the 85th percentile speedas a measure of the operating speed
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DESIGN
SPEED
CONSIDERATIONS
1. Functional classification of the highway
2. Characteristics of the terrain
3. Density and characteristics of adjacent land uses
4. Traffic volumes expected to use the highway
5. Economic and environmental considerations
GEOMETRIC DESIGN OF ROADSGDR710S TOMEKA C. L. T. G.