02 design speed and sight distances

8/19/2019 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).

GEOMETRIC DESIGN OF ROADSGDR710S TOMEKA C. L. T. G.


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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


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,


,


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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.

–


.


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IMPORTANT VEHICLE CHARACTERISTICS

.

WEIGHT OF THE VEHICLEi.

.

ACCELERATION CAPABILITIESi.

DYNAMIC CHARACTERISTICS3.

AIR RESISTANCEi.

GRADE RESISTANCEii.

ROLLING RESISTANCEiii.


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)



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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


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


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


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


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


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


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


02 design speed and sight distances

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