highway engg

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Prepared by , khushboo patel ( B.E civil engg.)

Highway engineering

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SIGHT DISTANCE

Definition : The actual distance along the road surface , which a driver

from a specified height above the carriageway has visibility of stationary or moving object is known as sight distance.

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Restrictions to sight distance may be caused as shown in fig.

• Sight distance at horizontal curve

http://www.tpub.com/inteng/11.htm16.gif

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• Sight distance at vertical curve

http://www.msha.gov/training/surfhaul/images/slide6.jpg

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• Sight distance at intersection

http://onlinemanuals.txdot.gov/txdotmanuals/rdw/images/3-2.png

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Types of

sight distanc

e

Stopping sight

distance

Overtaking sight

distance

Site distance at intersectio

n

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Stopping sight distance (SSD)Definition : The minimum sight distance available on a

highway to stop a vehicle travelling at design speed , safely without collision with any other obstruction is xalled stopping distance.

https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcQFKpIxAAAbJH3n64ECpY2EHIf-mjYUXedmQ0WUPheL5hqfngHS

8http://www.ite.org/css/online/img/Figure10-5.jpg

• For the purpose of measuring stopping sight distance , IRC has suggest clear sight triangle as shown in fig. ,

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Factors affecting stopping sight distance

Speed of

vehicle

Efficiency of brakes

Total reaction time of driver

Friction resistance

between road & tyres

Gradient of the road

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1) speed of vehicle :

https://encrypted/tbn1.gstatic.com/images?q=tbn:ANd9GcTJk56HWGT5GXbpWSsGFElb5384HgIeyKfqcB4cqKwu9xy69dSm

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2) total reaction time of the driver :

http://www.hk/phy.org/contextual/mechanics/kin/eq_motion/6-01.gif

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3) friction resistance between the road & tyres :

https://www.koroad.or.kr/images/en/img_know38.jpg

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4) gradient of road :

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http://webarchive.nationalarchives.gov.uk/20061101091400/http://ordnancesurvey.co.uk/images/userImages/misc/education/gradient.gif

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Analysis of SSD:

Lag distanc

e

Breakin

g distance

Stopping sight distance

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Lag distanceDefinition : The distance travelled by the vehicle during

the total reaction time is known as lag distance .

Reaction time of the driver is the time taken from the instant the object is visible to the driver to the instant the brakes are effectively applied.

http://www.hk/phy.org/contextual/mechanics/kin/eq_motion/6-01.gif

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What is PIVE Theory ?Acc. To PIVE theory the total reaction time of the driver is

split into four parts .

Perception time

Intellection time

Emotion time

Volition time

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1) Perception time : It is the time required for the sensations received by the eyes

or ears to be transmitted to the brain through the nervous system .

2) Intellection time : It is the time required for understanding the situation.

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http://www.arrivealive.co.za/images/stoppingdistance.jpghttps://m2.behance.net/rendition/pm/703187/disp/859cc9a079eca4b5556af04b3aa6215c.png

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3) Emotion time : it is time elapsed during emotional sensations & Disturbance

such as fear , anger ,etc. with reference to the situation . 4) Volition time : volition time is the time taken for the final action.

Lag distance = v * t

Where, v = speed of vehicle in m/s t = total reaction time (s) [ as per IRC t = 2.5 s ]

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Have a nice day.