Highway Alignments

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The Geometric design of a highway consist of horizontal alignment, vertical alignment and cross-sectional elements.
Horizontal alignment of a highway defines its location and orientation in plan view. Vertical alignment of a highway deals with its shape in profile.


Stationing

Distance along a horizontal alignment is measured in terms of stations. A full station is defined as 100 ft (30.48 m) and a half station as 50 ft (15.24 m). This distance is measured horizontally along the centerline of the roadway, whether it is a tangent, a curve, or a combination of these.

Stopping Sight Distance

Stopping Sight Distance or SSD is the distance needed
between a vehicle and an arbitrary object (at some point down the road) to permit a driver to stop a vehicle safely before reaching the obstruction.
For crest vertical curves, AASHTO defines the minimum length Lmin
, ft (m), of crest vertical curves based on a required sight distance S, ft (m), as that given by

1) For S
Lmin=(AS2)/[100 (?2H1 +?2H2)2]

2)For S>L
Lmin=25-[200((?H1+?H2)2]/AS2

where
A= algebraic difference in grades, percent, of the tangents to the vertical curve

H1= eye height, ft (m), above the pavement

H2= object height, ft (m), above the pavement

Design controls for vertical curves can be established in terms of the rate of vertical curvature K defined by

K=L/A

where
L= length, ft (m), of vertical curve
. K is useful in determining the minimum sight distance, the length of a vertical curve from the PVC to the turning point (maximum point on a crest and minimum on a sag). This distance is found by multiplying K by the approach gradient.
Recommended values of K for various design velocities and stopping sight distances for crest and sag vertical curves are published by AASHTO.

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Comments
  • Ravi Shankar July 2, 2008 at 3:33 am

    Sir/Madam,

    Presently i am doing the alignment for road of 1047 km on the left bank of river Ganges in India. This is now in the planning stages, the alignment is marked on the Survey of India map of scale 1:50,000. I would like to know that is it possible to mark or peg the centre line without carrying out detail survey; i suppose it is not possible as prelim svy is required with all control work and also as the length is long, we ar bound to get erros in projection on the ground as the map is in Lambert conical ,wheras we plan to carryout the stretch in TM projection. Kindly do let me know how you would like to carryout the greenfield project implementation of the centre line marking on the grd once the aligment has been frozen on the map.

    Regards
    Ravi Shankar

    • nilesh November 12, 2010 at 2:29 pm

      Hi, yes you can do it without doing any prelim svy…u just try “Google Earth pro” with Autodesk Civil 3D 2011 …results are not that much accurate but they are sufficient for start of project …these are some advance tech’s …advantage is that u will get dynamic model of your project without doing any “physical” survey….and if u look according to hand on project and presentation they are just crazy……try it…for this u will need some guy who having good hand on these soft.

  • Aditya Pathania August 27, 2009 at 5:02 pm

    Sir ,
    My name is Aditya Pathania. I am doing B.Tech in Civil Engineering , i am in 3rd year. Sir can u tell me which site can i join to become a member of Civil engineering site, without any cost.
    So please let me know the best site for it. If there is any indian site for it then please tell me that also.

    Sincere Regards
    Aditya Pathania

  • alhassan abubakar February 23, 2010 at 3:58 pm

    Hello pls link me with civil engineering website bcus i want 2 start my PGD in CIVIL, HERE IN NIGERIA am HND holder.Thank 4rm Alhassan Abubakar.

  • sreedharan.s February 27, 2010 at 2:03 pm

    sir, My name is sreedharan, i am a land surveyor since 1986 onwads. now i am doing total station survey also,LISCAD WHICH IS THE SOFT WARE USE TO DOWN LOAD the work, i need to learn proper, so for that which site can i select. sincerly SREEDHARAN

  • mya April 20, 2010 at 11:15 pm

    halo..i juz wanna to know the procedure to carry out detail survey

  • Arivazhagan April 19, 2012 at 3:03 am

    i’m arivazhagjan presently working as land surveyor.i would like to work land survey with using software.kindly send me which are the software need for surveying?.

  • Tariq August 26, 2013 at 11:06 am

    Wht is alignment? I want answere in hindi and in simple term

  • Post a comment

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