sight distance and vertical alignment i5

Chapter four: Sight Distance And Vertical Alignment Highway Engineering Dr. Raad Awad Kattan

1

Sight distance:

Is the length of the roadway a driver can see ahead at any particular time

stopping sight distance:

is the minimum sight distance required for a driver travel at near the design speed to stop a vehicle after seeing a stationary object in the vehicles path without hitting that object.

Minimum stopping sight distance( S.S.D)= distance travelled during perception reaction

Time + distance travelled during braking

S.S.D=v.t+

Substituting for g( acceleration due to gravity) = 9.81m/sec2

S.S.D=0.28 v.t+

S.S.D in meters v= speed km/hr t= perception- reaction time (usually=2.5sec)

f= friction G= slope (up +ve, down ve)

AASHTO represents the friction coefficient (f) by ( a/g) where a= constant rate of deceleration =3.4m/sec2 and when g= 9.81 m/sec2 then a/g= 0.35

Example :

Find the minimum S.S.D for a vehicle travelling at a speed of 70km/hr on a highway having a slope of 5%

Passing sight distance:

Is the minimum sight distance required on a 2-lane 2-way highway that will permit a driver to complete a passing maneuver without colliding with an opposing vehicle and without cutting off the passed vehicle. (Single passes are considered)

Certain assumptions are made:

1- The passing vehicale is travelling at the same speed as the impeder (the vehicle being passed)

Chap

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3- Th

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3

Example: find the minimum passing sight distance at a section of highway assuming

v= 70 km/hr, m=16 km/hr, a=2.3 km/hr/sec, t1 =4sec, t2 =10 sec

d1=0.278 t1 (v-m+a. t1/2)

= 0.28x 4( 70- 16 +2.3x4/2)= 66m

d2=0.28 v t2= 0.28x70x10 = 196m

d3 = assumed to be 55m for this speed

d4=2/3 d2= 2x196/3= 131m

Total passing sight distance = 448m

The vertical alignment

The vertical alignment of a highway consists of straight sections known as grades or tangents connected by vertical curves.

Grades:

Maximum Grade limits on highway range btween 4 % to 12%

4% grade is the maximum desirable grade on major highways

Grade has greater effect on trucks than on passenger cars..

Trucks speed may increase by up to 5% on down grades and may decrese by 7% on up

grades use climbing lanes for trucks

Minimum grade depends on the drainage conditions

Zero% grade is used on uncurbed pavement having adequate cross slope

Minimum of 0.5% grade is used in curbed pavement to let surface watwe flow.


4

Vertical curves

Vertical curves are used to provide a gradual change from one tangent grade to another.

These curves are usually parabolic curves.

I +G2

+G1 -G1 +G1

L/ -G1

L -G2

Crest vertical curves

+G2

-G1 I +G1

+G1

-G1 -G2 Sag vertical curves


5

The parabola I

p ax2

b.x

BOVC X EOVC

L

Crest vertical curve

Equation of the parabola:

Elev.p= Elev. BOVC+b.x +ax2

b is the left hand grade a=

example: a parabolic vertical curve having a length= 400m used to connect two grades the first= +4% , the second=-5%. elevation of the point of intersection =30m. find the elevation of points on the curve every 50m from BOVC

Elev. BOVC= 30- = 22m b= 4% a=

=-9/80000

x ( m) bx ( m) ax2 ( m) elev p (m)

0 0 0 22.00 50 2 -0.28 23.72 100 4 -1.13 24.87 150 6 -2.53 25.47 200 250 300 350 400

Find the elevation of the highest point on the curve?

bx

Chap

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7

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Determine 00km/hr,

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the minimf=0.35, t=

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8

Location of the headlight above ground= 0.6m

Angle at which headlight is inclined upward= 1 S = sight distance

2-Rider comfort: this criteria is based on the fact that when vehicle travels on a sag v.c, both the gravitational and centrifugal forces act together. This will result in a greater effect on the passengers.

comfort ride will be provided if acceleration is not greater than 0.3m/sec2. The expression used for comfort control is :

L=. v is the design speed

3-Control of drainage a: a minimum slope of 0.35% is required

4- General appearance :

The minimum length of the v.c required L=30A if A>2

L=60m if A< 2

Example:

Find the minimum length of a sag v.c connecting a -3% grade to a +3% grade. The design speed= 65km/hr. f=0.32, perception- reaction time = 2.5 sec

Solution:

SSD= 102.9m

Head light distance L=125.78m for S>L . not ok

L=132.31m for S 2

Therefore the minimum length that satisfy all criteria = 180m

Chap

Sight

Strucheke

L=

L=2S

C= ve

h1

h2=

Exam

1-

2-

pter four: Sig

t distance

uctures maed

S-

ertical clea

1 = height

= height o

mple: G1=-

For S

sight distance and vertical alignment i5

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