The design for a four lane highway has 20m platform width in

The design for a four lane highway has 20m platform width including shoulders on each side of the pavement.(each lane width:3.5m,median:2m)Cut slopes(accepted as obstructs) are located 0.7meter beyond shoulders.

a)what is the recomended maximum design speed if \'\'e\'\'(superelevation) is decided as 8% and D(degree of curve ) is 11 degrees ?

b)would the foregoing speed be limited by the sight distance under given conditions ?explain and show,why?

Solution

step 1: Given data:

width of each lane= 3.5 m

total width=3.5*4=16 feet

for 4 lanes 2 medians are rquired

so total width=20 feet (this length includes the shoulders also)

step 2:

(a) design of maximum speed:

e+f=V²/127R

f is the coefficient of lateral friction assume it as 0.10

first find out the radius of curve

D=1720/R

11=1720/R

R=156.36m

e+f=V²/(127x156.36)

(0.08+0.1)x127x156.36=V²

V=59.78 Kmph

V=60 kmph

step 3:

(b) SSD:

V=60 Kmph

V=37.158 MPH

a=11.2 ft/sec²

SSD=1.47x37.158x2.5+[37.158x37.158/30(11.2/32.2)]

SSD=192.62 feet

SSD=58.216 m

From the relationship we can see that, the sight distance is a function of reaction time, speed and acceleartion due to gravity

the acceleration due to gravity remains constant, but the reaction time and the velocity may be variable

so the SSD is going to limit the velocity, it depends on the condition which the driver encounters