hre reading materials
minimum gradient
is important only at locations where surface drainage is important, camber will take care of the lateral drainage, but the longitudinal drainage along the side drains requires some slope for smooth flow of water
overtaking sight distance
is measured along the center line of the road over which a driver with his eye level 1.2 m above the road surface can see the top of an object 1.2 m above the road surface
intermediate sight distance
is often referred to as combined distance travelling at design speed needs to stop before reaching a stationary object
horizontal alignment
is one of the most important features influencing the efficiency and safety of a highway
horizontal transition curve
is provided to change the horizontal alignment from straight to circular curve gradually and has a radius which decreases from infinity at the straight end tangent point to the desired radius of the circular curve at the other end curve point
sight distance
is the actual distance along the road surface, over which a driver from a specified height above the carriage way has visibility of stationary or moving objects
setback distance or clearance distance
is the distance required from the centreline of a horizontal curve to an obstruction on the inner side of the curve to provide adequate sight distance at a horizontal curve
lag distance
is the distance the vehicle travelled during the reaction time and is given by velocity multiply by time
breaking distance
is the distance travelled by the vehicle during braking operation
headlight sight distance
is the distance visible to a driver during night driving under the illumination of head lights
ruling gradient
is the maximum gradient with which the designer attempts to design the vertical profile of the road
overtaking sight distance
is the minimum distance open to the vision of the driver of a vehicle intending to overtake the slow vehicle ahead safely against the traffic in the opposite direction
overtaking zones
the desirable length of overtaking zones is 5 times OSD and the minimum is three times OSD
speed the vehicle
affects the sight distance., higher the speed, more time will be required to stop the vehicle
off tracking
when a vehicle negotiates a horizontal curve, the rear wheels follow a path of shorter radius than the front wheels
design consideration
are comfort and security of the driver, and the appearance of the profile alignment, among these, a sight distance requirement for the safety is most important on summit curves
overtaking zones
are provided when OSD cannot be provided throughout the length of the highway
sag curve or valley curve
are vertical curves with convexity downwards
summit curve
are vertical curves with gradient upwards
exceptional gradient
are very steeper gradients given at unavoidable situations, in mountainous and steep terrain, the gradient is restricted to 2.5%
gradient up to 7%
can have considerable effect on the speeds of the passenger cars
vertical alignment
consists of gradients and vertical curves, the vertical alignment is usually drawn as a profile, which is a graph with elevation as vertical axis and the horizontal distance along the centre line of the road as the horizontal axis
centrifugal force
depends on speed and radius of the horizontal curve and is counteracted to a certain extent by transverse friction between the tyre and pavement surface
efficiency of brakes
depends upon the age of the vehicle, vehicle characteristics etc.
headlight sight distance
ensures that the roadway ahead is illuminated to at least the stopping distance
A = g1% + g2%
formula for A
H = [ L ( g1 + g2 ) ] / 8
formula for H, symmetrical
H = [ ( L1 ) ( L2 ) ( g1 + g2 ) ] / [ 2 ( L1 + l2 ) ]
formula for H, unsymmetrical
L = ( A S² ) / [ 100 ( √ 2 h1 + √ 2 h2 )² ]
formula for S < L, crest
L = [ S² ( g2 - g1 ) ] / [ 2 ( S θ + h ) ] g2 - g1 = decimal, with sign θ = multiply to π / 180 for rad
formula for S < L, headlamp
L = ( A S² ) / ( 122 + 3.5 S )
formula for S < L, sag
L = 2 S - [ 200 ( √ h1 + √ h2 )² ] / A
formula for S > L, crest
L = [ 2 ( S θ + h ) ] / ( g2 - g1 ) g2 - g1 = decimal, with sign θ = multiply to π / 180 for rad
formula for S > L, headlamp
L = 2 S - ( 122 + 3.5 S ) / A
formula for S > L, sag
S1 = [ ( g1 ) ( L1 )² ] / 2 H S2 = L - S1 or S2 = [ ( g2 ) ( L2 )² ] / 2 H S1 = L - S2
formula for S1 or S2, with H, unsymmetrical
S1 / g1 = L / ( g1 + g2 ) or S2 / g2 = L / ( g1 + g2 )
formula for S1 or S2, with L, symmetrical
S = V t + V² / [ 2 G ( f + g1 ) ]
formula for V, crest
L = ( A V² ) / 395
formula for V, sag
g3% ( L1 + L2 ) = +- g1% ( L1 ) +- g2% ( L2 )
formula for g3, unsymmetrical
add (if going up) or subtract (if going down) the area below to grade diagram
how to get elevation
add (if going right) or subtract (if going left) the length on horizontal plane
how to get station
horizontal curve design elements
include design of super elevation, extra widening at horizontal curves, design of transition curve, and set back distance
horizontal alignment design
involves the understanding on the design aspects such as design speed and the effect of horizontal curve on the vehicles
overtaking sight distance
is the minimum sight distance on two-way single roadway roads that must be available to enable the driver of one vehicle to pass another vehicle safely without interfering with the speed of an oncoming vehicle travelling at the design speed
gradient
is the rate of rise or fall along the length of the road with respect to the horizontal
design speed
is the single most important factor in the design of horizontal alignment
reaction time of the driver
is the time taken from the instant the object is visible to the driver to the instant when the brakes are applied
intermediate sight distance
it allows two drivers travelling towards each other at design speed to stop before meeting and allows overtaking with safety when no opposing traffic in view
frictional resistance
plays an important role to bring the vehicle to stop
g1 / S1 = g2 / S2
ratio and proportion
centrifugal force
reactive force acting outward on a vehicle negotiating it
extra widening
refers to the additional width of carriageway that is required on a curved section of a road over and above that required on a straight alignment
length of transition curve
should be determined as the maximum of rate of change of centrifugal acceleration, rate of change of super elevation, and an empirical formula
critical length of grade
the maximum length of the ascending gradient which a loaded truck can operate without undue reduction in speed is called critical length of the grade
grade compensation
the maximum length of the ascending gradient which a loaded truck can operate without undue reduction in speed is called critical length of the grade
psychological widening
there is a tendency for the drivers to drive close to the edges of the pavement on curves. some extra space is to be provided for more clearance for the crossing and overtaking operations on curves
limiting gradient
this gradient is adopted when the ruling gradient results in enormous increase in cost of construction, on rolling terrain and hilly terrain it may be frequently necessary to adopt this gradient
mechanical widening
to provide the same clearance between vehicles travelling in opposite direction on curved roads as is provided on straight sections, there must be extra width of carriageway available
