Highway Geometric Design MCQ Quiz - Objective Question with Answer for Highway Geometric Design - Download Free PDF

Explanation:

On a horizontal curve, a vehicle is subjected to centrifugal force acting outward. To ensure safe travel, two critical stability conditions must be satisfied:

1. Condition to Avoid Overturning:

• Overturning happens when the moment due to centrifugal force exceeds the restoring moment due to the vehicle’s weight.

• The condition to avoid overturning is centrifugal ratio should always be less than  

2. Condition to Avoid Lateral Skidding:

• Skidding occurs if the centrifugal force exceeds the maximum lateral frictional force that can be developed between the tyres and the road.

• So, the condition is the centrifugal ratio should always be less than coefficient of lateral friction.

Additional Information

• Centrifugal Force and Horizontal Curves
  When a vehicle negotiates a horizontal curve, it experiences an outward centrifugal force, which increases with the square of its speed and decreases with a larger radius. This force acts horizontally through the center of gravity and can lead to either skidding or overturning if not balanced by friction or geometry.

• Overturning Criteria
  Overturning occurs when the moment generated by the centrifugal force about the outer wheel exceeds the restoring moment due to vehicle weight. The critical ratio b2h\frac{b}{2h}2hb​ is derived from equating these moments. A wider wheel track and lower center of gravity enhance stability against overturning.

• Skidding Resistance and Friction
  Skidding happens when the lateral frictional force between the tires and the pavement is exceeded by the centrifugal force. The coefficient of lateral friction, denoted as $f$, provides a threshold for safe travel, and it depends on pavement conditions, tire quality, and vehicle speed.

Explanation:

• Gradient refers to the longitudinal slope along the road, expressed as a percentage (or ratio), representing the rise or fall per unit length of the road.

• Camber is the cross slope provided to the road surface, so that water drains off the pavement quickly. It is also expressed as a percentage or ratio.

• The relation between gradient and camber is Gradient = Camber 

Additional InformationRuling Gradient

• This is the maximum gradient which is commonly adopted in the design of road alignment under normal conditions.

• It provides an economical balance between construction cost and operational efficiency of vehicles.

• It depends on terrain — in plain areas, ruling gradient is flatter, in hilly terrain, it can be steeper.

Limiting Gradient

• This is steeper than the ruling gradient.

• Used when topography, construction cost, or other constraints make it impossible to follow ruling gradient.

Exceptional Gradient

• The steepest allowable gradient, used only in very short lengths (say 100 m or less).

• Used when terrain is extremely difficult and no other option is feasible.

• Not recommended for general use because it causes driver fatigue and affects vehicle operation.

Explanation:

• A camber is the transverse slope provided to the road surface, mainly to facilitate drainage of rainwater and prevent water from stagnating on the road.
• For earthen roads, which are more susceptible to damage from water, a steeper camber like 1 in 30 ensures quick and effective drainage, thus maintaining the serviceability of the road.

 Additional InformationCamber to be provided for different types of pavement are as follows:

Explanation:

Concept:

While negotiating curve on road by vehicle, the required centripetal force is provided by the frictional force between the road and tyre.

Centripetal force =  and $\mu mg$

 For avoiding car to skid the centripetal force must be equal to the frictional force ⇒    

∴ Maximum speed with which the cyclist can travel without the fear of skidding  

Solution:

Given, Radius of corner = 50 m,  

coefficient of friction between the tires and track =  0.2,

 acceleration due to gravity = 10 m/s2

∴ Maximum speed with which the cyclist can travel without the fear of skidding 

  m/s

Explanation:

The extra width of a carriageway that is required on a curved section of a road over and above that is required on a straight alignment is known as Extra Widening.

Mechanical widening: The widening required to account for the off-tracking due to rigidity of wheelbase is known as Mechanical widening. It can be calculated as:

n = Number of lanes

l = length of wheelbase = 6.1 m

R = Radius of circular curve

Psychological Widening: Extra width of pavement provided for psychological reasons such as overhangs of vehicles, greater clearance for crossing, etc is known as Psychological Widening.

Concept:

The centrifugal force exerted on the vehicles while traversing through the curves is counteracted by providing superelevation, which is given by:​​

However, it is assumed that the centrifugal force is completely nullified if the vehicle is travelling at its 75% of the vehicle design speed.

∴ 

According to IRC,

Concept:

Lag distance = 0.278 × V × t_R

Where,

V = Speed in Kmph

t_R = Reaction time in sec

Calculation:

Lag distance = 0.278 × 47 × 2.5 = 32.665

For two way traffic on a single lane lag distance = 2 × 32.665 = 65.33 m

Explanation:

Transitions curve:

(i) When a vehicle traveling on a straight road enters into a horizontal curve instantaneously, it will cause discomfort to the driver. To avoid this, it is required to provide a transition curve. This may be provided either between a tangent and a circular curve or between two branches of a compound or reverse curve.

Different types of transition curve:

The types of transition curves commonly adopted in horizontal alignment highway are

(i) Spiral or clothoid

(ii) Bernoulli’s Lemniscate

(iii) Cubic parabola

(a) All the three curves follow almost the same path up to deflection angle of 4°, and practically there is no significant difference between even up to 9°. In all these curves, the radius decreases as the length increases.

(b) According to IRC ideal shape for transition curve is spiral because rate of change of radial acceleration remains constant. Generally, spiral curve provided on hilly road.

(c) Cubic parabola is provided for the railway.

Given:

Ruling gradient of hilly road = 6%

Radius of curve (R) = 60 m

Calculation:

Grade compensation = 

=  = 1.5%

This should not be more than

=  =  = 1.25%

Compensated gradient = Ruling gradient – Grade compensation

= 6% – 1.25% = 4.75%

Concept:

Design Speed:

• It is defined as the maximum speed at which vehicles can continuously travel safely under favorable conditions is called design speed. 
• It may also be thought of as the maximum approximate speed that will be adopted by most drivers, Choice of design speed has to be made carefully, so as to match the terrain condition and also to be acceptable to most road users.
• It is the basic parameter that determines all other geometric design features.

Design speeds for various classes of Roads are given below in the table:

Hairpin bends:

• A hairpin bend is a sharp curve and it is located on a hillside having the minimum slope and maximum stability.
• It must also be safe from the viewpoint of landslides and groundwater.
• For reducing the construction problems and expensive protection works, the hairpin bends should be provided with long arms and farther spacing. 
• The minimum design speed adopted where hairpin bends are provided at hill roads is 20 Kmph.

Explanation

Given,

 Upgrade of 2.0 % followed by the downgrade of 2 % 

 N1 = 2 % , N2 = - 2%

  Rate of change of grade is 0.05 % per 20 m chain.

 Total change in grade (N) = N1 – N2

 N= 2 % - (- 2%) = 2% + 2%

 N = 4 %

Width of formation or roadway width:

It is the sum of the widths of pavements or carriageways including separators and shoulders. This does not include the extra land of formation/cutting.

These values suggested by IRC:

∴ The width of National & State Highways in plain and rolling terrain for the two-lane is 12 m.

Concept:

The height of the crown with respect to edges is given by

Calculation:

Given: Width of road = 8 m, camber (or) cross fall = 1 in 50 = 1/50 = 0.02

⇒ Height of crown = 0.08 m

Additional Information

Camber of pavement depends on

• Type of pavement
• Intensity of rainfall (Light/Heavy rain)

Explanation:

The extra width of carriageway that is required on a curved section of a road over and above that is required on a straight alignment is known as Extra Widening.

Mechanical widening: The widening required to account for the off-tracking due to rigidity of wheelbase is known as Mechanical widening. It can be calculated as:

n = Number of lanes

l = length of wheelbase = 6.1 m

R = Radius of circular curve

Psychological Widening: Extra width of pavement provided for psychological reasons such as overhangs of vehicles, greater clearance for crossing, etc is known as Psychological Widening.

V = Design speed (kmph)

Concept:

• If the pavement is rotated about the inner side, Then rise of outer edge = e × W
• If the pavement is the rotated about center line, Then rise of the outer edge =  

Where e = Super elevation and W = Width of pavement

Calculation:

Given: Rise of outer edge with respect to inner = 40 cm, W = 10 m = 1000 cm

It is given outer edge is 40 cm higher with respect to inner edge

40 = e × 1000

The required superelevation is 1 in 25

Additional Information

Superelevation:

To counteract the effect of centrifugal force and to reduce the tendency of the vehicle to overturn or skid, the outer edge of the pavement is raised with respect to the inner edge, thus providing a transverse slope throughout the length of the horizontal curve. This transverse inclination to the pavement surface is known as superelevation or cant.