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  REHABILITATION OF ROAD TRAFFIC

BY INTRODUCING FLYOVERS
 

Flyover: - It is a bridge that carries one road or railway line above another either with or without subsidiary roads, for communication between the two.
 
 Reason behind going for a flyover: - As the traffic on the road goes on increasing and we don’t have any space left in both the dimensions, then the only option left will be to go to the third dimension and that is done through flyover construction.

Design of flyover: -

All roads, flyovers, etc are designed solely to accommodate the heaviest and tallest vehicle: the fully loaded truck, thus resulting in a gross over design for cars. The slab of a flyover is designed to withstand the 20 tonne weight, (dynamic load) of a fully loaded truck. This results in a slab with box girders that are about 2-3metres deep. Similarly the clear height of the flyover slab is based on the requisite 7-9metres clearance for a truck. The finished slab of the flyover is at +9-12metres. The slope of the flyover ramp is also calculated on the basis of a 1:20 gradient, on the basis of a truck. However, a slope of 1:10 would provide comfortable access for cars. The cost saving alone in the embankments would be considerable. Similarly, by factoring segregation of traffic into design for each aspect, there would be enormous design optimisation and concomitant cost saving.

Classification: -

    At Crossings:

       Railway Crossing: - At railway crossing where there is high traffic congestion in terms of the frequency of trains passing by or the traffic on the road, in both the cases the flyover should be provided along the road. Here the flyover becomes indispensable.

      Road crossing: - There are two types of flyovers which are used for traffic management at road crossings.

                Simple Flyovers: - In this case, the main road is used for fast traffic, which is made to pass at a high level by a bridge, providing ramps on both the approaches; and the slow traffic is made to pass underneath. Thus the traffics pass at two different levels, and leave no chance for an accident

N.B. Flyover bridges are not well suited to roads where separate provision for cyclists is to be made, because climbing a flyover becomes difficult for a cyclist

 Even after providing the flyover, as we have to continue providing the traffic lights at the intersection beneath the flyover so as to make a person approaching from any direction reach every other direction, the flyover is not replacing the traffic lights.
 In the figure (of the Simple Flyover enclosed) if we see point A- there are two directions of traffic flow at two different times. This we cannot afford if both the roads are having heavy and fast traffic. So we have to find some other option so that no one will be interrupted. The option is to construct a cloverleaf junction.
 
Grade Seperator: - The Rotary Grade Separator dovetails the benefits of a rotary with the concept of a flyover. It is essentially a multi-level rotary with traffic segregation at distinct vertical levels on the basis of mode of traffic and not direction alone. We already accept the horizontal segregation of traffic in separate lanes based on direction and within lanes based on speed of travel. The Rotary Grade Separator carries this idea of segregation through to a traffic crossing. While the flyover focuses on enabling fast movement of traffic, it ignores the pedestrians' difficulty in negotiation. The biggest benefit of the Rotary Grade Separator is that it is designed around the human being - the pedestrian and providing him safe and secure movement and access.

             Cloverleaf Junction: -It is also a type of grade separator. It was first used in America. It requires a very large area of land. All conflicting streams of traffic are avoided, and so traffic can move at its own speed. This is more advantageous than a roundabout, as there is no necessity for weaving and slowing down of traffic.
For any person approaching the intersection there are three ways through any of which he may pass and there are four ways through which he can approach. So, totally there should be 4 x 3 = 12 connectivities, which can be seen in the figure (of the cloverleaf junction) enclosed.
Cloverleaf junction(Top view)
 

Cloverleaf junction(Isometric view)

The cloverleaf is (on paper) the simplest way to connect two freeways. The only bridges required are to separate the two roadways. If land is expensive, so too can be the cloverleaf, which becomes a choice between tight turning radii (and lower design speed) or lots of consumed land. We can notice that most loop ramps are banked to counteract centrifugal forces.
    A small advantage that "falls out of the design" is the "second chance:" if we miss the first ramp to the right, we can simply take three loops in a row to get back on track. (We can also use two loops of a cloverleaf to make a U-turn.)

A disadvantage to the plain cloverleaf is the "weaving" process, where drivers exiting one loop have to merge and cross other drivers entering the next one. Weaving, which causes bottlenecks and accidents, is the primary reason cloverleafs are now deprecated in designs for new or revamped interchanges.
One way to improve a cloverleaf is to add collector/distributor (C/D) roads, which run parallel to the freeway and isolate it from the weaving action at the loops. Traffic exits the freeway onto the C/D road, and then can decide which direction to take. Likewise, onramps from both directions of the other road merge together first, then merge onto the primary road. The weaving problem still exists, but has been moved to a lighter-traffic side road; and the freeway now has two ramps (entrance and exit) to deal with instead of four. Where two freeways intersect, each one may have C/D roads.
 

Alternate routes:-  One purpose of flyovers is to provide alternate routes from one place to another. But in this case it may happen that the flyover may add to the traffic as such increasing the usual traffic. For example there is a crossing connecting four roads and there is a place, which can be reached through one of these roads by crossing a railway line. In order to prevent crossing the railway track and also to provide an alternate route, if a flyover is constructed between the crossing and that place then there will be one more road added at the crossing. With this the management of traffic becomes very difficult at the crossing as the traffic will be increased by a considerable amount by its introduction. So, here the flyover is adding to the traffic rather than reducing it.
 
 

Advantages of flyover:-
 


Flyovers have many advantages, but shortcomings arise only because of some mistakes committed during their construction or due to improper planning, etc.

  Shortcomings:
 

Conclusion:- After discussing the various facts what we can say is that flyovers should be constructed only if the benefits from it are far greater than the cost incurred. Also proper location must be decided in constructing it with proper management. It should be seen that as far as possible the flyovers purpose is consummated.
 

Reference:
A Text Book of Highway Engineering By Prof. Shahane & Prof. Iyengar.
Newspaper of The Hindu.
www.anangpur.com
 

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