TRAFFIC AND TRANSPORT-I
Assignment-3rd
Submissions: *Grade separation system and its advantages and disadvantages: *Consideration for making an selection for particular type of road Design of junctions: *General observed principles of road junction:
SUBMITTED TO:T.& TLECTURER
PROF. MRS. RICHA TIWARI
SUBMITTED BY:
Anil Sharma, 8007,9th sem.
GRADE SEPERATION SYSTEM INTRODUCTION Traffic problems are a bugbear in most metropolises around the world. Statistics are routinely circulated about the city worst hit by traffic congestion . However, even periodic traffic surveys and studies, as well as the imposition of regulations fail to yield appropriate solutions. In this grim scenario, the transportation engineer came upwith grade separators concept, becoming very common now. Grade separation is the process of aligning a junction of two or more transport axes at different heights (grades) so that they will not disrupt the traffic flow on other transit routes when they cross each other.
The composition of such transport axes does not have to be uniform; it can consist of a mixture of roads, footpaths, railways, canals, or airport runways. Bridges, tunnels, or a combination of both can be built at a junction to achieve the needed grade separation.
The grade seperators are mainly divided in to two categories i.e.
OVERPASSES
UNDERPASSES
OVERPASSES term used to describe the roads crossing over any other road. It can be a bridge, flyover or any other similar structure. UNDERPASSES used to describe a passage underneath something, especially a section of road that passes under another road or a railroad. An intersection formed in this way.
GRADE SEPARATOR
A grade separator can either be a flyover or an underpass, that is any structure which separates the uniform grade of a running highway into two different grades. A bridge is a structure built to span a gorge, valley, road, railroad track, river, body of water, or any other physical obstacle, for the purpose of providing passage over the obstacle. Grade separators can be bridges, but not all bridges are grade separators.
An potential grade in the
example of the complexity of separation, seen Circle Interchange in Chicago
Types Fully separated These junctions connect two roads : Stack interchange :- two-level, three-level, or four-level stack, depending on how many levels cross at the central point Cloverleaf interchange :- A cloverleaf interchange is a two-level interchange in which left turns (in countries that drive on the right) are handled by loop roads Compact grade-separation:- whereby the two roads are linked by a compact "connector road", with majorminor priority junctions at each of its ends; usually a variant of the cloverleaf type interchange, but only involving two quadrants rather than four.
Partially separated
These junctions connect two roads, but only one is fully grade-separated, i.e. traffic on one road does not have to stop at yield lines or signals on one road, but may have to do so when switching to the other: •
Diamond interchange
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Parclo interchange
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Single-point urban interchange
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Roundabout interchange Other variants These junctions terminate one road into another:
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Trumpet interchange Directional-T interchange 4 level stack interchange between the M25 (in the foreground) and M23 in England.
Stack interchange
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A stack interchange is a four-way interchange whereby left turns are handled by semi-directional flyover/under ramps. To go left (right in countries with left-hand drive), vehicles first turn slightly right (on a right-turn off-ramp) to exit, then complete the turn via a ramp which crosses both highways, eventually merging with the right-turn on-ramp traffic from the opposite quadrant of the interchange.
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A stack interchange, then, has two pairs of left-turning ramps, of which can be stacked in various configurations above or below the two interchanging highways.
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Stacks do not suffer from the problem of weaving, but require massive construction work for their flyovers. A standard stack interchange includes roads on four levels. This is not only expensive, but also creates an eyesore among local residents.
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1-Six-level urban stack interchange in Puxi, Shanghai, China (Nanbei Elevated Road at Yanan Middle Road) 2-The High Five Interchange in Dallas, Texas, United States, is a five level interchange
Cloverleaf interchange •
A cloverleaf interchange is a two-level interchange in which left turns (in countries that drive on the right) are handled by loop roads (U.S.: ramps, UK: slip roads).
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To go left (in right-hand traffic)vehicles first pass either over or under the other road, then turn right onto a one-way three-fourths loop ramp (270°) and merge onto the intersecting road.
A typical cloverleaf interchange with collector/distributor roads
ADVANTAGE The major advantage of cloverleaf's is that they require only one bridge, which makes such junctions inexpensive as long as land is plentiful. Problems •
A point of conflict is the merging of exiting and entering traffic in the same lane, known as weaving.
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Weaving is a consequence of having too many grade separated junctions on a road in a short distance, where traffic wanting to leave the grade-separated road at the next junction has to fight for road space with traffic which has just entered from the previous one.
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Weaving can be alleviated by using collector/ distributor roads to separate entering and exiting traffic.
Diamond interchange •
A diamond interchange is a common type of road junction, used where a freeway crosses a minor road.
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The freeway itself is grade-separated from the minor road, a bridge being provided for one or the other. Approaching the interchange from either direction, an off-ramp diverges only slightly from the freeway and runs directly across the minor road, becoming an on-ramp that returns to the freeway in similar fashion.
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The Diamond interchange uses less space than most types of freeway interchange, & avoids the interweaving traffic flows that occur in interchanges such as cloverleaf.
A typical diamond interchange
partial cloverleaf interchange or parclo interchange
partial cloverleaf interchange or parclo interchange •
A partial cloverleaf interchange or parclo interchange is a modification of a cloverleaf interchange. It was developed by the Ministry of Transportation as a replacement for the cloverleaf on Highways, removing the dangerous weaving patterns and allowing for more acceleration and deceleration space on the freeway.
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A diamond interchange has four ramps. A cloverleaf interchange has eight ramps, as does a stack interchange . A parclo is then somewhere in between, although six is the most common.
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Various forms of parclo interchanges are used in the Philippines' North Luzon Expressway.
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The configuration of parclo interchanges (particularly those of the "folded diamond" type) allows for the consolidation of toll barriers at points where onramps and offramps run alongside each other.
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A single large barrier can serve each onramp/offramp pair simultaneously, reducing construction and operation expenses.
Single-point urban interchange •
A single-point urban interchange (SPUI, pronounced also called a single-point interchange (SPI) or singlepoint diamond interchange (SPDI), is a type of highway interchange. The design was created in order to help move large volumes of traffic through limited amounts of space safely and efficiently.
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A single point urban interchange is similar in form to a diamond interchange, but has the advantage of allowing opposing left turns to proceed simultaneously by compressing the two intersections of a diamond into one single intersection over or under the free-flowing road. Advantages
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The most commonly cited advantages of single point urban interchanges are improved operation efficiency and safety as well as reduced right-of-way requirements compared to other interchange forms.
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Left turning traffic from both directions of the intersecting roadways are able to turn simultaneously without crossing the path of the opposing left turns. Because traffic passing through the interchange can be controlled by a single signal.
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vehicles can clear the intersection much more quickly than in a diamond interchange (which requires two sets of traffic signals). Disadvantages
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The major disadvantage of single point urban interchanges over other types of road junctions is the increased cost due to the need for a longer or wider bridge.
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A freeway-under SPUI (as in the upper diagram) requires a wider bridge over the free-flowing road to make room for the compressed on- and off-ramps.
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However, this disadvantage poses less of a problem in cases where the arterial, or non-freeway road already requires a very wide bridge. Roundabout
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A roundabout is a type of circular junction in which road traffic must travel in one direction around a central island. Signs usually direct traffic entering the circle to slow down and give the right of way to drivers already in the circle.
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These junctions are sometimes called modern roundabouts in order to emphasise the distinction from older circular junction types which had different design characteristics and rules of operation. Older designs, called traffic circles or rotaries, are typically larger, operate at higher speeds, and often give priority to entering traffic. In some cases, the term "traffic circle" has been used to describe roundabouts in North America,but generally "roundabout" is used by engineers.
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In countries where people drive on the right, the traffic flow around the central island of a roundabout is anticlockwise (counter clockwise). In countries where people drive on the left, the traffic flow is clockwise.
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Statistically, roundabouts are safer for drivers and pedestrians than both traffic circles and traditional intersections Because low speeds are required for traffic entering roundabouts they are not designed for high-speed motorways (expressways). When such roads are redesigned to take advantage of roundabout principles, steps are taken to reduce the speed of traffic, such as curving the approaches.
Safety •
Roundabouts are safer than both traffic circles and traditional junctions—having 40% fewer vehicle collisions, 80% fewer injuries and 90% fewer serious injuries and fatalities (according to a study of a sampling of roundabouts in the United States, when compared with the junctions they replaced). Roundabouts also reduce points of conflict between pedestrians and motor vehicles and are therefore
considered to be safer for them. However, roundabouts, especially large fast moving ones, are unpopular with some cyclists.
Advantages of Roundabouts •
Roundabouts are safer than signal controlled junctions, with accidents usually occurring at a slower speed and at a slight angle instead of right-angle or rear end collisions at junctions.
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Roundabouts allow U-turns within the normal flow of traffic, which are often not possible at other forms of junction.
Trumpet interchange
Trumpet interchanges have been used where one highway terminates at another highway. These involve at least one loop ramp connecting traffic either entering or leaving the terminating expressway with the far lanes of the continuous highway.
These interchanges are useful for highways as well as toll roads, as they concentrate all entering and exiting traffic into a single stretch of roadway, where toll booths can be installed. A double-trumpet interchange version can be found where a toll road meets another toll road or a free highway.
DIRECTIONAL interchange A directional T interchange uses flyover/under ramps in all directions at a three-way interchange. A semidirectional T does the same, but some of the splits and merges are switched to avoid ramps to and from the passing lane. Directional T interchanges are very efficient, but are expensive to build compared to other three-way interchanges. They also require three levels, which can be an eyesore for local residents.
ROTARY GRADE SEPERATORS The Rotary Grade Separator 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. A bus and truck way, car way and pedestrian way have been provided at three separate levels, irrespective of direction of traffic. The three rotaries are also arranged concentrically, with the car way providing the innermost ring, followed by the bus and truck way and finally the pedestrian rotary.
Bus stops are provided at points just outside the rotary. The commuter can now alight at these points, climb down approximately 3.0metres, cross over to any other bus stop and take a connecting change of route within 200.00metres.
For the optimum Rotary Grade Separator, a diameter of 100M for the outermost rotary is proposed for several reasons.
It enables a comfortable turning radius (several times the minimum specification) for even the longest transport carriers.
Effects OF GRADE SEPERATORS Advantages Roads with grade separation generally allow traffic to move freely, with fewer interruptions, and at higher overall speeds; this is why speed limits are typically higher for grade-separated roads. In addition, less conflict between traffic movements reduces the capacity for accidents. Motorways, though having higher average speeds, usually have much lower accident rates per distance travelled than roads which are not grade separated. DISAdvantages However, grade-separated junctions are very space-intensive, complicated and costly, due to the need for large physical structures such as tunnels, ramps and bridges. Their height can be obtrusive and this, combined with the large traffic volumes that grade-separated roads attract, tend to make them unpopular to nearby landowners and residents. New grade-separated road plans can receive significant opposition from local groups for these reasons.
*THE FACTORS SHOULD UNDERTAKEN WHILE DESIGN ROAD JUNCTION WITH RESPECT ROAD TYPE:
The choice of a junction design depends upon several factors, whose relative importance varies between cases and should be assessed. The most important ones are :
Traffic safety Road type and function Number of concurring legs Traffic volume and type Design and operating speed Priority setting Terrain Available room Adjacent land use Service to neighbouring population Network considerations (design consistency)
Environmental concerns Cost
Safety of motorists, cyclists and pedestrians When operating within their capacity limitations, roundabouts offer superior safety to traffic signals for motorists. Numerous studies indicate lower casualty accidents for roundabouts. The situation is less definitive for cyclists and pedestrians but it appears that if traffic volumes are moderate the splitter islands at roundabouts provide adequate crossing opportunities for cyclists and pedestrians. However, at higher traffic flows, particularly at dual lane roundabouts, the more positive control of signal operation may provide safer crossing opportunities, particularly for the very young and the aged. Partly for this reason there is a general preference to provide traffic signals in town centres. Operating efficiency The volume and distribution of turning traffic are the prime considerations in determining relative operating efficiency between roundabouts and signals. Traffic signals are clearly favoured when through flows predominate, whereas roundabouts perform best when right turning flows are relatively heavy. A potential problem with roundabouts is the monopolisation of capacity which arises when one movement predominates. Traffic flow characteristics required for satisfactory operation of downstream intersections. Uncontrolled downstream intersections (typically T junctions) may operate more satisfactorily if traffic signals are installed upstream. This is because the exit flow from roundabouts tends to be uniform and lacks the interruption caused by signals. Requirement to accommodate long vehicles Long vehicles, because of their wide swept path when turning, may not be readily accommodated through small roundabouts. Public transport requirements The need to accommodate public transport routes may influence the choice of control type. Bus priority measures can be provided at signalised intersections. Speed of traffic Roundabouts, as well as controlling the speed of traffic, are inherently safer in high speed environments for the reasons mentioned previously. Construction costs Construction costs are normally site specific but in green fields situations traffic signals and roundabouts with similar capacity have been shown to favour roundabout construction primarily due to their adaptability for staging purposes. Environmental factors The operation of traffic signals causes stop-start traffic at all times of the day which may be environmentally intrusive whereas roundabouts permit un-interrupted flow during much of the day and night. However, any form of control may increase noise levels over the uncontrolled situation. Civic design considerations Traffic signals, while introducing obtrusive elements in the form of lanterns allow the continuity of through pavements to be maintained which may be of importance on ceremonial routes to maintain vistas and permit the passage of processions. Roundabouts, because of the requirement to install regulatory and warning signs and street lighting, do interrupt vistas along routes. The requirement to maintain sight distance across the roundabout limits the opportunity to ameliorate these aesthetic defects with landscaping except on larger roundabouts.
Adjacent land use Adjacent land use may influence control type. For example, if a school or elderly citizen facilities are nearby, the speed reduction effected by a roundabout may indicate preference for this form of control. Consistency with adjacent intersections In choosing a junction type, account must be taken of adjacent intersections for operational as well as aesthetic conditions. Traffic signals are readily linked to provide progression in the roundabout within a system of linked signals is inappropriate. Similarly, a traffic signal adjacent to a roundabout may cause operational problems due to queues from the signals banking up and disrupting flow on the roundabout. Principles of road junction Junctions, at-grade or grade separated, are locations of high accident concentration. In most countries 40 60% of the total number of accidents occurs at junctions. Consequently, special attention should be given in determining the type, the shape of junctions, as well as the number of junctions along a road axis and the efficient design of each one. Minimization of traffic conflicts locations: A junction has a set of conflict points between vehicle paths, and a good design should aim at minimizing the severity of potential accidents at these points. Sufficient sight distances: Appropriate sight distances, both while approaching the junction area and being at the junction are of major importance for the safe operation of the junction. The creation of junctions in vertical sag curves is considered to be favourable. Another important parameter concerns the prompt perception and comprehension of the junction's layout and operation by drivers, particularly those who are not regular users of the junction, and therefore the selection of appropriate path and travel speed, on which drivers can be assisted by improved horizontal and vertical road marking and appropriate junction layout Longitudinal section and transverse gradients design: It is important to design the longitudinal section of the road in the junction area and access areas, in order to achieve a smooth transition of transverse gradients and effective drainage. Ideal location for a junction is on a tangent. Location in curves may cause problems, as visibility is reduced, conflict points increase and superelevation and lane widening make the situation more complicated. Left turning movements: The locations where left turns and U-turns are allowed should be specifically determined and appropriately configured, by means of traffic islands and/or marked lanes. It should be noted that left turns are high risk movements on a level junction. Research results in Great Britain have shown that these movements (right turns in Great Britain) are responsible for around 70% of all accidents on three-arm junctions Minimization of weaving areas
