Intersection designs

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Intersection Designs By Mahesh

Transcript of Intersection designs

Page 1: Intersection designs

Intersection Designs

ByMahesh

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Intersection is an area shared by two or more roads.

This area is designated for the vehicles to turn to different directions to reach their desired destinations.

This is because vehicles moving in different direction want to occupy same space at the same time.

Intersection of road

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There are two main types of intersection of roads.

Grade – separated intersections or interchanges.

At – grade intersections.

Types

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It is a bridge that eliminates crossing conflicts at intersections by vertical separation of roadways in space.

Route transfer at grade separations is accommodated by interchange facilities consisting of ramps.

The interchange configurations are designed in such a way to accommodate economically the traffic requirements of flow, operation on the crossing facilities, physical requirements of the topography, adjoining land use, type of controls, right-of-way and direction of movements.

Grade separated intersection or interchanges

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Objective The ultimate objective of grade separated

intersections is to eliminate all grade crossing conflicts and to accommodate other intersecting maneuvers by merging, diverging and weaving at low relative seed.

Grade separated intersection or interchanges

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One of the distinctions made in type of interchange is between the directional and the non directional.

Directional interchanges are those having ramps that tend to follow the natural direction of movement.

Non directional interchanges require a change in the natural path of traffic flow.

Classification of Grade Separated Intersection

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Underpass Overpass Trumpet Interchange Diamond Interchange Cloverleaf Interchange Partial cloverleaf Interchange Directional Interchange Bridged Rotary

Major Interchanges

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An underpass or a tunnel is an underground passageway, completely enclosed expect for openings for ingress and egress, commonly at each end.

A tunnel may be for foot or vehicular road traffic, for rail traffic.

Underpass

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An overpass also known as a flyover, is a bridge, road, railway or similar structure that crosses over another road or railway.

A pedestrian overpass allows pedestrians safe crossing over busy without impacting traffic.

Overpass

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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.

The principal advantages are low construction cost and are useful for highways as well as toll roads.

Trumpet interchanges

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A diamond interchange is a common type of road junction, used where a freeway crosses a minor road.

The diamond interchange uses less space than most types of freeway interchange, and avoids the interweaving traffic flows that occur in interchanges such as the cloverleaf.

Diamond interchanges are most effective in areas where traffic is light.

Diamond interchange

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A cloverleaf interchange is a two-level interchange in which left turns are handled by ramp roads.

To go left(in right-hand traffic), vehicles first continue as one road passes over or under the other, then exit right onto a one-way three-fourths loop ramp (270) and merge onto the intersecting road.

Cloverleaf interchange

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Partial clover leaf is a modification that combines some elements of a diamond interchange with one or more lops of a cloverleaf to eliminate only the more critical turning conflicts.

It provides more acceleration and deceleration space on the freeway.

Partial cloverleaf interchange

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A Directional interchange provides direct for left turns.

These interchanges contain ramps for one or more direct or semi direct left turning movements.

Interchanges of two freeways or interchanges with one or more very heavy turning movements usually warrant direct ramps, which have higher speeds of operation and higher capacities, compared to loop ramps.

Directional interchange

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At-grade intersections in which all the exchanges between the roads take place on the sane plane.

These are of two types Standard at-grade intersections Round about at-grade intersections.

At grade intersections

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Simple intersections maintain the street’s typical cross-section and number of lanes throughout the intersection, on both the major and minor streets.

Simple intersections are best-suited to locations where auxiliary (turning) lanes are not needed to achieve the desired level-of-service.

Simple intersections

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Vehicles approaching an intersection are directed to definite paths by islands, marking etc. and this method of control is called channelization.

Channelized intersection provides more safety and efficiency.

It reduces the number of possible conflicts by reducing the area of conflicts available in the carriageway.

If no channelizing is provided the driver will have less tendency to reduce the speed while entering the intersection from the carriageway.

Channelized intersections

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Design parameters of intersections

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Crossings and pedestrian curb cut Ramp Locations

Walking Speed

Pedestrian Flow Capacity

Traffic Control, Yielding and Delay

Pedestrians

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Cross-section- Bicyclists position themselves for their intended destination regardless of the presence of bike lanes or shoulders.

Operating Speed- At un signalized intersections, an average bicycle speed of 15 miles per hour can be assumed on the major project.

Bicyclists

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Bicycle Capacity - The number of bicycles per hour that can be accommodated by the facility under normal conditions.

Traffic Control - Bicyclists are required by law to obey control devices at intersections. Therefore, traffic control devices need to account for bicycle activity.

Bicyclists

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Design Vehicle *The largest type of motor vehicle that is normally expected

to be accommodated through the intersection.

*At intersections, the most important attribute of design vehicles is their turning radius, which in turn influences the pavement corner radius and therefore the size of the intersection.

*Lane width, another feature related to the design vehicle, has some impact on intersection design.

* The design vehicle appropriate for most types of transit service is the “city-Bus” as defined by AASHTO.

Motor vehicles

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Passive control> No control> Traffic signs> Traffic signs plus marking.

Levels of intersection control

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Semi control Channelization Traffic rotaries Active control Traffic signals Grade separated intersections

Levels of intersection control

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Intersection area is paved and there is absolutely no restriction to vehicles to use any part of intersection area.

Hence the un channelized (all-paved) intersections are the lowest class of intersection, easiest in the design but most complex in traffic operations

Resulting in maximum conflict area and more number of accidents, unless controlled by traffic signals or police.

Unchannelized Intersections

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Plain intersection: No provision for additional pavement width for turning movements

Flared intersection: Provision for additional pavement width for turning movements

Types of unchannelized intersection:

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A small raised area in the middle of a road which provides a safe place for pedestrians to stand and marks a division between two opposing streams of traffic.

(or) A traffic island is a solid or painted object in a road

that channelizes traffic. It can also be a narrow strip of island between roads that intersect at an acute angle.

If the island uses road markings only, without raised kerbs or other physical obstructions, it is called a painted island. Traffic islands can be used to reduce the speed of vehicles driving through.

Traffic island

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Divisional Islands: Divisional islands are dividing the highway in two one way road way so that head on collision are eliminated and accidents are reduced.

Channelizing Islands: Channelizing islands guide the traffic into proper channel through the intersection area.

Pedestrian loading Islands: They are provided at regular bus stops and similar laces for the protection of passengers.

Rotary Islands: Rotary islands is the large central island of a rotary intersection.

Types of Traffic islands

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Rotary Islands Rotary intersections or

round abouts are special form of at-grade intersections laid out for the movement of traffic in one direction around a central traffic island.

The vehicles entering the rotary are gently forced to move in a clockwise direction in orderly fashion.

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Width of the Rotary IRC suggest that a two-lane of 7m width should be kept as

7m for urban roads and 6.5 for rural roads. Further for a three-lane road ofb10.5m is to be reduced to 7m

and 7.5m respectively for urban and rural roads. The width of weaving section should be higher than the

width at entry and exit. The weaving with is given as,

Where e1 = width of carriageway at the entry e2 = width of carriageway at the exit

W weaving= {(e1+e2)/2} +3.5 m

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The capacity of rotary is determined by the capacity of each weaving section. Transportation road research lab (TRL) proposed the following empirical formula to find the capacity of the weaving section.

where is the average entry and exit width, is the weaving width, is the length of weaving, and is the proportion of weaving traffic to the non-weaving traffic.

four types of movements at a weaving

section,  and  are the non-weaving traffic and are the weaving traffic.

Capacity

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“Determine the radius of a circle in which lengths of two chords ‘a’ and ‘b’ are known and the chords meet at any point on the periphery of the circle” ?..

The equation derived as solution of the problem is given below. It is a quadratic equation in terms of R2.

New Concept to Design Rotary at Road Intersections

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i. Length of Intersecting straights with minimum site distances recommended by I.R.C. are: 50 km/h.,

ii. Minimum Sight distance 15 m along minor roads S.K. Mahajan et al. / Procedia - Social and Behavioral Sciences 96 ( 2013 ) 2791 – 2799 2795

iii. Weaving Distance = 110 m, 145 m, 180 m and 220 m for speeds 50 km/h., 65 km/h., 80 km/h, 100 km/h. respectively. Minimum Weaving length: 30 to 50 m

iv. All traffic islands are constructed 200 mm higher than road level. They are provided with lawn, statue or fountains, painted with colours in contrast with the road pavement.

Design Consideration

{1-(a/b)2} R4-{b2-5a2/4} R2 –(a2b2/2) = 0

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