Intersection Ass

7/28/2019 Intersection Ass.

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2012-2013

Bermido, Marie Fe F.

BSCE-04

Engr. Henry P. Turalde

Instructor

ENGINEERING

TRANSPORTATION INTERSECTION


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Intersection is a road junction where two or more roads either meet or cross at

grade (they are at the same level). An intersection may be 3-way aT junctionorfork,

4-way - a crossroads, or 5-way or more. It may often be controlled by traffic lights, and

may be a roundabout (traffic circle in America).

Intersections are classified as 3-way, 4-way, 5-way, 6-way, etc. depending on the

number of road segments (arms) that come together at the intersection.

3-way intersectio n- A junction between three road segments (arms) is a T

junction (two arms form one road) or a Y junction.

4-way intersection susually involve a crossing over of two streets or roads. In

areas where there are blocks and in some other cases, the crossing streets or roads

are perpendicularto each other. However, two roads may cross at a different angle.

In a few cases, the junction of two road segments may be offset from each when

reaching an intersection, even though both ends may be considered the same

street.

5-way intersection sare less common but still exist, especially in urban areas with

non-rectangular blocks.

6-way intersection susually involve a crossing of three streets at one junction; for

example, a crossing of two perpendicular streets and a diagonal street is a rather

common type of 6-way intersection.

Seven or more approaches to a single intersection, such as at Seven Dials,

London, are rare.

Intersection along the Veterans Memorial Parkway, an at-grade limited-access

road inLondon, Ontario.
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Another way of

Uncontrol led intersect ions , without signs or signals (or sometimes with a

warning sign). Priority (right-of-way) rules may vary by country: on a 4-way

intersection traffic from the right often has priority; on a 3-way intersection either

traffic from the right has priority again, or traffic on the continuing road. For trafficcoming from the same or opposite direction, that which goes straight has priority

over that which turns off.

Yield-control led intersect ionsmay or may not have specific "YIELD"

signs (known as "GIVE WAY" signs in some countries).

Stop-control led intersect ionshave one or more "STOP" signs. Two-way stops

are common, while some countries also employ four-way stops.

Signal-controlled intersections depend on traffic signals, usually electric, which

indicate which traffic is allowed to proceed at any particular time.

Atraff ic circleis a type of intersection at which traffic streams are directed

around a circle. Types of traffic circles include roundabouts, 'mini-roundabouts','rotaries', "STOP"-controlled circles, and signal-controlled circles. Some people

consider roundabouts to be a distinct type of intersection from traffic circles (with

the distinction based on certain differences in size and engineering).

A box junct ioncan be added to an intersection, generally prohibiting entry to the

intersection unless the exit is clear.

Some intersections employ indirect left turns to increase capacity and reduce

delays. The Michigan left combines a right turn and a U-turn. Jughandlelefts

diverge to the right, then curve to the left, converting a left turn to a crossing

maneuver. These techniques are generally used in conjunction with signal-

controlled intersections, although they may also be used at stop-controlled

intersections.

Turns

At same-grade intersections, turns are usually allowed, but often regulated to avoid

interference or collision with other traffic. If the crossing street is a one-way street, or

other cases of flow limitation, certain turns may be not allowed or may be limited by

regulatory signs or signals.

In the absence of lane markings indicating otherwise, left turns should be made from

the leftmost lane and right turns from the rightmost lane to avoid collision or blocking

of traffic going straight.

Turn lanes

At intersections with large proportions of turning traffic, turn lanes (also known as

turn bays)[1]may be provided where the street/road approaches the intersection. For

example in the intersection shown in the following diagram, there are left turn lanes

in the east-west street for traffic approaching the intersection in the eastbound and

westbound directions.
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Turn lanes may be provided to allow vehicles to turn across oncoming traffic (i.e., a

left turn in right-side driving countries, or a right turn in left-side driving countries), or

to turn off the road without crossing traffic (i.e., a right turn in right-side driving

countries, or a left turn in left-side driving countries). Absence of a turn lane does not

normally indicate a prohibition of turns in that direction. Instead, traffic control signs

are used to prohibit turns.

Turn lanes may be provided to increase the capacity of an intersection, or to

improve safety. Turn lanes can have a dramatic effect on the safety of a junction. In

rural areas, crash frequency can be reduced by up to 48% if left turn lanes are

provided on both main-road approaches at stop-controlled intersections. At

signalized intersections, crashes can be reduced by 33%. Results will be slightly

lower in urban areas.[2]

Turn lanes are marked with an arrow bending into the direction of the turn which is

to be made from that lane. Multi-headed arrows indicate that vehicle drivers may

travel in any one of directions shown.

Traffic signals facing vehicles in turn lanes often have arrow-shaped indications.

Green arrows indicate protected turn phases, when vehicles may turn unhindered by

oncoming traffic. Red arrows may be displayed to prohibit turns in that direction.

Red arrows may be displayed along with a circular green indication to show that

turns in the direction of the arrow are prohibited, but other movements are allowed.

In some jurisdictions, a red arrow prohibits a turn on red,[3]

while in others, it does

not.

Disadvantages to turn lanes include increased pavement area, with associated

increases in construction and maintenance costs, as well as increased amounts

ofstormwaterrunoff. They also increase the distance that pedestrians crossing the

street are exposed to vehicle traffic.
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Diagram of an example intersection of two-way streets as seen from above (traffic

flows on the right side of the road).

The East-West Street has left turn lanes from both directions, but the North-South

street does not have left turn lanes at this intersection. The East-West street traffic

lights also have green left turn arrows to show when unhindered left turns can bemade. Some possible markings for crosswalks are shown as examples.

A fork (literally "fork in the road") is a type of intersection. When a road splits, the

main road steers to the left or right, depending of what side you drive on, and the

smaller road heads straight. It is common for 2 lane roads. Heading toward the main

road, the traveler must turn left or right. If a road has a curb that sticks out, it is not

classified as a fork.

In some places, wider white stop lines (see preceding diagram) indicate where

vehicles should stop at an intersection when there is a stop sign or a red light in a

traffic signal facing them. Some intersections have pedestrian

crosswalks designated on the street pavement. Some possible markings for

crosswalks are shown as examples. Note that the stop line is positioned to not allow

stopped vehicles to block the crosswalk.
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(AT-GRADE INTERSECTION)- intersections in the same

plane. This intersection is the intersection of the mostcommon / prevalent, both in urban areas or in aerah out oftown. This intersection is the meeting of two roads (or more)located in the same plane.

(GRADE SEPARATED INTERSECTION)- intersectionsin the area that is not the same.This intersection is not apiece of a meeting between the road with high traffic road

with high traffic / other low. Plot was made with a view toreducing / eliminating conflict points, thus the high roadberlalulintas still have a high capacity
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Type Of Intersection according to theOperation.

Unchannelized Intersection-intersections without canalization (Intersection without

canalization used on a minor road traffic of relatively

low volume. However, in urban areas usually still be

found even though the intersection without

canalization lalulintasny have high volume. In general,

to cope with the traffic signal is used masalan or

STOP signs.

Channelized Intersection-intersections with canalization. The goal is to reduce

points of conflict, by separating traffic movements, soas not to interfere with each other.

Separation can be done by creating a traffic island

(traffic island), safety beam / column separator, and

so on.
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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. The

presence of traffic islands, markings etc. forces the driver to reduce the speed

and becomes more cautious while maneuvering the intersection. A

channelizing island also serves as a refuge for pedestrians and makespedestrian crossing safer. Channelization of traffic through a three-legged

intersection (refer figure 1) and a four-legged intersection (refer figure 2) is

shown in the figure.

Figure 1: Channelization of traffic through a three-legged intersection
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Conflicts at an intersectionConflicts at an intersection are different for different types of intersection.

Consider a typical four-legged intersection as shown in figure. The number of

conflicts for competing through movements are 4, while competing right turn

and through movements are 8. The conflicts between right turn traffics are 4,

and between left turn and merging traffic is 4. The conflicts created by

pedestrians will be 8 taking into account all the four approaches. Diverging

traffic also produces about 4 conflicts. Therefore, a typical four legged

intersection has about 32 different types of conflicts. This is shown in figure 1.

Conflicts at intersection

The essence of the intersection control is to resolve these conflicts at theintersection for the safe and efficient movement of both vehicular traffic andpedestrians. Two methods of intersection controls are there: time sharing andspace sharing. The type of intersection control that has to be adopteddepends on the traffic volume, road geometry, cost involved, importance ofthe road etc.

Levels of intersection controlThe control of an intersection can be exercised at different levels. They can be

either passive control, semi control, or active control. In passive control, there

is no explicit control on the driver . In semi control, some amount of control on

the driver is there from the traffic agency. Active control means the movement

of the traffic is fully controlled by the traffic agency and the drivers cannot

simply maneuver the intersection according to his choice.

Passive control

When the volume of traffic is less, no explicit control is required. Here the road

users are required to obey the basic rules of the road. Passive control like

traffic signs, road markings etc. are used to complement the intersection

control. Some of the intersection control that are classified under passive

control are as follows:
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1. No control If the traffic coming to an intersection is low, then byapplying the basic rules of the road like driver on the left side of the roadmust yield and that through movements will have priority than turningmovements. The driver is expected to obey these basic rules of theroad.

2. Traffic signs: With the help of warning signs, guide signs etc. it is ableto provide some level of control at an intersection. Give way control,two-way stop control, and all-way stop control are some examples. TheGIVE WAY control requires the driver in the minor road to slow down toa minimum speed and allow the vehicle on the major road to proceed.Two way stop control requires the vehicle drivers on the minor streetsshould see that the conflicts are avoided. Finally an all-way stop controlis usually used when it is difficult to differentiate between the major andminor roads in an intersection. In such a case, STOP sign is placed onall the approaches to the intersection and the driver on all theapproaches are required to stop the vehicle. The vehicle at the rightside will get priority over the left approach. The traffic control at 'at-grade' intersection may be uncontrolled in cases of low traffic. Here theroad users are required to obey the basic rules of the road. Passivecontrol like traffic signs, road markings etc. are used to complement theintersection control.

3. Traffic signs plus marking: In addition to the traffic signs, roadmarkings also complement the traffic control at intersections. Some ofthe examples include stop line marking, yield lines, arrow marking etc.

Semi control

In semi control or partial control, the drivers are gently guided to avoid

conflicts. Channelization and traffic rotaries are two examples of this.

1. Channelization: The traffic is separated to flow through definite pathsby raising a portion of the road in the middle usually called as islandsdistinguished by road markings. The conflicts in traffic movements arereduced to a great extent in such a case. In channelized intersections,as the name suggests, the traffic is directed to flow through different

channels and this physical separation is made possible with the help ofsome barriers in the road like traffic islands, road markings etc.

2. Traffic rotaries: It is a form of intersection control in which the traffic ismade to flow along one direction around a traffic island. The essentialprinciple of this control is to convert all the severe conflicts like throughand right turn conflicts into milder conflicts like merging, weaving anddiverging. It is a form of at-grade' intersection laid out for the


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movement of traffic such that no through conflicts are there. Free-leftturn is permitted where as through traffic and right-turn traffic is forcedto move around the central island in a clock-wise direction in an orderlymanner. Merging, weaving and diverging operations reduces theconflicting movements at the rotary.

Active control

Active control implies that the road user will be forced to follow the path

suggested by the traffic control agencies. He cannot maneuver according to

his wish. Traffic signals and grade separated intersections come under this

classification.

1. Traffic signals: Control using traffic signal is based on time sharingapproach. At a given time, with the help of appropriate signals, certain

traffic movements are restricted where as certain other movements arepermitted to pass through the intersection. Two or more phases may beprovided depending upon the traffic conditions of the intersection. Whenthe vehicles traversing the intersection is very large, then the control isdone with the help of signals. The phases provided for the signal maybe two or more. If more than two phases are provided, then it is calledmultiphase signal.

The signals can operate in several modes. Most common are fixed timesignals and vehicle actuated signals. In fixed time signals, the cycle

time, phases and interval of each signal is fixed. Each cycle of thesignal will be exactly like another. But they cannot cater to the needs ofthe fluctuating traffic. On the other hand, vehicle actuated signals canrespond to dynamic traffic situations. Vehicle detectors will be placed onthe streets approaching the intersection and the detector will sense thepresence of the vehicle and pass the information to a controller. Thecontroller then sets the cycle time and adjusts the phase lengthsaccording to the prevailing traffic conditions.

2. Grade separated intersections: The intersections are of two types.

They are at-grade intersections and grade-separated intersections. Inat-grade intersections, all roadways join or cross at the same verticallevel. Grade separated intersections allows the traffic to cross atdifferent vertical levels. Sometimes the topography itself may be helpfulin constructing such intersections. Otherwise, the initial constructioncost required will be very high. Therefore, they are usually constructedon high speed facilities like expressways, freeways etc. These type of


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intersection increases the road capacity because vehicles can flow withhigh speed and accident potential is also reduced due to verticalseparation of traffic.

Grade separated intersectionsAs we discussed earlier, grade-separated intersections are provided toseparate the traffic in the vertical grade. But the traffic need not be thosepertaining to road only. When a railway line crosses a road, then also gradeseparators are used. Different types of grade-separators are flyovers andinterchange. Flyovers itself are subdivided into overpass and underpass.When two roads cross at a point, if the road having major traffic is elevated toa higher grade for further movement of traffic, then such structures are calledoverpass. Otherwise, if the major road is depressed to a lower level to cross

another by means of an under bridge or tunnel, it is called under-pass.

Interchange is a system where traffic between two or more roadways flows atdifferent levels in the grade separated junctions. Common types ofinterchange include trumpet interchange, diamond interchange , andcloverleaf interchange.

1. Trumpet interchange: Trumpet interchange is a popular form of threeleg interchange. If one of the legs of the interchange meets a highway atsome angle but does not cross it, then the interchange is called trumpet

interchange. A typical layout of trumpet interchange is shown in figure 1.

Figure 1: Trumpet interchange
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Diamond interchange: Diamond interchange is a popular form of four-leginterchange found in the urban locations where major and minor roadscrosses. The important feature of this interchange is that it can be designedeven if the major road is relatively narrow. A typical layout of diamondinterchange is shown in figure 1.

Figure 1: Diamond interchange

Clover leaf interchange: It is also a four leg interchange and is used whentwo highways of high volume and speed intersect each other withconsiderable turning movements. The main advantage of cloverleafintersection is that it provides complete separation of traffic. In addition, highspeed at intersections can be achieved. However, the disadvantage is thatlarge area of land is required. Therefore, cloverleaf interchanges are provided

mainly in rural areas. A typical layout of this type of interchange is shown infigure 2.
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Figure 1: Channelization of traffic through a three-legged intersection

Figure 2: Channelization of traffic through a four-legged intersection

SummaryTraffic intersections are problem spots on any highway, which contribute to a

large share of accidents. For safe operation, these locations should be kept

under some level of control depending upon the traffic quantity and behavior.

Based on this, intersections and interchanges are constructed, the differenttypes of which were discussed in the chapter.

Documents

Intersection Ass