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INTERSECTION

An intersection is the area where two or more streets join or cross

at-grade. The intersection includes the areas needed for all modes

of travel: pedestrian, bicycle, motor vehicle, and transit (Ben

Thagesen. Thus, the intersection includes not only the pavement

area, but typically the adjacent sidewal!s and pedestrian curb cut

ramps. They can be any of three types namely:

"ignalised #ntersections

"top-$ontrolled intersections

%oundabouts

#ntersections are a !ey feature of street design in four respects:

Focus of activity - intersections are usually very congested as

they are where two or more streets meet, which ma!es it a

concentration of different travel destinations.

Conflicting movements - &edestrian crossings and motor vehicle

and bicycle turning and crossing movements are typically

concentrated at intersections.

Traffic control - At intersections, movement of users is assigned

by traffic control devices such as yield signs, stop signs, and traffic

signals. Traffic control often results in delay to users travelling along

the intersecting roadways, but helps to organi'e traffic and decrease

the potential for conflict.

Capacity - #n many cases, traffic control at intersections limits the

capacity of the intersecting roadways, defined as the number of

users that can be accommodated within a given time period.

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STREET INTERSECTION USERS

This comprises of all roadway users as intersections are very

important to every commuter. They are therefore affected in the

following ways:

Pedestrians ey elements affecting intersection performance for

pedestrians are:

). Amount of right-of-way provided for the pedestrian including

both sidewal! and crosswal! width, accuracy of slopes and

cross slopes on curb cut ramps and wal!ways, audible and*or

tactile cues for people with limited sight, and absence of

obstacles in accessible path+

. $rossing distance and resulting duration of eposure to

conflicts with motor vehicle and bicycle traffic+. /olume of conflicting traffic+ and

0. "peed and visibility of approaching traffic.

!icyclists ey elements affecting intersection performance for

bicycles are:

). 1egree to which pavement is shared or used eclusively by

bicycles+

. %elationship between turning and through movements for

motor vehicles and bicycles+

. Traffic control for bicycles+

0. 1ifferential in speed between motor vehicle and bicycle traffic+

and

2. /isibility of the bicyclist.

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"otor ve#icles ey elements affecting intersection performance

for motor vehicles are:

). Type of traffic control+

. /ehicular capacity of the intersection, determined primarily

from the number of lanes and traffic control (although there

are other factors+

. Ability to ma!e turning movements+

0. /isibility of approaching and crossing pedestrians and bicycles+

and

2. "peed and visibility of approaching and crossing motor

vehicles.

Transit 3hen transit operations involve buses, they share the

same !ey characteristics as vehicles. #n addition, transit operations

may involve a transit stop at an intersection area, and influence

pedestrian, bicycle, and motor vehicle flow and safety.

CONCEPT OF $ESI%N OF INTERSECTION

The functional area on the approach to an intersection or driveway

consists of three

Basic elements:

). &erception reaction distance+ the total time re4uired for a

driver to react to a stimulus.

. 5anoeuvre distance+ The manoeuvre distance includes the

length needed for both bra!ing and lane changing when there

is a left or right turning lane. #n the absence of turn lanes, the

manoeuvre distance is the distance to bra!e to a comfortable

stop.

. 6ueue storage distance.

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INTERSECTION TER"INO&O%'

The major street is typically the intersecting street with greater

traffic volume, larger cross-section, and higher functional class.

The minor street is the intersecting street li!ely to have less traffic

volume, smaller cross-section and lower functional classification

than the major street.

The term intersection encompasses not only the area of pavement

jointly used by the intersecting streets, but also those segments of

the intersecting streets affected by the design. Thus, those

segments of streets adjacent to the intersection for which the cross-

section or grade has been modified from its typical design are

considered part of the intersection.

The angle of intersection is formed by the intersecting streets7

centrelines. 3here the angle of intersection departs significantly(more than approimately 8 degrees from right angles, the

intersection is referred to as a skewed intersection(

Intersection legs are those segments of roadway connecting to

the intersection. The leg used by traffic approaching the intersection

is the approach leg, and that used by traffic leaving is the

departure leg(

Sidewalks) crosswalks and pedestrian curb cut ramps are

considered to be within the intersection. T#e pavement edge

corner is the curve connecting the edges of pavement of the

intersecting streets.

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Channelizing and divisional islands may be added to an

intersection to help delineate the area in which vehicles can

operate, and to separate conflicting movements. #slands can also

provide for pedestrian refuge.

* turning roadway is a short segment of roadway for a right turn,

delineated by channeli'ing islands. Turning roadways are used

where right-turn volumes are very high, or where s!ewed

intersections would otherwise create a very large pavement area.

SI%N*&IS*TION

The design of an intersection cannot be complete without proper

signalisation, in terms of traffic devices, such as all signs, mar!ings

and signals.

"ignalisation design includes both physical installation and signaloperating plan. #t is important that both be compatible with each

other and with the geometric configuration. To achieve this proper

use of traffic-control devices are important.

TR*FFIC - CONTRO& $E+ICES

Traffic control devices include all signs, mar!ings, and signals placed

on or adjacent to a street or highway by public agencies in order to

regulate, warn, or guide traffic.

Traffic control devices assign right of way, to both motori'ed and

non-motori'ed traffic and include traffic signals, pavement

mar!ings, "T9& signs, #;

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Before the installation of traffic signals at any intersection the

following data must be thoroughly determined and eamined.

). To critically eamine the eisting traffic flow, characteristics of

vehicles,traffic capacity and safety around the intersections.

. To conduct traffic volume studies so as to determine certain

volume characteristics which include the Average 1aily Traffic

(A1T, the &ea! =our /olume (&=/, vehicle $lassification (/$,

the /ehicle ilometers of Travel (/T and the 9rigin

1estination "tudy (91".

. To analy'e the sight distances at the intersections for possible

installation of stop or yield signs.

0. To ascertain the necessity and basis for the installation of

traffic signals.

2. To carry out the redesign of the intersections to guarantee the

easy movement of commuters and pedestrians plying the

route.

>. To design the signal settings for intersections so that driverswill not encounter problems at the intersection.

T#e ,ey decisions affecting t#e signal design include

). . "election of controller type+

?. "election of the phasing plan+@. Assignment of movements to timing functions+

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. 1evelopment of the timing plans for arterial coordination+

)8. 1etermination of detector locations+

)).

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The intersection under study is a signali'ed intersection with some

traffic streams

allowed to have simultaneous right-of-way while other streams are

stopped.

The major roadway which is the intersecting roadway with the

greatest traffic volume and this was observed to be =erbert-

5acaulay %oad with a volume of 0 vehicles passing the

intersection under study. 9n the other hand the minor street, which

is the intersecting street with the least traffic volume was found to

be 5urtala 5ohammed way characteri'ed by only vehicles passing

the intersection during the study duration.

/ehicles passing the intersection and approaching 5urtala

5ohammed 3ay were allowed to move in only one direction i.e.

towards 5urtala 5ohammed 3ay, thus resulting in the lowest traffic

volume through the intersection. #n other words, 5urtala

5ohammed 3ay is strictly a departure leg as traffic only

approaches, but does not leave the roadway segment. 3e alsoobserved that turning roadways were added to the intersection to

help delineate the area in which vehicles can operate and to

separate conflicting movements.

1uring our study, we observed that there were three channels: one

lin!ing the Agege 5otor %oad way to the 5urtala 5ohammed 3ay,

one lin!ing the =erbert 5acaulay %oad to #!orodu %oad, and the

other lin!ing #!orodu %oad to Agege 5otor %oad.

/ehicle classifications were also studied along the intersection

segment of the highway and the results showed that private owned

vehicles mainly plied the roadway lin!ing =erbert 5acaulay %oad

with Agege 5otor %oad. Buses were also observed to pass the

intersecting roadway (channeli'ing island connecting =erbert

.5acaulay 3ay with #!orodu %oad.