chapter 3 junction design

CHAPTER 3.0 JUNCTION DESIGN

CC503 – TRAFFIC ENGINEERING

JUNCTION

• A road junction is a

location where

vehicular traffic going in

different directions can

proceed in a controlled

manner designed to

minimize accidents.

• In some cases, vehicles

can change between

different routes or

directions of travel.

INTERSECTION

• An intersection is a road junction where two or more roads either meet or cross at grade (they are at the same level).

• Such a road junction may also be called a crossroads.

FACTORS THAT AFFECT JUNCTION LOAD.

• At grade intersections present a driver with several points of conflict with other vehicles.

• The aims of intersection design are to improve traffic flow and reduce the like hood of accidents.

FACTORS THAT AFFECT JUNCTION LOAD.

• Factors that affect junction load are:-

i. Geometrical design and traffic control devices

ii. Total or length wide

iii. Distribution design

iv. Turning radius

v. Traffic volume and characteristic

TYPE OF JUNCTION AT GRADE JUNCTION

• Junction without signage/control

• Priority Junction

• Priority Junction with routing

• Roundabout with or without traffic light

• Traffic light controlled junction

• Tiered junction

GRADE SEPARATED OR INTERCHANGED

• Trumpet interchange:

• Diamond interchange:

• Clover leaf interchange

Junction without signage/control

Priority Junction

Priority Junction with routing

Roundabout with or without traffic light

Traffic light controlled junction

Tiered junction

DIAGRAM TO DETERMINE INTERSECTION TYPE (ARAHAN TEKNIK JALAN 11/87)

Trumpet Interchange

Diamond interchange

Cloverleaf Interchange

TYPES OF MOVEMENTS AT THE JUNCTION

a. Turning

b. Intersect

a. Turning

b. Intersect/crossing

c. Link/merge

d. Diverges

e. weaves

• Link / merge

• Diverges

• Weave

CONFLICT POINT AND CONFLICT AREA AT AN INTERSECTION

• Conflict points are commonly used to explain the accident potential of a roadway.

• Access management strategies are typically designed to reduce the number and density of conflict points.

• A conflict point is the point at which a highway user crossing, merging with, or diverging from a road or driveway conflicts with another highway user using the same road or driveway


Here, how conflict happen:-

• Vehicles travel from two streams and meets with each other

• Where roads cross at an acute angle or the opposing legs of an intersection are offset, excessive intersection area results

• In general, large areas of uncontrolled pavement invite dangerous vehicle maneuvers and should be eliminated


Total

Junction

Intersect

Conflict

Link

Conflict

Diverge

Conflict

Total

Conflicts

3 3 3 3 9

4 16 8 8 32

5 49 15 15 79

6 124 24 24 172

• Figure 1.0: Conflict at 4 legs-junction and No right turn method to solve conflict

6 link conflicts

8 Intersect conflicts

6 diverge conflicts

SKETCH THE METHOD TO REDUCE CONFLICT AT 4-LEG JUNCTION BELOW:-

1. No right turn

2. One of the road become one way movement

3. Both road become one way movement

4. Change 4 leg cross section become staggered intersection

5. Use channelized

ANSWER OF METHOD TO REDUCE CONFLICT AT JUNCTION

One of the road become one way movement

7 Intersects

2 Diverges

2 Merges

Both road become one way movement

4 Intersects

2 Diverges

2 Merges

Change 4 leg cross section become staggered intersection

6 Intersects

6 Merges

6 Diverges

Use Channelization

• Channelization of traffic through a three-legged intersection

• Channelization of traffic through a four-legged intersection

SELECTION FACTORS OF JUNCTION TYPE:-

i. Volume, composition and speed of traffic in each stream

ii. Hierarchy of roads that intersect with each other

iii. Clarity of vision in different weather condition

iv. Type and rate of control of traffic entering

v. Area and the cost of land use

vi. Total conflict at intersection

vii. Construction and maintenance cost

viii. Accident data (to be used for intersection’s planning)

TYPE OF SIGHT DISTANCE AT JUNCTION

A. Sight triangle

B. Sight distance for approach

• No Stop or Signal Control at Intersection

• Signalized intersection

• Stop controlled intersection

C. Sight distance for departure


Figure 2.1: Intersection Sight Triangle


Figure 2.2: No Stop or Signal Control at Intersection


Figure 2.3: Stop Control on Minor Road

SIGNALIZED INTERSECTION

• The sight distance is the sum of a distance travelled during the total reaction time which is the interval between the instant that the driver recognizes the traffic signals of the intersection ahead and the instant that the driver actually applies the brakes, and a distance to stop the vehicle at the stop line with applying brake.

S = (Vt/3.6) + [ (1/2)*(V/3.6)2 ] Where, t = 10 sec. (rural), t = 6 sec (urban), = 0.2 x g = 0.2 x 9.8 = 1.96 m/sec2

STOP CONTROLLED INTERSECTION • In this case, time for decision making as in

signalized intersection is not necessary because every driver must stop.

S = (Vt/3.6) + [ (1/2)*(V/3.6)2 ] Where, V= speed (km/h), t = 2 sec,

= 1.96 m/sec2

SIGHT DISTANCE FOR DEPARTURE

• At an intersection where traffic is controlled by STOP signs on the minor road it is necessary for the driver of a stopped vehicle to see enough of the major road to be able to cross before a vehicle on the major road reaches the intersection

d = 0.28V (J + ta) Where, d = minimum sight distance along the major road from the intersection,

meters. V = design speed of major road, km/hr. J = sum of perception time and the time required to shift to first gear or

actuate an automatic shift. (Assume J = 2 sec, Urban or sub-urban = 1 ½ sec to 1 sec))

ta = time required to accelerate and traverse the distance S to clear the major road, seconds.

SIGHT DISTANCE FOR DEPARTURE

S = D + W+ L

Where,

S = Distance for vehicle must travel to cross the major road

D = distance from near edge of pavement of front of stopped vehicle

W= width of pavement along path of crossing vehicle.

L = overall length of vehicle

Example

• A vehicle travel before hit to collision point has 2 sec of reaction time to stop, deceleration of vehicle is 1.96 m/s2 and the speed of vehicle is 100 km/hr. Determine the sight distance for a vehicle before it hit the collision point.

Solution S = (Vt/3.6) + [ (1/2)*(V/3.6)2 ] = (100 x 2)/3.6 + [(1/2 x 1.96) x (100/3.6)2] = 252.4 m

TRAFFIC LIGHT DESIGN AND PHASES

• Traffic lights which may also be known as stoplights, traffic lamps, traffic signals, stop-and-go lights, robots or semaphore, are signaling devices positioned at road intersections, pedestrian crossings and other locations to control competing flows of traffic.

ADVANTAGES & DISADVANTAGES TRAFFIC LIGHT

ADVANTAGES

i. Provide orderly movement of traffic

ii. Minimize the number of conflicting movements

iii. Increase capacity

iv. Interrupt heavy traffic to allow other traffic to enter or cross.

v. Coordinated continuous movement of traffic at a desired speed

vi. Promote driver confidence

DISADVANTAGES

i. Increase total intersection delay especially during off peak periods

ii. Probable increase in certain types of accidents

iii. Interrupt the progressive flow of traffic on a route cause delay & stopping

iv. When improperly located causes unnecessary delay

v. When improperly timed, causes excessive delay, increasing driver irritation

REQUIREMENT OF TRAFFIC INSTRALLATION

• THREE (3) warrant should be considered before installing traffic signal are:-

Warrant 1: Vehicular Operations

Warrant 2: Pedestrian Safety (S)

Warrant 3: Accident Experience

PHASING ELEMENTS

• Definition:

A signal phase is a part of the cycle length allocated to a traffic movement receiving the right of way simultaneously during one or more intervals

• A traffic movement is a single vehicular movement, a single pedestrian movement, or a combination of vehicular and pedestrian movements

• Cycle length = the sum of all traffic phases

TYPE OF PHASING

N-S: Red N-S: Green (all movements)

E-W: Green (all movements) E-W: Red

N-S:Red N-S: Red N-S: Green (all movements) E-W:Green(TM) E-W: Red (TM) E-W: Red : Red (RT) : Green (RT)

TRAFFIC LIGHT TERM

1. LOST TIME, L

It indicates the time during which the intersection is not effectively utilized for any movement.

2. INTERGREEN PERIOD OR INTERVAL, I

Intergreen period is defined as the period between one phase losing right of way and the next phase gaining right of way, or the time between the end of green on one phase and the start of green on the next.

TRAFFIC LIGHT TERM

3. ACTUAL GREEN TIME, GT

The time within a cycle in which an approach has the green indication (express in second and symbol GT).

4. EFFECTIVE GREEN, g

Effective green defined as the length of time that would be required to get a given discharge rate over the stop-line if the flow commenced and finished simultaneously and instantaneously on the change of colour as displayed on the signal head

Formula: g = Co – L

TRAFFIC LIGHT TERM

5. RED TIME, RT

The time within a cycle in which an approach has the red indication (express in second and symbol RT).

6. AMBER OR YELLOW TIME, a

The time within a cycle in which an approach has the yellow indication (express in second and symbol a)

TRAFFIC LIGHT TERM

7. CYCLE LENGTH, C

One complete sequence ( for all approaches of signal indications [green, yellow, red] ).

Cycle time maximum (Cm) is 120 second (2 minutes) is considered as good practice. Normally, the cycle time will lie within the range of 30 – 90 s.

chapter 3 junction design

Engineering