NNAMDI AZIKIWE UNIVERSITY, AWKA

FACULTY OF ENGINEERING,

DEPARTMENT OF CIVIL ENGINEERING.

CVE 471: Highway and Transportation Engineering I

A PROJECT REPORT

ON

DESIGN OF A ROTARY AND TRAFFIC SIGNALLING SCHEME.

AT

ESTHER OBUAKOR AND CPHA AVENUE INTERSECTION,

AWKA, ANAMBRA STATE.

17TH JANUARY, 2025

 GROUP 3

1 UDEMBA KOSISOCHUKWU MAUREEN 2021224028

2 NSOFOR CHUKWUEBUKA HENRY 2021224029

3 NNACHOR VICTORY CHINECHEREM 2021224030

4 EZE ERNEST CHINONSO 2021224031

5 ANUNIKE CHUKWUDUM JOHN 2021224032

6 OPIAH DORIS SOMTOCHUKWU 2021224034

7 UMEADI KENECHUKWU EMMANUEL 2021224035

8 NNACHEBE JOHN CHUKWUDILE 2021224037

9 NGODDY CHUKWUNONSO MICHAEL 2021224039

10 UWALAKA PAUL IKENNA 2021224040

Introduction to Rotary Intersection and Traffic Signal Controlled Intersection.

 Rotary Intersection
Rotary intersections, commonly known as roundabouts, are designed to enhance traffic flow and safety
by allowing vehicles to move in one direction around a central island. This design minimizes severe
conflicts, such as those between vehicles going straight and those making right turns, by converting
them into less dangerous merging and diverging situations.
In a roundabout, vehicles approaching the intersection yield to those already circulating, which helps to
maintain a continuous flow of traffic. The gentle curvature of the road encourages drivers to navigate
the intersection in a clockwise manner, reducing the likelihood of high-speed.

The advantages of rotary intersections include:

1. Traffic flow is regulated in one direction, which eliminates severe conflicts between crossing
movements.
2. Vehicles entering the rotary are naturally slowed down, allowing them to continue moving without
needing to stop, unlike signalized intersections.
3. The lower speeds and reduced conflicts lead to fewer accidents and less severe collisions.
4. Rotaries are self-governing and require minimal control from police or traffic signals.
5. They work well for moderate traffic, especially in areas with irregular geometry or intersections with
more than three or four approaches.

However, there are some limitations to consider:

1. All vehicles must slow down to negotiate the intersection, leading to higher cumulative delays
compared to channelized intersections.
2. Even with low traffic, vehicles are still required to reduce speed.
3. Rotaries need a large area of relatively flat land, making them costly to implement in urban areas.
4. Since vehicles do not typically stop and can exit at higher speeds, they are not ideal in locations with
high pedestrian traffic.

The design of rotary intersections aims to enhance safety, improve traffic flow, and reduce congestion.
Key features of rotary intersections include:
i. Yield Control: Vehicles entering the roundabout must yield to those already circulating, which
minimizes the potential for high-speed collisions.
ii. Deflection: The circular path encourages slower speeds, reducing the severity of accidents.
iii. Multi-Lane Options: Depending on traffic volumes, roundabouts can be designed with multiple
lanes, allowing for efficient movement of vehicles.

The design process involves careful consideration of various factors, including traffic volumes, vehicle
types, pedestrian access, and landscaping. Properly designed rotary intersections can significantly
improve safety and efficiency compared to traditional signalized intersections.

 Traffic Signal Controlled Intersections

Signal-controlled intersections regulate traffic flow using traffic lights. They can effectively manage
heavy traffic volumes and ensure that all directions receive equal opportunity to proceed.
Signal-controlled intersections utilize traffic lights to manage vehicle and pedestrian movements. They
offer several benefits:
1. Structured Traffic Flow: Signals provide a clear framework for managing complex traffic
patterns, especially in high-volume areas.
2. Pedestrian Safety: Signals can incorporate dedicated phases for pedestrian crossings, enhancing
safety for foot traffic.
3. Flexibility: Traffic signal timing can be adjusted based on real-time conditions, using
technologies like adaptive signal control.

Despite these advantages, signal-controlled intersections can lead to delays due to stop-and-go traffic
especially during off-peak hours or when vehicles are waiting for the light to change, increased
emissions, and potential for accidents like rear-end collisions during signal changes.
Signalized intersections are crucial for managing traffic flow, and understanding their operation is key
for effective traffic modeling. The aspects of traffic signals in different regions, such as the U.K.,
ensure that vehicles know when to stop and go.
Effective green time, which is the period when traffic can move, is influenced by factors like reaction
time, meaning it starts and ends later than the actual green signal. The phases of a traffic signal
determine the timing for each movement through the intersection, and it's important that the total
duration of these phases is significantly less than the total signal cycle. This difference accounts for lost
time during signal changes and pedestrian phases.

Traffic signalling schemes are essential for managing the flow of vehicles and pedestrians at
intersections.
Effective traffic signal design involves several critical components:
i. Signal Phasing: This determines the sequence in which different movements (e.g., left turns,
straight-through traffic, and pedestrian crossings) receive green lights.
ii. Timing Plans: The duration of each signal phase must be carefully calculated based on traffic
volumes, ensuring that green lights are long enough to accommodate vehicles while minimizing
delays.
iii. Detection Systems: Advanced technologies, such as inductive loops and cameras, can be
employed to detect vehicle presence and adjust signal timing dynamically, enhancing efficiency.
iv. Pedestrian Considerations: Signals must also accommodate pedestrian safety, with features like
countdown timers and audible signals to assist visually impaired individuals.

The design of traffic signalling schemes is a complex process that requires an understanding of traffic
flow principles, human behavior, and engineering practices. When implemented effectively, these
schemes contribute to safer and more efficient transportation networks.
 FEATURES OF THE ASSIGNED INTERSECTION
1. Meridian Divider:

A meridian divider, often called a median divider, is a physical barrier or strip that separates lanes of
traffic moving in opposite directions on a road. It can be made of concrete, grass, or any other material.

The main purpose of a median divider is to enhance road safety by preventing head-on collisions. It
also helps in controlling access to the roadway, as it can restrict where vehicles can enter or exit.

Its benefits includes:

a. By reducing the likelihood of head-on collisions, it significantly improve safety for drivers and
passengers.
b. They can help manage traffic flow by organizing how vehicles enter and exit the roadway.
c. Medians can provide safe crossing points for pedestrians.

2. A T junction:

It is also known as a T intersection, it is a type of road junction where one road meets another road at a
right angle, forming the shape of the letter "T." In this configuration, one road (the stem of the T)
continues straight while the other road (the top of the T) intersects it, leading to three possible
directions for vehicles: straight, left, or right.

3. Traffic Control:

Absence of traffic signals; The intersection operates without traffic signals, relying on standard right-
of-way rules to manage the flow of vehicles. Drivers are expected to yield or give way to traffic as per
the rules of the road, with the major road (Esther Obuakor Road) having priority over CPHA Avenue.
This system works well during the day, but it may require careful attention from drivers at peak hours.

4. Drainage Systems:

The intersection is equipped with well-designed drainage systems to manage rainwater runoff and
prevent flooding. These systems include culverts and open drains along the sides of the roads, ensuring
that water is channeled away from the road surface effectively. This helps to maintain the integrity of
the roads, reducing the chances of erosion and damage to the pavement. In addition, the drainage setup
minimizes the risk of water pooling, which could lead to accidents or discomfort for pedestrians.

5. Surrounding Environment:

The area surrounding the Esther Obuakor Road and CPHA Avenue intersection is a mix of residential
and commercial spaces, making it a hub for both local living and business activity. Nearby residential
buildings cater to the growing population of Awka, while small businesses and shops are dotted along
the roads, contributing to the local economy. This urban environment increases foot traffic, making
pedestrian crossings at the intersection more important. There is a continuous flow of people,
especially during peak hours, which underscores the need for the well-maintained pedestrian walkways
and street lighting to ensure safety.
 Junction Assessment Report

Location: Esther Obuakor Road and CPHA Avenue intersection.

Time Frame: 8:36 AM - 9:36 AM (1 hour)
Vehicle Count: 758 vehicles

TABLE SHOWING THE TRAFFIC FLOW AT ESTHER OBUAKOR ROAD AND CPHA AVENUE
ROAD INTERSECTION OBSERVED ON JANUARY 17, 2025.

APPROACH LEFT TURN STRAIGHT AHEAD RIGHT TURN

Bus Cars Tricycle Heavy Bus Cars Tricycle Heavy Bus Cars Tricycle Heav

North - - - - 18 69 118 3 4 28 61 0

South 3 34 28 0 19 63 117 5 - - - -

West 3 26 67 1 - - - - 6 50 27 0

SUMMARY OF THE PCU FACTORS FOR ROTARY

Vehicles Roundabout/rotary
Bus 2.8
Heavy 2.8
Cars 1.0
Tricycle 0.75

PASSENGER CAR UNIT FLOW

APPROACH Left turn Straight Right turn
North - 217PCU 85PCU
South 64PCU 218PCU -
West 88PCU - 88PCU
 ROTARY INTERSECTION DESIGN

i. The design speed = 30KPH

ii. The radius at entry = 25m
iii. The radius at exit = 40m
iv. The radius of the central island = 35m
v. The weaving length = 70m

Weaving Proportion (P)

a= 88
b= 217
c= 88
a+ b
P=
a+b+ c
 88+ 217
P= = 0.78
88+217+ 88
P= 0.78
(0.4 < P < 1.0)
P IS OK
Entry/ Exit Width (e)
e 1+e 2
e=
2
e1= 10m, e2= 10m
10+10
e=
2
e= 10
Width of Weaving Section (w)
e 1+e 2
W= +3.5(safety factor)
2
W=10+3.5 ≈14m
w = 14.0m OK
Length of weaving section
Let L=70.0m
10
e/w= =0.71 ( 0.4< e/w < 1.0) ………………OK
14
14
w/L = =0.2 ( 0.13< w/L < 0.4)……………. OK
70
USING WARDROPE’S EQUATION
e P
280 w(1+ )(1 − )
w 3
Wp=
w
(1+ )
L
10 0.78
280× 14(1+ )×(1 − )
14 3
Wp =
14
1+
70
Wp= 4134 Pcu/hr.
a+b+c= 393
Wp= 4134> (a+b+c=393)---------- OK
 DESIGN OF TRAFFIC CONTROL SYSTEM
Note: All the traffic legs are assumed having zero gradient.

SUMMARY OF THE PCU FACTORS FOR TRAFFIC SIGNAL SCHEME

Vehicles Traffic Signals
Bus 1.75
Heavy 2.25
Cars 1.0
Tricycle 0.33

PASSENGER CAR UNIT FLOW

APPROACH Left turn Straight Right turn
North - 147PCU 56PCU
South 49PCU 147PCU -
West 56PCU - 70PCU

Leg A Leg B
NORTH SOUTH WEST
Flow (q) = (56×1.75) +147 (49×1.75) +147 (56×1.75)+(70×1.75)

245PCU 233PCU 221PCU

Where leg A has a width of 16.0m and leg B has a width of 10.7m
Saturation; N S W
8400 8400 5618

Q ⁄S;
245
For NORTH; = 0.03
84 00
23 3
For SOUTH ; = 0.028
84 00
221
For WEST ; =0.04
5618

NORTH SOUTH WEST

Flow (Q) 245 233 221
Saturation( S) 8400 8400 5618
Q/S 0.03 0.028 0.04

Y =0.03+0.04≤ 1
0.07 ≤ 1……………….ok
LAB = (IG –a) +SS
Assuming amber period = 3 seconds
Starting and stopping loss = 2seconds
IG = 11seconds.
LAB = (11-3) +2
= 10seconds
IG= 2 Seconds
LBA = (2-3) +2
= 6seconds
L = LBA + LAB (L; loss time)
10+6 = 16seconds
1.5 L+5
Co =
1−Y
1.5(16)+5
= 32sec
1− 0.07
20≤ Co ≤ 120
Total green time (Ge) = Co – L
32 - 16 = 16sec
For north - south phase
 Ya
Ga = ×≥¿
Y
0.03
× 16 = 6.9sec
0.07
Effective green time for north - south phase
Geff = Ga –a + Ss
(6.9-3) +2 = 5.9sec
For west phase
Yb
Gb = ×≥¿
Y
0.04
×16 = 9.14sec
0.07

Effective green time of west phase

Geff = Gb–a + Ss
(9.14-3) +2 = 8.14sec

Determination of peak 15 minutes

Time interval (am) Volume of flow
8:36 - 8:51 221
8:51 - 9:06 180
9:06 - 9:21 178
9:21 - 9:36 179

From the table above it is clear that the peak 15 minutes for the intersection occur between 8:36am-
8:51am. It is the time with the highest volume of flow of 221pcu.

Conclusion
After the course of the design, the Rotary controlled scheme seems to be the most appropriate for the
intersection at this time. Due to the following reasons;
1. Capacity for Moderate Traffic Flow: Rotaries are well-suited for handling moderate traffic
volumes, such as 758 vehicles per hour. They ensure a steady flow of traffic without requiring
vehicles to stop completely, which reduces delays.
2. Improved Safety: Rotaries reduce the likelihood of severe collisions such as T-bone (right-angle)
crashes and head-on collisions by promoting low-speed, one-way traffic flow.
3. Continuous Traffic Flow: Unlike signal-controlled intersections, rotaries do not require vehicles
to stop for red lights. This continuous movement decreases travel times and reduces congestion
during moderate traffic conditions.
4. Lower Operational Costs: Rotaries do not rely on traffic signals or electronic systems, eliminating
maintenance and energy costs associated with signal-controlled intersections.
5. Efficiency During Off-Peak Hours: During low-traffic periods, vehicles at rotaries face virtually
no delays, unlike signalized intersections, where vehicles might stop unnecessarily at red lights.