CIV2282 Transport and Traffic Engineering

CIV2282 Transport and Traffic Engineering WITH ANSWERS
Prac Class 7 – Unsignalised Intersections Page 1 of 6

MONASH UNIVERSITY
DEPARTMENT OF CIVIL ENGINEERING
CIV2282: Transport and Traffic Engineering
Semester 2/2020, Week 7 beginning Monday 14 September
Practical Class 7 – Unsignalised Intersections
With Answers
The exercises in this practical class draw on the material presented in the lecture slides
“Unsignalised Intersections (1) – Capacity and Analysis, and “Unsignalised Intersections (2)
– Roundabouts”, as well as the lecture notes ‘Topic 6: Foundations of Unsignalised
Intersection Analysis’ and ‘Topic 7: Analysis of Unsignalised Intersection Capacity.’
You will probably need a spreadsheet such as Excel to complete these exercises,
remember to bring a computer with spreadsheet software into the practical class.
Exercise 1 – Unsignalised Intersection Analysis
Figure 1 shows the current layout of the intersection of a major road with a minor road. The
intersection is controlled by a GIVE WAY sign, and Figure 1 shows the current A.M. peak
period traffic movements. There is negligible traffic turning from the major road into the minor
road. Of the 440 vehicles per hour approaching on the minor road, 264 turn left and 176 turn
right. All traffic arrivals can be treated as random. You have been asked to analyse the
performance of the existing layout (Figure 1) and a revised design (Figure 2).
Figure 1: Existing Intersection Design
440 veh/h
CIV2282 Transport and Traffic Engineering WITH ANSWERS
Prac Class 7 – Unsignalised Intersections Page 2 of 6
A worksheet (Table 1) has been prepared which identifies the key formula and helps the
calculation process. Hint: Retain as much precision as possible in your calculations, otherwise
rounding errors will give an incorrect result.
PART A: Analysis of Existing Conditions
We first need to examine the current operation of the intersection using gap acceptance and
queuing models. In your analysis you should use design values for the critical gap and followup headway of 4 seconds and 2 seconds for the left turn and 5 seconds and 3 seconds for the
right turn, respectively. Assume queuing service rates (practical capacities) equal 90 per cent
of the corresponding theoretical maximum absorption capacities.
Table 1 – Unsignalised Intersection Analysis Worksheet (combined lane)

1 0




qT
qT
qe
C
e
C C P  0.9 D C HOQ P 1 1
T
L PL R PR
C
P C P C


D C HOQ C   1 T    ? w  

  


w  61.687 s
 
2
Q w  
  
 


CIV2282 Transport and Traffic Engineering WITH ANSWERS
Prac Class 7 – Unsignalised Intersections Page 3 of 6

t w D   HOQ
N t  
 
 

log
log
N



 
  
 
 


PART B: Effect of Intersection Redesign
A proposed redesign of the intersection is shown in Figure 2 and involves a provision for two
turning lanes. The volumes are assumed to remain the same as in Part A. Use the same design
values for the critical gaps and follow-up gaps as used in Part A. Likewise, assume that queuing
service rates equal 90 per cent of the theoretical maximum absorption capacities.
In addition, you have been asked for your professional opinion on the following:
1. If the intersection was reconstructed in line with the proposed design, would you
expect the volumes from the minor street to remain the same?
We would expect the volumes from the minor street to increase if the intersection was
reconstructed in line with the proposed design, since it is likely to attract additional traffic
from other roads.
We should re-analyse the intersection performance using an estimate of this increased
traffic demand.
Induced traffic is important – just think about the amount of development that has
occurred in Melbourne’s Eastern suburbs since the EastLink tollway was opened.
Figure 2: Redesigned Intersection
900 veh/h
300 veh/h
440 veh/h
L = Left Turn
Lane Length
CIV2282 Transport and Traffic Engineering WITH ANSWERS
Prac Class 7 – Unsignalised Intersections Page 4 of 6
Table 2 – Unsignalised Intersection Analysis Worksheet (separate lanes)

1
D C HOQ P     ? w  

  


w  2.544 s
w 13.384 s
t w D   HOQ
t p t p t     L L R R N t  
 
 

log
log
N



 
  
 
max ,   N N   L R
CIV2282 Transport and Traffic Engineering WITH ANSWERS
Prac Class 7 – Unsignalised Intersections Page 5 of 6
Exercise 2 – Roundabout Analysis
A four-approach roundabout has been experiencing considerable delays on some of its
approaches. There is one lane on each approach and one circulating lane.
A turning movement survey was conducted giving the following results:
Table 3 – Roundabout turning movement survey (vehicles per hour)

CIV2282 Transport and Traffic Engineering WITH ANSWERS
Prac Class 7 – Unsignalised Intersections Page 6 of 6
1. Calculate the total approach volume, the circulating volume, and the exit volume in
front of each approach (in vehicles per hour).
Table 4 – Roundabout approach, circulating and exit volumes (vehicles per hour)

Assuming the queues at each approach can be represented by the M/M/1 queuing process
model, and critical gaps and follow-up gaps are all 5 seconds and 3 seconds respectively,
calculate the following for each approach (one approach has been completed already):
2. Probability of vehicles having to stop (%)
3. Average waiting time in the queue (s)
4. Average total queuing delay (s)
5. Average total number of vehicles at each approach (vehicles)
6. Queue length that would only be exceeded 5% of the time (vehicles)

Two approaches have delays that are excessive, but for different reasons
7. What are the two different reasons that delays are very large at those two approaches?
On the North approach, the delays are caused by the large arrival flow entering the approach.
On the East approach, the delays are caused by the large circulating flow in front of the
approach (mainly from the North approach). This is a classic case of unbalanced flow at a
roundabout causing delays on other approaches.