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1. 1


A Report on
Road Intersections
Course No: CE 454
Course Title: Transportation Engineering Sessional II






Submitted by:
Nazifa Tabassum (Mou)
Student ID: 1204010
Level/Term: 4/2
Section: A

Department Of Civil Engineering
Bangladesh University of Engineering & Technology

2. 2

Chapter 1
Introduction

1.1 Background: Speed is an important transportation consideration because it relates to safety,
time, comfort, convenience, and economics. Spot speed studies are used to determine the speed
distribution of a traffic stream at a specific location. The data gathered in spot speed studies are
used to determine vehicle speed percentiles, which are useful in making many speed-related
decisions.


1.2 Rationale: The importance of spot speed studies lies in:

i. Determining existing traffic operations and evaluation of traffic control devices.
ii. Establishing roadway design elements.
iii. Assessing roadway safety questions.
iv. Monitoring traffic speed trends by systematic ongoing speed studies.
v. Measuring effectiveness of traffic control devices or traffic programs, including signs and
markings, traffic operational changes, and speed enforcement programs.


1.3 Objectives of the study: The specific objectives of traffic speed include:
 To present detailed diagram of spot speed calculations.
 To calculate spot speeds and prepare tables for statistical analysis of spot speeds.
 To plot histograms, frequency curves and cumulative frequency curves of spot speeds.
 To determine weighted average speed, pace, modal speed, speed limit (85th percentile
speed), design speed, etc. of spot speeds.

3. 3


Chapter 2
Literature Review

2.1 Traffic Survey
Traffic engineers and planners need information about traffic. They need information to design
and manage road and traffic system. They use the information for planning and designing traffic
facilities, selecting geometric standards, economic analysis and in determination of priorities.
They use this to justify warrant of traffic control devices such as signs, traffic signals, pavement
markings, school and pedestrian crossings. They also use this information to study the
effectiveness of introduced schemes, diagnosing given situations and finding appropriate
solutions, forecasting the effects of projected strategies, calibrating and validating traffic models.

Transportation system is a dynamic system. Information about traffic must be regularly updated
to keep pace with ever-changing transportation system. Data must be collected and analyzed
systematically to get representative information. Traffic surveys are the means of obtaining
information about traffic. This is a systematic way of collecting data to be used for various traffic
engineering purposes.

2.2 Main purposes of traffic survey:
The main purposes of traffic survey are: traffic monitoring, traffic control and management,
traffic enforcement, traffic forecasting, model calibration and validating etc.

2.3 Parts of traffic studies: Traffic studies include:
 Inventory of road traffic physical features
 Traffic stream characteristics- volume, speed, density, occupancy studies etc.
 Capacity studies of streets and intersections
 System usage studies- Travel time and delay, O-D survey
 Travel demand- home interview survey
 Road users cost- Value of travel time, vehicle operating cost
 Parking supply & demand studies
 Axle load survey
 Mass transit performance and usage studies
 Traffic accidents studies
 Environmental impact studies of transport

4. 4



2.4 Traffic Speed Study
Traffic speed data are needed in research, planning, designing and regulation phases of traffic
engineering and are also used in establishing priorities and schedules of traffic improvements.
The traffic engineer must acquire general knowledge of traffic speeds in order to set different
limits, setting different distances i.e. passing sight distance, stopping sight distance etc.

2.5 Definition of Speed:
In simple words, speed is defined as the distance travelled in a unit time. Speed is expressed in
m/s, fps, mph etc. units. Speed is given by:

V= x/t (2.1)
V= dx/dt (2.2)

Where,
x = Distance (mile or meter or feet)
t = Time (second or hour)
Speed acquired by using Eq 2.1 will give the average speed. If Eq 2.2 is used the instantaneous
speed will be found.

Graphically speed can be measured from time space diagram. Time space diagram is a diagram
in which position of a vehicle is plotted against time chronologically. A qualitative time-space
diagram is shown in Fig 2.1.




Fig 1: A qualitative time-space diagram

5. 5


Types of speed:
 Spot speed
 Space-Mean speed
 Time-Mean speed
 Free flow speed
 Travel speed


2.6. Spot speed
Spot speed is the instantaneous speed of a vehicle as it passes a specified point along a road.
Spot speeds may be determined by manually measuring (with use of electronic or
electromechanical devices like pneumatic tube detectors or radars) the time required for a vehicle
to traverse a relatively short specified distance.
Methods for spot speed data collection: There are several methods for collecting spot speed data.
Some of them are:
i. Stopwatch method
ii. Radar meter method
iii. Pneumatic method

2.6. i. Stopwatch Method
The stopwatch method can be used to successfully complete a spot speed study using a small
sample size taken over a relatively short period of time. The stopwatch method is a quick and
inexpensive method for collecting speed data.

Key Steps to a Stopwatch Spot Speed Study
A stopwatch spot speed study includes five key steps:
1. Obtain appropriate study length.
2. Select proper location and layout.
3. Record observations on stopwatch spot speed study data form.
4. Calculate vehicle speeds.
5. Generate frequency distribution table and determine speed percentiles.

Table 1: Recommended Spot Speed Study Lengths

Traffic Stream Average Speed
Recommended Study Length(feet)
Traffic Stream Average Speed
Recommended Study Length(feet)
Below 25 mph 88
25–40 mph 176
Above 40 mph 264

6. 6



A radar meter is a commonly used device for directly measuring speeds in spot speed studies.
This device may be hand-held, mounted in a vehicle, or mounted on a tripod. The effective
measuring distance for radar meters ranges from 200 feet up to 2 miles. A radar meter requires
line-of-sight to accurately measure speed and is easily operated by one person.




Fig 2: A radar meter Fig 3: Recording data with radar meter method.



2.7. Space-Mean-Speed (SMS)
Space-Mean-Speed is the average of vehicle speeds weighted according to how long they remain
on the section of road. Mathematically it is harmonic mean of the observed speeds. It is given by:




Where,

ti = observed time for the i th vehicle to travel distance d
N or n = number of vehicles observed
d= length of roadway section

7. 7

2.8. Time-Mean-Speed (TMS)
The time mean speed Ut, is the arithmetic mean of spot speeds of all vehicles passing a point
during a specified interval of time. It is given by,



Where,
Ui or ui = observed speed of i-th vehicle
N or n = number of vehicles observed

Space-mean speed and time-mean speed are not equal. For general usage, no distinction is
normally made between both speeds, for theoretical and research purposes WARDROP has
shown in his calculations that:



Design speed
Design speed is defined as the maximum safe speed that can be maintained over a specified
section of highway when conditions are so favorable that the design features of the highway
govern (ITE 1999). This definition implies that the design speed should be selected based on
drivers expectations, the type of highway and terrain and topography.

The 1997 Highway Capacity Manual notes that speed is a major indicator of service quality to
drivers. Freedom to maneuver within the traffic stream and proximity to other vehicles are
equally important to a driver. Further, the density increases as the flow increases up to capacity
to a broad range of flows. Thus, density is the primary performance measure used to provide an
estimate of the level of service. The following table shows the density ranges for each level of
service:

8. 8

Level of Service Density Range (pc/mi/in)
A 0-10.0
B 10.1-16.0
C 16.1-24.0
D 24.1-32.0
E 32.1-45.0
F >45.0
Table 2: Density range for each level of service

9. 9

Chapter Three

Methodology


Methods of Spot Speed Study

Methods of Conducting Spot Speed Studies are divided into two main categories:

1. Manual
2. Automatic
I. Road Detectors
II. Doppler-Principle Meters
III. Electronic-Principle Detectors

Manual Method - Stopwatch method
The stopwatch method can be used to successfully complete a spot speed study using a small
sample size taken over a relatively short period of time. The stopwatch method is a quick and
inexpensive method for collecting speed data. However, it is the least accurate of the methods.


Fig 4: Stopwatch Spot Speed Study Layout

Road Detectors (Automatic)

1) Pneumatic Road Tubes or Induction Loops.
2) Can be used to collect data on speeds at the same time as volume data are being collected.

The advantage of the detectors is that human errors are considerably reduced. The disadvantages
are that they are expensive and may affect the driver behavior. Pneumatic Road Tubes are laid
across the lane in which data are to be collected.

Doppler-Principle Meters (Automatic)

Doppler meters work on the principle that when a signal is transmitted onto a moving vehicle,
the change in frequency between the transmitted signal and the reflected signal is proportional to

10. 10

the speed of the moving vehicle. The difference between the frequency of the transmitted signal
and that of the reflected signal is measured by the equipment, then converted to speed in mph or
km/h.

A radar meter is based on such Doppler principle and used to measure spot speed. This device
may be hand-held, mounted in a vehicle, or mounted on a tripod. The effective measuring
distance for radar meters ranges from 200 feet up to 2 miles. A radar meter requires line-of-sight
to accurately measure speed and is easily operated by one person. If traffic is heavy or the
sampling strategy is complex, two radar units may be needed.

Electronic-Principle Detectors (Automatic)
The presence of vehicles is detected through electronic means, and information on these vehicles
is obtained, from which traffic characteristics such as speed, volume, queues, and headways are
computed. The most promising technology using electronics is video image processing,
sometimes referred to as a machine-vision system.

The main advantage of method is that, it gives a permanent record with 100% sample obtained.
This method is quite expensive and generally adopted in a situation where evidence is required.
Even video recorder can be used which give more accurate result.



Here we used the manual stopwatch method as it was the easiest and the simplest and in the
absence of modern equipment.

We had six enumerators for the purpose of this experiment and Tk. 500 was allocated for each
enumerator.

Methodology of reconnaissance survey

Before starting the actual work we have conducted a reconnaissance survey. We visited the spot
of study and then we selected the reference points where we collected speed data.

11. 11


Chapter Four

Data Collection


Methodology of reconnaissance survey

Before starting the actual work we have conducted a reconnaissance survey. We visited the spot
of study and then we selected the reference points where we collected speed data.

Location: Location of the spot for traffic speed survey was chosen to be from Panthapath
intersection to Russel Square. Vehicles from Panthapath to Russel Square were counted.
We stood by the side of the road and data of different vehicles were collected by different
persons.

Date: Data for speed study was collected on 23 March 2017. It was Thursday and it was a
weekday.

Time: Time of data collection for volume study was different for different groups however for
group-1 the time was from 8:30 am to 8:40 am

Weather Condition: It was a sunny day.

Observation: Classified Vehicle Counts.

12. 12



CAR Spot Speed Study

CAR Distance(ft)
Time
(sec)
Spot
Speed
(mph)
1 44 1.65 18.1818
2 44 1.32 22.7273
3 44 1.20 25.0000
4 44 1.10 27.2727
5 44 1.43 20.9790
6 44 1.70 17.6471
7 44 1.80 16.6667
8 44 1.05 28.5714
9 44 1.77 16.9492
10 44 1.52 19.7368
11 44 1.62 18.5185
12 44 1.80 16.6667
13 44 1.24 24.1935
14 44 1.39 21.5827
15 44 1.70 17.6471
16 44 1.39 21.5827
17 44 1.67 17.9641
18 44 1.59 18.8679
19 44 1.41 21.2766
20 44 1.06 28.3019
21 44 1.60 18.7500
22 44 1.45 20.6897
23 44 1.40 21.4286
24 44 1.27 23.6220
25 44 1.07 28.0374
Table 3: Summary table showing car spot speed data of group- 1

13. 13

Chapter Four

Data Analysis

From Spot speed data of all 8 groups, following table is prepared:


Table 4: Statistical calculation table of spot speed data of all groups

Speed
Range
(mph)
No. of
Vehicle
observed
(f)
Mid-
Speed,V
(mph)
%
Frequenc
y
Cumulativ
e %
Frequenc
y
Frequenc
y * Mid-
speed,
f*V
Average
Velocity,
V
avg
(mph)
0 - 7
41
3.5 7.2 7.2 143.5
7 - 14
298
10.5 52.29 59.49
3129
14 - 21
144
17.5 25.27 84.76
2520
21 - 28
50
24.5 8.78 93.54
1225
28 - 35
30
31.5 5.27 98.81
945
35 - 42 7 38.5 1.23 100.04 269.5
Total
570 8232
14.45

14. 14


Fig 5: % Frequency vs Spot speed curve

 Modal speed 11 mph for this road and traffic condition. This is the speed which has the
highest frequency and therefore indicates the optimum speed which drivers want to
maintain in the roadway under concern.
 Pace is in between 6mph and 16mph. That is most drivers are seen driving with a 10 mph
difference, and in between these speeds.
As the curve is nearly symmetric, so median speed should be close to 11mph



Fig 6: % Cumulative frequency vs Spot speed curve
0
10
20
30
40
50
60
0 5 10 15 20 25 30 35 40 45
% Frequency
Spot Speed (mph)
Frequency Distribution For Spot Speed
Modal Speed = 11 mph
Pace = 6 ~ 16

15. 15

 From the cumulative frequency graph, the 98
th
percentile correspond to the design speed.
So the design speed for this road from Panthapath to Russell Square section is 29.1 mph.
 The 15
th
percentile speed is 4.8mph and the 85
th
percentile speed is 17.5mph. These are
the lower and upper speed limits. The speed range is therefore 4.8mph- 17.5mph.



Fig 7: Combination of % Frequency vs Spot speed curve and Cumulative frequency vs Spot
speed curve

Superimposing the Cumulative Frequency Distribution and the Frequency Distribution, we can
see that,
26% to 80% vehicles travel in Pace Speed
62% vehicles travel at Modal Speed

16. 16


Fig 8: Speed Histogram

 From the histogram it can be observed that most vehicles have speed range between 7-
14mph. This is around 52.3%. Vehicles having speed in the 14-21mph range was also
significant with a percentage frequency of around 26%.
 Velocity less than 7mph is very rare.
 Very few vehicles have speed greater than 35mph and indicates only a mere 1.23%.




Table 5: Standard Deviation

From the table we see the standard deviation is 1.68 which is good.

Speed
Range
(mph)
No. of
Vehicle
observed
(f)
Mid-
Speed,V
(mph)
Average
Velocity,
V
avg
(mph)
Variance
SD
0 - 7
41
3.5
119.9025
7 - 14
298
10.5
15.6025
14 - 21
144
17.5
9.3025
21 - 28
50
24.5
600.25
28 - 35
30
31.5
290.7025
35 - 42 7 38.5
578.4025
Total
570
1614.163
14.45
1.68

17. 17

Chapter Five

Conclusion and Recommendation

Findings from our Study:
 Average Speed of the stream, Vavg = 14.45 mph,
 Design Speed (98th Percentile) = 29.1 mph
 Maximum Speed (85th Percentile) = 17.5 mph
 Minimum Speed (15th Percentile) = 4.8 mph
 Modal Speed = 11 mph
 Pace Speed = 6 ~ 16 mph
 26% to 80% vehicles travel in Pace Speed
 62% vehicles travel at Modal Speed
 Standard Deviation of Spot Speeds, σ = 1.68


Recommendations
1. The buses we observed on the road were too old that they could not maneuver easily. So
replacing these old buses with new ones is highly recommended.
2. Bicycle should have specific lanes of their own which typically is placed beside the
footpath/shoulder. But there was not any specific lane in the road we studied. So it is
recommended that a lane system should be introduced to increase efficiency of the road at the
same time there should be a bicycle specific lane.
3. NMT or electrical low speed vehicles should not be permitted in this type of arterial road.
Although they typically travel on the left lane but they create a drag force which slows down the
high speed vehicles which creates congestion.

18. 18

References

Robertson, H. D. 1994. Spot Speed Studies. In Manual of Transportation Engineering Studies,
ed. H. D. Robertson, J. E. Hummer, D. C. Nelson. Englewood Cliffs, N.J.:
Prentice Hall, Inc., pp. 33–51.

Currin, T. R. 2001. Spot Speed Study. In Introduction to Traffic Engineering: A Manual for Data
Collection and Analysis, ed. B. Stenquist. Stamford, Conn.: Wadsworth Group,
pp. 4–12.

Homburger, W. S., J. W. Hall, R. C. Loutzenheiser, and W. R. Reilly. 1996. Spot Speed Studies.
In Fundamentals of Traffic Engineering. Berkeley: Institute of Transportation Studies,
University of California, Berkeley, pp. 6.1–6.9.

Parma, K. 2001. Survey of Speed Zoning Practices: An Informational Report. Washington,
D.C.: Institute of Transportation Engineers.

Persaud, Bhagwant, Parker, Martin Jr., and Gerald Wilde. 1997. Safety, speed and speed
management. Transportation Canada Repor. Ottawa Canada.

Pline, James L. editor. 1999. Traffic Engineering Handbook. Institute of Transportation
Engineers (ITE), 5th Edition.

Mannering, Fred L. Walter P. Kilarski. 1998. Principles of Highway Engineering and Traffic
Analysis. Wiley, New York. 2nd Edition:340.

National Research Council. 1998. Highway Capacity Manual; Special Report 209. 3
rd
Edition
Washington, D.C.