Lecture 6 & 7 Transportation and Traffic Engineering (CE 351)
SPEED STUDIES
Introduction
o It is a basic measure of traffic and roadway performance
o Influence the traveler in selecting routes or transportation modes.
Speed – The rate of movement of a vehicle, generally expressed in fps, kph or
mph.
Objectives
The main objectives of speed studies are:
o Capacity studies
o Geometric design
o Accident analysis
o Economic studies
o Performance study of a traffic control measure (before and after studies)
o Planning and designing traffic control measures viz.
Traffic signal design
Designing road signs
Designing parking crossings
Establishing speed limits
Speed zoning
Placing speed breakers etc.
1 Md. Yusuf Ali
Lecture 6 & 7 Transportation and Traffic Engineering (CE 351)
Factors Affecting Speeds
Vehicles’ speed depends on:
o Roadway geometric, operating and surface conditions
o Amount of side frictions
o Traffic conditions
o Location along the road
o Time of the day
o Weather conditions.
Types of Speeds
a. Time-mean speed: The speed obtained when the sum of all values is
divided by the number of observation. It gives arithmetic mean of the spot
speeds of vehicles passing a point. Useful for traffic regulating purposes.
vt = individual speeds
n = no. of observations
b. Space-mean speed: The speed obtained when speeds are averaged over
space. It gives the harmonic mean of the speeds of vehicles passing a point,
on a highway, during an interval. Useful for establishing Speed-Flow
relationships.
S = space distance
T = mean travel time
2 Md. Yusuf Ali
Lecture 6 & 7 Transportation and Traffic Engineering (CE 351)
Problem 1:
The following travel times were observed for 4 vehicles traversing a 1 mile
segment of highway.
Vehicle Time (min)
1 1.6
2 1.2
3 1.5
4 1.7
Calculate the Space mean and Time mean speeds of the vehicles.
Solution:
Time mean speed = [(1/1.6)+(1/1.2)+1/1.5)+(1/1.7)]/4
= 0.68 miles per minute
= 40.8 mph
Space mean speed = 4/91.6+1.2+1.5+1.7)
= 0.67 miles per minute
= 40.0 mph
(Ans: TMS – 40.8 mph & SMS – 40 mph).
3 Md. Yusuf Ali
Lecture 6 & 7 Transportation and Traffic Engineering (CE 351)
Different Forms of Speed
o Spot speed: Instantaneous speed of a vehicle at a point.
o Running speed: Average speed while in motion.
o Journey speed: Overall speed including stops.
o Operating speed: The highest overall speed.
o Free Flow speed: When drivers face no restriction in driving and can
maintain their desired speeds.
o Safe speed: 85th percentile speed.
o Design speed: 98th percentile speed.
o Median speed: 50th percentile speed.
o Modal speed: Speed at the highest frequency.
o Pace: It is a range of speed, usually taken in 10 mph or 15 mph increment.
o Speed limits: Upper limit at 85th percentile speed.; Lower limit at 15th
percentile speed.
4 Md. Yusuf Ali
Lecture 6 & 7 Transportation and Traffic Engineering (CE 351)
Problem 2:
Following data was collected when conducting spot speed studies at certain stretch of a
road within the urban area. Determine:
a) Average speed of traffic stream
b) Modal speed
c) Design speed
d) Upper & Lower values of the speed limits.
Speed No. of Vehicle Mid- Speed, V % Cumulative %
Range
Observed (f) (mph) Frequencies Frequencies
(mph)
0-5 0 2.5 0.00 0.00
5 - 10 10 7.5 1.20 1.20
10 - 15 20 12.5 2.41 3.61
15 - 20 40 17.5 4.82 8.43
20 - 25 110 22.5 13.25 21.69
25 - 30 200 27.5 24.10 45.78
30 - 35 250 32.5 30.12 95.90
35 - 40 120 37.5 14.46 90.36
40 - 45 40 42.5 4.82 95.18
45 - 50 20 47.5 2.41 97.59
50 - 55 10 52.5 1.20 98.80
55 - 60 7 57.5 0.84 99.64
60 - 65 3 62.5 0.36 100.00
65 - 70 0 67.5 0.00 100.00
Total 830
Vavg =Sum (f*V)/Sum (f)
= 30.2 mph
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Lecture 6 & 7 Transportation and Traffic Engineering (CE 351)
(Problem 2)
Frequency Distribution for Spot Speed
35.00%
30.00%
Modal Speed= 32.0 mph
% Frequency
25.00%
20.00%
Pace= 22~37 mph
15.00%
10.00%
5.00%
0.00%
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
Spot Speed, mph
Cumulative Frequency Distribution of Spot Speed
100%
Cumulative Frequency, %
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
Spot Speed, mph
98th percentile = 46.2 mph
85th percentile = 35.5 mph
15th percentile = 17.2 mph
6 Md. Yusuf Ali
Lecture 6 & 7 Transportation and Traffic Engineering (CE 351)
DELAY STUDIES
Introduction
o It is a modified form of the speed studies
o It measures the average journey time.
Objectives
o To find out the locations, causes and duration of the delay/congestions
o To asses the quality/efficiency of road network.
o To prescribe the remedied measures of congestions.
o For economic evaluation of improvement measures
o To evaluate the performance of particular measures.
o For travel demand management /route guidance.
Causes of Delay
o Fixed Causes/Geometric delay: Delay incurred due to roadway geometry. It
occurs mainly due to road intersections/signals, narrow road/bridges, sharp
horizontal and vertical curves etc. It is unavoidable and almost same for all
vehicles.
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Lecture 6 & 7 Transportation and Traffic Engineering (CE 351)
o Operational Delay: It is mainly caused by road-side and inter-vehicular
frictions such as:
Road side parking
Road side non-motor activities.
Road side pedestrian movements and random crossing.
Bus stoppage near intersections.
High traffic volume and lack of roadway capacity.
Merging, turning, lane changing/overtaking or weaving
maneuvers etc.
Delay/Congestions
It indicates the situation when extra time is needed to travel through a roadway
facility.
Types of Delay/Congestion
o Non-recurrent Delay: Which is unpredictable in nature and occurs
due to the following random events.
Flow break down at peak periods (make shock wave –
which quickly amplify and propagate to down stream).
Accidents
Special events (sports, national events).
Signal Malfunctions etc.
o Recurrent Delay: Which occurs periodically at particular locations
and particular time of a day.
Daily morning/evening peak period congestions due to
rush commuter movements.
Congestions other than the peak periods on all purpose
roads due to increased flow level.
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