Short Notes for Traffic Engineering | Short Notes for Civil Engineering - Civil Engineering (CE) PDF Download

 Page 1


 ? Introduction
 ? Traf? c studies
 ? Traf? c speed studies
 ? Speed and delay studies
 ? Origin and destination studies
 ? Accident studies
 ? Traf? c ? ow characteristics
 ? Speed–Flow–Density relations
 ? Derived characteristics
 ? Traf? c stream models
 ? Traf? c regulation and control
 ? Types of traf? c signals
 ? Design of rotary
 ? Road intersections
 ? Interchanges
 ? Parking
 ? Highway lighting
CHAPTER HIGHLIGHTS
Chapter 4
Introduction
Traffi  c Engineering is that branch of engineering which 
deals with the improvement of traffi  c performance on road 
network and terminals through systematic traffi  c studies, 
scientifi c analysis and engineering applications. It also 
includes planning and geometric design on one hand and 
regulation and control on other.
Traffic Studies
Traffi  c Volume Study ( q)
Traffi  c volume or fl ow is expressed as number of vehicles 
that pass across a given transverse line of the road during 
unit time. In this study the type and the number of vehicles 
are counted either manually or by using mechanical or auto-
matic counters.
Method of Volume Count
Manual Count (Best Method) By manual count, it gives 
data which cannot be collected by mechanical or automatic 
counts. 
 1. Classifi cation of diff erent vehicles and their counts at 
required time. 
 2. Direction wise and turning movements at 
intersections. 
 3. Details of pedestrian volume counts. 
Draw back: It is not practicable to carry out manual count 
of diff erent vehicle classes during 24 hours of a day for 365 
days.
Automatic Counters
 1. Photo electric cell 
 2. Electrical method 
 3. Pneumatic method 
 • Advantage of it is that it can work throughout the day and 
night for desired period, recording exact time at which 
each vehicle crosses. Lane occupancy and speeds of
diff erent vehicles can be recorded. 
 • Drawback is that it cannot classify and record every
vehicle type and details of turning movements and also 
pedestrian fl ow. 
 • Suitable for long counts and for permanent installations
 • In view of variety of vehicles, based on the vehicle 
speed, space and characteristics a number is assigned to 
the vehicle in comparison to car termed as ‘Equivalent 
passenger car unit’ (PCU). Using the values in table, the 
Traffi  c Engineering
Part III_Unit 11_Chapter 04.indd   974 5/31/2017   6:44:15 PM
Page 2


 ? Introduction
 ? Traf? c studies
 ? Traf? c speed studies
 ? Speed and delay studies
 ? Origin and destination studies
 ? Accident studies
 ? Traf? c ? ow characteristics
 ? Speed–Flow–Density relations
 ? Derived characteristics
 ? Traf? c stream models
 ? Traf? c regulation and control
 ? Types of traf? c signals
 ? Design of rotary
 ? Road intersections
 ? Interchanges
 ? Parking
 ? Highway lighting
CHAPTER HIGHLIGHTS
Chapter 4
Introduction
Traffi  c Engineering is that branch of engineering which 
deals with the improvement of traffi  c performance on road 
network and terminals through systematic traffi  c studies, 
scientifi c analysis and engineering applications. It also 
includes planning and geometric design on one hand and 
regulation and control on other.
Traffic Studies
Traffi  c Volume Study ( q)
Traffi  c volume or fl ow is expressed as number of vehicles 
that pass across a given transverse line of the road during 
unit time. In this study the type and the number of vehicles 
are counted either manually or by using mechanical or auto-
matic counters.
Method of Volume Count
Manual Count (Best Method) By manual count, it gives 
data which cannot be collected by mechanical or automatic 
counts. 
 1. Classifi cation of diff erent vehicles and their counts at 
required time. 
 2. Direction wise and turning movements at 
intersections. 
 3. Details of pedestrian volume counts. 
Draw back: It is not practicable to carry out manual count 
of diff erent vehicle classes during 24 hours of a day for 365 
days.
Automatic Counters
 1. Photo electric cell 
 2. Electrical method 
 3. Pneumatic method 
 • Advantage of it is that it can work throughout the day and 
night for desired period, recording exact time at which 
each vehicle crosses. Lane occupancy and speeds of
diff erent vehicles can be recorded. 
 • Drawback is that it cannot classify and record every
vehicle type and details of turning movements and also 
pedestrian fl ow. 
 • Suitable for long counts and for permanent installations
 • In view of variety of vehicles, based on the vehicle 
speed, space and characteristics a number is assigned to 
the vehicle in comparison to car termed as ‘Equivalent 
passenger car unit’ (PCU). Using the values in table, the 
Traffi  c Engineering
Part III_Unit 11_Chapter 04.indd   974 5/31/2017   6:44:15 PM
Chapter 4 
¦
 Traffic Engineering | 3.975
traffic can be expressed uniformly in terms of passenger 
car unit (PCU) per hour or per day 
Vehicle Type PCU
Passenger car, Tempo, Jeep, Auto 
rickshaw, Van, Agricultural tractor 
1
Bus, Truck, Agricultural tractor trailer 3
Motor cycle, scooter and cycle 0.5
Cycle rickshaw 1.5
Horse drawn vehicle 4
Bullock cart 8
Small bullock cart 6
Hand cart 6
 • As per IRC, traffic counts are taken twice in a year 
for a village. One during peak season of harvesting 
and marketing and other during the lean season. It 
shall be taken for 7 consecutive days and 24 hours 
each day (24 × 7).
Presentation of Traffic Volume Data
 1. Average annual daily traffic (AADT): It is the 
average daily (24 hours) traffic volume recorded for 
all the 365 days of the year.
 2. Average daily traffic (ADT): Average daily traffic 
volume recorded for a period less than a year 
(6 months, a season, a month, a weak, or 2 days)
 3. Trend charts: Shows volume trends over period of 
years. It is used in estimating the rate of growth and 
for planning future expansion, design and regulation. 
 4. V ariation charts: V ariation of flow (hourly, daily and 
seasonally) are prepared. These help in facilitating or 
regulating traffic at peak traffic periods.
 5. Traffic flow maps: Traffic flow lines along the routes 
are drawn. The thickness of the lines represents the 
traffic volume to any desired scale. 
 6. V olume flow diagram at intersections to a scale.
 7. 30th highest hourly volume: The hourly volume 
that will be exceeded only 29 times in a year and all 
other hourly volumes of the year will be less than this 
volume.
 • Congestion only during 29 hours in the year. 
 • Generally taken as hourly volume for design. 
 • Highway facilities are designed for this volume, as 
it is satisfactory from both facility and economic 
considerations.
 8. Peak hour factor: It is in terms of traffic volume 
during peak hour and it is expressed as percentage 
of AADT. This peak hour factor is used in the 
design of transportation facilities of urban transport 
infrastructure projects.
Traffic Speed Studies
 1. Spot speed: It is the instantaneous speed of a vehicle 
at specified cross section or location. 
 2. Average speed: It is the average of spot speeds of all 
the vehicles passing a given point on the highway.
 3. Running speed: It is the average speed maintained 
by a vehicle over a particular stretch of road (delays 
are excluded). 
 4. Travel/overall/journey speed: It is the effective 
speed of travel all along the route between two 
terminals (delays are included).
Methods for Spot Speeds
 • Radar speed meter method (speed gun and is most 
efficient and easy to use).
 • Electronic meter method. 
 • Photo-electric meter.
 • Enoscope/mirror box method.
 ¦ Mirrors are arranged at 45°.
 ¦ Simple cheap and easy to use. 
 ¦ Difficult for heavy multilane traffic and slow method 
 • Time lapse method 
 • Pressure contact strip method 
Presentation of Spot Speed Data
Speed (kmph)
Frequency (%)
Modal speed
Frequency distribution diagram 
 • Modal speed: speed at which greatest number of vehi-
cles travel. It is peak of frequency distribution curve.
Speed (kmph)
15%
85%
98%
Cumulative frequency (%)
98th Percentile speed
85th Percentile speed
Cumulative speed distribution diagram
Part III_Unit 11_Chapter 04.indd   975 5/31/2017   6:44:15 PM
Page 3


 ? Introduction
 ? Traf? c studies
 ? Traf? c speed studies
 ? Speed and delay studies
 ? Origin and destination studies
 ? Accident studies
 ? Traf? c ? ow characteristics
 ? Speed–Flow–Density relations
 ? Derived characteristics
 ? Traf? c stream models
 ? Traf? c regulation and control
 ? Types of traf? c signals
 ? Design of rotary
 ? Road intersections
 ? Interchanges
 ? Parking
 ? Highway lighting
CHAPTER HIGHLIGHTS
Chapter 4
Introduction
Traffi  c Engineering is that branch of engineering which 
deals with the improvement of traffi  c performance on road 
network and terminals through systematic traffi  c studies, 
scientifi c analysis and engineering applications. It also 
includes planning and geometric design on one hand and 
regulation and control on other.
Traffic Studies
Traffi  c Volume Study ( q)
Traffi  c volume or fl ow is expressed as number of vehicles 
that pass across a given transverse line of the road during 
unit time. In this study the type and the number of vehicles 
are counted either manually or by using mechanical or auto-
matic counters.
Method of Volume Count
Manual Count (Best Method) By manual count, it gives 
data which cannot be collected by mechanical or automatic 
counts. 
 1. Classifi cation of diff erent vehicles and their counts at 
required time. 
 2. Direction wise and turning movements at 
intersections. 
 3. Details of pedestrian volume counts. 
Draw back: It is not practicable to carry out manual count 
of diff erent vehicle classes during 24 hours of a day for 365 
days.
Automatic Counters
 1. Photo electric cell 
 2. Electrical method 
 3. Pneumatic method 
 • Advantage of it is that it can work throughout the day and 
night for desired period, recording exact time at which 
each vehicle crosses. Lane occupancy and speeds of
diff erent vehicles can be recorded. 
 • Drawback is that it cannot classify and record every
vehicle type and details of turning movements and also 
pedestrian fl ow. 
 • Suitable for long counts and for permanent installations
 • In view of variety of vehicles, based on the vehicle 
speed, space and characteristics a number is assigned to 
the vehicle in comparison to car termed as ‘Equivalent 
passenger car unit’ (PCU). Using the values in table, the 
Traffi  c Engineering
Part III_Unit 11_Chapter 04.indd   974 5/31/2017   6:44:15 PM
Chapter 4 
¦
 Traffic Engineering | 3.975
traffic can be expressed uniformly in terms of passenger 
car unit (PCU) per hour or per day 
Vehicle Type PCU
Passenger car, Tempo, Jeep, Auto 
rickshaw, Van, Agricultural tractor 
1
Bus, Truck, Agricultural tractor trailer 3
Motor cycle, scooter and cycle 0.5
Cycle rickshaw 1.5
Horse drawn vehicle 4
Bullock cart 8
Small bullock cart 6
Hand cart 6
 • As per IRC, traffic counts are taken twice in a year 
for a village. One during peak season of harvesting 
and marketing and other during the lean season. It 
shall be taken for 7 consecutive days and 24 hours 
each day (24 × 7).
Presentation of Traffic Volume Data
 1. Average annual daily traffic (AADT): It is the 
average daily (24 hours) traffic volume recorded for 
all the 365 days of the year.
 2. Average daily traffic (ADT): Average daily traffic 
volume recorded for a period less than a year 
(6 months, a season, a month, a weak, or 2 days)
 3. Trend charts: Shows volume trends over period of 
years. It is used in estimating the rate of growth and 
for planning future expansion, design and regulation. 
 4. V ariation charts: V ariation of flow (hourly, daily and 
seasonally) are prepared. These help in facilitating or 
regulating traffic at peak traffic periods.
 5. Traffic flow maps: Traffic flow lines along the routes 
are drawn. The thickness of the lines represents the 
traffic volume to any desired scale. 
 6. V olume flow diagram at intersections to a scale.
 7. 30th highest hourly volume: The hourly volume 
that will be exceeded only 29 times in a year and all 
other hourly volumes of the year will be less than this 
volume.
 • Congestion only during 29 hours in the year. 
 • Generally taken as hourly volume for design. 
 • Highway facilities are designed for this volume, as 
it is satisfactory from both facility and economic 
considerations.
 8. Peak hour factor: It is in terms of traffic volume 
during peak hour and it is expressed as percentage 
of AADT. This peak hour factor is used in the 
design of transportation facilities of urban transport 
infrastructure projects.
Traffic Speed Studies
 1. Spot speed: It is the instantaneous speed of a vehicle 
at specified cross section or location. 
 2. Average speed: It is the average of spot speeds of all 
the vehicles passing a given point on the highway.
 3. Running speed: It is the average speed maintained 
by a vehicle over a particular stretch of road (delays 
are excluded). 
 4. Travel/overall/journey speed: It is the effective 
speed of travel all along the route between two 
terminals (delays are included).
Methods for Spot Speeds
 • Radar speed meter method (speed gun and is most 
efficient and easy to use).
 • Electronic meter method. 
 • Photo-electric meter.
 • Enoscope/mirror box method.
 ¦ Mirrors are arranged at 45°.
 ¦ Simple cheap and easy to use. 
 ¦ Difficult for heavy multilane traffic and slow method 
 • Time lapse method 
 • Pressure contact strip method 
Presentation of Spot Speed Data
Speed (kmph)
Frequency (%)
Modal speed
Frequency distribution diagram 
 • Modal speed: speed at which greatest number of vehi-
cles travel. It is peak of frequency distribution curve.
Speed (kmph)
15%
85%
98%
Cumulative frequency (%)
98th Percentile speed
85th Percentile speed
Cumulative speed distribution diagram
Part III_Unit 11_Chapter 04.indd   975 5/31/2017   6:44:15 PM
3.976 | Part III 
¦
 Unit 11 
¦
 Transportation Engineering
 • 98th percentile speed: For the purpose of highway 
geometric design (design speed).
 • 85th percentile speed: The speed at or below which 85% 
of vehicles are passing a point on the highway or only 
15% of vehicles exceed the speed at that point (upper 
speed limit). 
  This is adopted for safe seed limit at this zone.
 • 15th percentile speed: The speed value which is used as 
minimum speed on major highways.
 • 50th percentile speed: The middle or 50th percentile 
of spot speed is called median/medium speed (median 
speed).
Types of Spot Speeds 
 1. Space mean speed (V
s
): It is the average speed of 
vehicles in a certain road length at any specified time 
period. This is the harmonic mean of spot speeds. 
 2. Time mean speed (V
t
): It is the average speed of 
vehicles passing a certain point over some specified 
time period on a highway.
   It is the average/mean of spot speeds:
 • (Average travel time of all vehicles)  
= 
1
Space mean speed
.
 • Space mean speed is slightly lower than time mean 
speed under typical speed conditions on rural 
highways
V
t
 = V
s
 +
s
2
V
s
	 	s
2
 = Standard deviation. 
  Both space and time mean speeds will be equal only 
when all vehicles are travelling at the same speed. 
Speed and Delay Studies
 • Floating car or riding check method 
 • License plate or vehicle number method 
 • Interview technique 
 • Elevated observations 
 • Photographic technique 
Origin and Destination Studies
 1. Methods of conducting O and D studies: 
 • Road side interview method 
 • License plate method 
 • Return post card method 
 • Tag on car method 
 • Home interview method 
 • Work place interview method 
 2. Applications:
 • Planning and design of highways (routes and 
terminals)
 • To improve existing road network. 
 • Estimation of future traffic needs. 
 • To locate terminal and intermediates stops and 
plan out facilities for public transport. 
 • To locate new bridge if demanded and planning for 
interchanges.
 3. Presentation of O–D data:
 • ‘O’ and ‘D’ tables showing number of trips between 
different zones.
 • Desire lines: (Graphical representation) These are 
straight line connecting origin with destination in 
different areas. 
  Width of the line represents number of trips.
  Desire line density map enables to decide the 
interest of road users and helps to find the necessity 
of new road link, derision, by pass or a new bridge. 
 • Pie charts: Circles are drawn and diameter is pro-
portional to number of trips.
 • Contour lines: Similar to topographic contours 
shape of contours would indicate general traffic 
needs of the area.
Accident Studies
 • Accident records are maintained giving all particulars of 
the accidents, location and other details. 
 • Collision diagrams are drawn which show the path of 
vehicle and pedestrians involved in accident. These dia-
grams are useful to compare the accident pattern before 
and after the remedial measures are taken. 
 • Individual and statistical analysis of accidents are done. 
Traffic Flow Characteristics
The basic traffic manoeuvres in a traffic stream are 
 1. Diverging 
 2. Merging
 3. Crossing manoeuvres 
Left
DIVERGING
Right (*)
Left
MERGING
Right (*)
Part III_Unit 11_Chapter 04.indd   976 5/31/2017   6:44:15 PM
Page 4


 ? Introduction
 ? Traf? c studies
 ? Traf? c speed studies
 ? Speed and delay studies
 ? Origin and destination studies
 ? Accident studies
 ? Traf? c ? ow characteristics
 ? Speed–Flow–Density relations
 ? Derived characteristics
 ? Traf? c stream models
 ? Traf? c regulation and control
 ? Types of traf? c signals
 ? Design of rotary
 ? Road intersections
 ? Interchanges
 ? Parking
 ? Highway lighting
CHAPTER HIGHLIGHTS
Chapter 4
Introduction
Traffi  c Engineering is that branch of engineering which 
deals with the improvement of traffi  c performance on road 
network and terminals through systematic traffi  c studies, 
scientifi c analysis and engineering applications. It also 
includes planning and geometric design on one hand and 
regulation and control on other.
Traffic Studies
Traffi  c Volume Study ( q)
Traffi  c volume or fl ow is expressed as number of vehicles 
that pass across a given transverse line of the road during 
unit time. In this study the type and the number of vehicles 
are counted either manually or by using mechanical or auto-
matic counters.
Method of Volume Count
Manual Count (Best Method) By manual count, it gives 
data which cannot be collected by mechanical or automatic 
counts. 
 1. Classifi cation of diff erent vehicles and their counts at 
required time. 
 2. Direction wise and turning movements at 
intersections. 
 3. Details of pedestrian volume counts. 
Draw back: It is not practicable to carry out manual count 
of diff erent vehicle classes during 24 hours of a day for 365 
days.
Automatic Counters
 1. Photo electric cell 
 2. Electrical method 
 3. Pneumatic method 
 • Advantage of it is that it can work throughout the day and 
night for desired period, recording exact time at which 
each vehicle crosses. Lane occupancy and speeds of
diff erent vehicles can be recorded. 
 • Drawback is that it cannot classify and record every
vehicle type and details of turning movements and also 
pedestrian fl ow. 
 • Suitable for long counts and for permanent installations
 • In view of variety of vehicles, based on the vehicle 
speed, space and characteristics a number is assigned to 
the vehicle in comparison to car termed as ‘Equivalent 
passenger car unit’ (PCU). Using the values in table, the 
Traffi  c Engineering
Part III_Unit 11_Chapter 04.indd   974 5/31/2017   6:44:15 PM
Chapter 4 
¦
 Traffic Engineering | 3.975
traffic can be expressed uniformly in terms of passenger 
car unit (PCU) per hour or per day 
Vehicle Type PCU
Passenger car, Tempo, Jeep, Auto 
rickshaw, Van, Agricultural tractor 
1
Bus, Truck, Agricultural tractor trailer 3
Motor cycle, scooter and cycle 0.5
Cycle rickshaw 1.5
Horse drawn vehicle 4
Bullock cart 8
Small bullock cart 6
Hand cart 6
 • As per IRC, traffic counts are taken twice in a year 
for a village. One during peak season of harvesting 
and marketing and other during the lean season. It 
shall be taken for 7 consecutive days and 24 hours 
each day (24 × 7).
Presentation of Traffic Volume Data
 1. Average annual daily traffic (AADT): It is the 
average daily (24 hours) traffic volume recorded for 
all the 365 days of the year.
 2. Average daily traffic (ADT): Average daily traffic 
volume recorded for a period less than a year 
(6 months, a season, a month, a weak, or 2 days)
 3. Trend charts: Shows volume trends over period of 
years. It is used in estimating the rate of growth and 
for planning future expansion, design and regulation. 
 4. V ariation charts: V ariation of flow (hourly, daily and 
seasonally) are prepared. These help in facilitating or 
regulating traffic at peak traffic periods.
 5. Traffic flow maps: Traffic flow lines along the routes 
are drawn. The thickness of the lines represents the 
traffic volume to any desired scale. 
 6. V olume flow diagram at intersections to a scale.
 7. 30th highest hourly volume: The hourly volume 
that will be exceeded only 29 times in a year and all 
other hourly volumes of the year will be less than this 
volume.
 • Congestion only during 29 hours in the year. 
 • Generally taken as hourly volume for design. 
 • Highway facilities are designed for this volume, as 
it is satisfactory from both facility and economic 
considerations.
 8. Peak hour factor: It is in terms of traffic volume 
during peak hour and it is expressed as percentage 
of AADT. This peak hour factor is used in the 
design of transportation facilities of urban transport 
infrastructure projects.
Traffic Speed Studies
 1. Spot speed: It is the instantaneous speed of a vehicle 
at specified cross section or location. 
 2. Average speed: It is the average of spot speeds of all 
the vehicles passing a given point on the highway.
 3. Running speed: It is the average speed maintained 
by a vehicle over a particular stretch of road (delays 
are excluded). 
 4. Travel/overall/journey speed: It is the effective 
speed of travel all along the route between two 
terminals (delays are included).
Methods for Spot Speeds
 • Radar speed meter method (speed gun and is most 
efficient and easy to use).
 • Electronic meter method. 
 • Photo-electric meter.
 • Enoscope/mirror box method.
 ¦ Mirrors are arranged at 45°.
 ¦ Simple cheap and easy to use. 
 ¦ Difficult for heavy multilane traffic and slow method 
 • Time lapse method 
 • Pressure contact strip method 
Presentation of Spot Speed Data
Speed (kmph)
Frequency (%)
Modal speed
Frequency distribution diagram 
 • Modal speed: speed at which greatest number of vehi-
cles travel. It is peak of frequency distribution curve.
Speed (kmph)
15%
85%
98%
Cumulative frequency (%)
98th Percentile speed
85th Percentile speed
Cumulative speed distribution diagram
Part III_Unit 11_Chapter 04.indd   975 5/31/2017   6:44:15 PM
3.976 | Part III 
¦
 Unit 11 
¦
 Transportation Engineering
 • 98th percentile speed: For the purpose of highway 
geometric design (design speed).
 • 85th percentile speed: The speed at or below which 85% 
of vehicles are passing a point on the highway or only 
15% of vehicles exceed the speed at that point (upper 
speed limit). 
  This is adopted for safe seed limit at this zone.
 • 15th percentile speed: The speed value which is used as 
minimum speed on major highways.
 • 50th percentile speed: The middle or 50th percentile 
of spot speed is called median/medium speed (median 
speed).
Types of Spot Speeds 
 1. Space mean speed (V
s
): It is the average speed of 
vehicles in a certain road length at any specified time 
period. This is the harmonic mean of spot speeds. 
 2. Time mean speed (V
t
): It is the average speed of 
vehicles passing a certain point over some specified 
time period on a highway.
   It is the average/mean of spot speeds:
 • (Average travel time of all vehicles)  
= 
1
Space mean speed
.
 • Space mean speed is slightly lower than time mean 
speed under typical speed conditions on rural 
highways
V
t
 = V
s
 +
s
2
V
s
	 	s
2
 = Standard deviation. 
  Both space and time mean speeds will be equal only 
when all vehicles are travelling at the same speed. 
Speed and Delay Studies
 • Floating car or riding check method 
 • License plate or vehicle number method 
 • Interview technique 
 • Elevated observations 
 • Photographic technique 
Origin and Destination Studies
 1. Methods of conducting O and D studies: 
 • Road side interview method 
 • License plate method 
 • Return post card method 
 • Tag on car method 
 • Home interview method 
 • Work place interview method 
 2. Applications:
 • Planning and design of highways (routes and 
terminals)
 • To improve existing road network. 
 • Estimation of future traffic needs. 
 • To locate terminal and intermediates stops and 
plan out facilities for public transport. 
 • To locate new bridge if demanded and planning for 
interchanges.
 3. Presentation of O–D data:
 • ‘O’ and ‘D’ tables showing number of trips between 
different zones.
 • Desire lines: (Graphical representation) These are 
straight line connecting origin with destination in 
different areas. 
  Width of the line represents number of trips.
  Desire line density map enables to decide the 
interest of road users and helps to find the necessity 
of new road link, derision, by pass or a new bridge. 
 • Pie charts: Circles are drawn and diameter is pro-
portional to number of trips.
 • Contour lines: Similar to topographic contours 
shape of contours would indicate general traffic 
needs of the area.
Accident Studies
 • Accident records are maintained giving all particulars of 
the accidents, location and other details. 
 • Collision diagrams are drawn which show the path of 
vehicle and pedestrians involved in accident. These dia-
grams are useful to compare the accident pattern before 
and after the remedial measures are taken. 
 • Individual and statistical analysis of accidents are done. 
Traffic Flow Characteristics
The basic traffic manoeuvres in a traffic stream are 
 1. Diverging 
 2. Merging
 3. Crossing manoeuvres 
Left
DIVERGING
Right (*)
Left
MERGING
Right (*)
Part III_Unit 11_Chapter 04.indd   976 5/31/2017   6:44:15 PM
Chapter 4 
¦
 Traffic Engineering | 3.977
Left
WEAVING
Right (*)
CROSSING (*)
 • The manoeuvres with (*) represent conflicting traffic. 
 • Merging from and diverging to left do not cause much 
problem.
 • Diverging to and merging from right create conflict points.
 • Weaving manoeuvre, i.e., merging and diverging in a 
short stretch always creates a conflict point. 
 • Crossing also causes a problem where road at a time 
results in considerable reduction in flow on one capacity 
of the intersection.
Number of Conflict Points on Cross Roads 
of Different Number of Lanes
Number of Lanes Number of Potential Conflicts
Road A Road B
Both 
Roads 
Two Way
A-One 
Way 
B-Two 
Way
Both 
Roads 
One Way
2 2 24 11 6
2 4 32 17 10
4 4 44 25 18
Conflicts on cross roads with one road as one way:
Major conflicts crossing = 7 
Minor conflicts = 4
Merging total = 11
Speed–Flow–Density Relations
 1. Traffic density (K): The number of vehicles 
occupying a unit length (1 km) of a lane of roadway 
at a given instant 
  S ? 1 vehicle 
  (1 km) = 1000 ? K =	?
  K = 
1000
S
 = Number of vehicle/km [per lane at an 
instance of time] 
  S = Average c/c distance between two successive 
vehicles.
  
 1.  Under free flow condition (with design speed) 
or road empty without traffic condition, density 
is least. 
 2.  A traffic jam condition before red signal (if all 
vehicles stopped on road) maximum density 
will be achieved called ‘Jam Density’ (K
max 
). 
NOTES
 2. Traffic volume 
Vehicle
hour
?
?
?
?
?
?
 ? 
q = V · K 
  Where 
   V = Speed of vehicles (km/hour)
   K = Traffic density (veh/km) 
 3. Capacity (C or q
max
): 
C = q
max
 = V 
1000
s
?
?
?
?
?
?
  It is the ability of the roadway to allow maximum 
traffic flow or traffic volume per unit time.
  Where 
   V = Design speed (km/h)
   S = c/c spacing of vehicles 
(a) Basic/theoretical/ideal capacity: The maximum 
number of vehicles that pass a given point on a 
lane or road way during one hour under ideal 
road way and traffic conditions. 
 S = L + 0.7 V in m
 S
L
 = Lag distance = 0.7 n 
 ? = Design speed (m/s) 
 L = Length of rigid wheel base (m) = 6.1 m
 t = Reaction time of ideal driver (0.7 second)
 • Two roads of same features will be same basic 
capacities 
(b) Practical/design capacity: The maximum 
number of vehicles that can pass a given 
point so that they may not cause unreasonable 
delay, hazards and restrictions to driver’s free-
dom to manoeuvres under the prevailing road 
Part III_Unit 11_Chapter 04.indd   977 5/31/2017   6:44:16 PM
Page 5


 ? Introduction
 ? Traf? c studies
 ? Traf? c speed studies
 ? Speed and delay studies
 ? Origin and destination studies
 ? Accident studies
 ? Traf? c ? ow characteristics
 ? Speed–Flow–Density relations
 ? Derived characteristics
 ? Traf? c stream models
 ? Traf? c regulation and control
 ? Types of traf? c signals
 ? Design of rotary
 ? Road intersections
 ? Interchanges
 ? Parking
 ? Highway lighting
CHAPTER HIGHLIGHTS
Chapter 4
Introduction
Traffi  c Engineering is that branch of engineering which 
deals with the improvement of traffi  c performance on road 
network and terminals through systematic traffi  c studies, 
scientifi c analysis and engineering applications. It also 
includes planning and geometric design on one hand and 
regulation and control on other.
Traffic Studies
Traffi  c Volume Study ( q)
Traffi  c volume or fl ow is expressed as number of vehicles 
that pass across a given transverse line of the road during 
unit time. In this study the type and the number of vehicles 
are counted either manually or by using mechanical or auto-
matic counters.
Method of Volume Count
Manual Count (Best Method) By manual count, it gives 
data which cannot be collected by mechanical or automatic 
counts. 
 1. Classifi cation of diff erent vehicles and their counts at 
required time. 
 2. Direction wise and turning movements at 
intersections. 
 3. Details of pedestrian volume counts. 
Draw back: It is not practicable to carry out manual count 
of diff erent vehicle classes during 24 hours of a day for 365 
days.
Automatic Counters
 1. Photo electric cell 
 2. Electrical method 
 3. Pneumatic method 
 • Advantage of it is that it can work throughout the day and 
night for desired period, recording exact time at which 
each vehicle crosses. Lane occupancy and speeds of
diff erent vehicles can be recorded. 
 • Drawback is that it cannot classify and record every
vehicle type and details of turning movements and also 
pedestrian fl ow. 
 • Suitable for long counts and for permanent installations
 • In view of variety of vehicles, based on the vehicle 
speed, space and characteristics a number is assigned to 
the vehicle in comparison to car termed as ‘Equivalent 
passenger car unit’ (PCU). Using the values in table, the 
Traffi  c Engineering
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traffic can be expressed uniformly in terms of passenger 
car unit (PCU) per hour or per day 
Vehicle Type PCU
Passenger car, Tempo, Jeep, Auto 
rickshaw, Van, Agricultural tractor 
1
Bus, Truck, Agricultural tractor trailer 3
Motor cycle, scooter and cycle 0.5
Cycle rickshaw 1.5
Horse drawn vehicle 4
Bullock cart 8
Small bullock cart 6
Hand cart 6
 • As per IRC, traffic counts are taken twice in a year 
for a village. One during peak season of harvesting 
and marketing and other during the lean season. It 
shall be taken for 7 consecutive days and 24 hours 
each day (24 × 7).
Presentation of Traffic Volume Data
 1. Average annual daily traffic (AADT): It is the 
average daily (24 hours) traffic volume recorded for 
all the 365 days of the year.
 2. Average daily traffic (ADT): Average daily traffic 
volume recorded for a period less than a year 
(6 months, a season, a month, a weak, or 2 days)
 3. Trend charts: Shows volume trends over period of 
years. It is used in estimating the rate of growth and 
for planning future expansion, design and regulation. 
 4. V ariation charts: V ariation of flow (hourly, daily and 
seasonally) are prepared. These help in facilitating or 
regulating traffic at peak traffic periods.
 5. Traffic flow maps: Traffic flow lines along the routes 
are drawn. The thickness of the lines represents the 
traffic volume to any desired scale. 
 6. V olume flow diagram at intersections to a scale.
 7. 30th highest hourly volume: The hourly volume 
that will be exceeded only 29 times in a year and all 
other hourly volumes of the year will be less than this 
volume.
 • Congestion only during 29 hours in the year. 
 • Generally taken as hourly volume for design. 
 • Highway facilities are designed for this volume, as 
it is satisfactory from both facility and economic 
considerations.
 8. Peak hour factor: It is in terms of traffic volume 
during peak hour and it is expressed as percentage 
of AADT. This peak hour factor is used in the 
design of transportation facilities of urban transport 
infrastructure projects.
Traffic Speed Studies
 1. Spot speed: It is the instantaneous speed of a vehicle 
at specified cross section or location. 
 2. Average speed: It is the average of spot speeds of all 
the vehicles passing a given point on the highway.
 3. Running speed: It is the average speed maintained 
by a vehicle over a particular stretch of road (delays 
are excluded). 
 4. Travel/overall/journey speed: It is the effective 
speed of travel all along the route between two 
terminals (delays are included).
Methods for Spot Speeds
 • Radar speed meter method (speed gun and is most 
efficient and easy to use).
 • Electronic meter method. 
 • Photo-electric meter.
 • Enoscope/mirror box method.
 ¦ Mirrors are arranged at 45°.
 ¦ Simple cheap and easy to use. 
 ¦ Difficult for heavy multilane traffic and slow method 
 • Time lapse method 
 • Pressure contact strip method 
Presentation of Spot Speed Data
Speed (kmph)
Frequency (%)
Modal speed
Frequency distribution diagram 
 • Modal speed: speed at which greatest number of vehi-
cles travel. It is peak of frequency distribution curve.
Speed (kmph)
15%
85%
98%
Cumulative frequency (%)
98th Percentile speed
85th Percentile speed
Cumulative speed distribution diagram
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 • 98th percentile speed: For the purpose of highway 
geometric design (design speed).
 • 85th percentile speed: The speed at or below which 85% 
of vehicles are passing a point on the highway or only 
15% of vehicles exceed the speed at that point (upper 
speed limit). 
  This is adopted for safe seed limit at this zone.
 • 15th percentile speed: The speed value which is used as 
minimum speed on major highways.
 • 50th percentile speed: The middle or 50th percentile 
of spot speed is called median/medium speed (median 
speed).
Types of Spot Speeds 
 1. Space mean speed (V
s
): It is the average speed of 
vehicles in a certain road length at any specified time 
period. This is the harmonic mean of spot speeds. 
 2. Time mean speed (V
t
): It is the average speed of 
vehicles passing a certain point over some specified 
time period on a highway.
   It is the average/mean of spot speeds:
 • (Average travel time of all vehicles)  
= 
1
Space mean speed
.
 • Space mean speed is slightly lower than time mean 
speed under typical speed conditions on rural 
highways
V
t
 = V
s
 +
s
2
V
s
	 	s
2
 = Standard deviation. 
  Both space and time mean speeds will be equal only 
when all vehicles are travelling at the same speed. 
Speed and Delay Studies
 • Floating car or riding check method 
 • License plate or vehicle number method 
 • Interview technique 
 • Elevated observations 
 • Photographic technique 
Origin and Destination Studies
 1. Methods of conducting O and D studies: 
 • Road side interview method 
 • License plate method 
 • Return post card method 
 • Tag on car method 
 • Home interview method 
 • Work place interview method 
 2. Applications:
 • Planning and design of highways (routes and 
terminals)
 • To improve existing road network. 
 • Estimation of future traffic needs. 
 • To locate terminal and intermediates stops and 
plan out facilities for public transport. 
 • To locate new bridge if demanded and planning for 
interchanges.
 3. Presentation of O–D data:
 • ‘O’ and ‘D’ tables showing number of trips between 
different zones.
 • Desire lines: (Graphical representation) These are 
straight line connecting origin with destination in 
different areas. 
  Width of the line represents number of trips.
  Desire line density map enables to decide the 
interest of road users and helps to find the necessity 
of new road link, derision, by pass or a new bridge. 
 • Pie charts: Circles are drawn and diameter is pro-
portional to number of trips.
 • Contour lines: Similar to topographic contours 
shape of contours would indicate general traffic 
needs of the area.
Accident Studies
 • Accident records are maintained giving all particulars of 
the accidents, location and other details. 
 • Collision diagrams are drawn which show the path of 
vehicle and pedestrians involved in accident. These dia-
grams are useful to compare the accident pattern before 
and after the remedial measures are taken. 
 • Individual and statistical analysis of accidents are done. 
Traffic Flow Characteristics
The basic traffic manoeuvres in a traffic stream are 
 1. Diverging 
 2. Merging
 3. Crossing manoeuvres 
Left
DIVERGING
Right (*)
Left
MERGING
Right (*)
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Left
WEAVING
Right (*)
CROSSING (*)
 • The manoeuvres with (*) represent conflicting traffic. 
 • Merging from and diverging to left do not cause much 
problem.
 • Diverging to and merging from right create conflict points.
 • Weaving manoeuvre, i.e., merging and diverging in a 
short stretch always creates a conflict point. 
 • Crossing also causes a problem where road at a time 
results in considerable reduction in flow on one capacity 
of the intersection.
Number of Conflict Points on Cross Roads 
of Different Number of Lanes
Number of Lanes Number of Potential Conflicts
Road A Road B
Both 
Roads 
Two Way
A-One 
Way 
B-Two 
Way
Both 
Roads 
One Way
2 2 24 11 6
2 4 32 17 10
4 4 44 25 18
Conflicts on cross roads with one road as one way:
Major conflicts crossing = 7 
Minor conflicts = 4
Merging total = 11
Speed–Flow–Density Relations
 1. Traffic density (K): The number of vehicles 
occupying a unit length (1 km) of a lane of roadway 
at a given instant 
  S ? 1 vehicle 
  (1 km) = 1000 ? K =	?
  K = 
1000
S
 = Number of vehicle/km [per lane at an 
instance of time] 
  S = Average c/c distance between two successive 
vehicles.
  
 1.  Under free flow condition (with design speed) 
or road empty without traffic condition, density 
is least. 
 2.  A traffic jam condition before red signal (if all 
vehicles stopped on road) maximum density 
will be achieved called ‘Jam Density’ (K
max 
). 
NOTES
 2. Traffic volume 
Vehicle
hour
?
?
?
?
?
?
 ? 
q = V · K 
  Where 
   V = Speed of vehicles (km/hour)
   K = Traffic density (veh/km) 
 3. Capacity (C or q
max
): 
C = q
max
 = V 
1000
s
?
?
?
?
?
?
  It is the ability of the roadway to allow maximum 
traffic flow or traffic volume per unit time.
  Where 
   V = Design speed (km/h)
   S = c/c spacing of vehicles 
(a) Basic/theoretical/ideal capacity: The maximum 
number of vehicles that pass a given point on a 
lane or road way during one hour under ideal 
road way and traffic conditions. 
 S = L + 0.7 V in m
 S
L
 = Lag distance = 0.7 n 
 ? = Design speed (m/s) 
 L = Length of rigid wheel base (m) = 6.1 m
 t = Reaction time of ideal driver (0.7 second)
 • Two roads of same features will be same basic 
capacities 
(b) Practical/design capacity: The maximum 
number of vehicles that can pass a given 
point so that they may not cause unreasonable 
delay, hazards and restrictions to driver’s free-
dom to manoeuvres under the prevailing road 
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conditions. This is of primary interest to the 
designers 
 S = L + SSD
 SSD = Stopping sight distance 
 L = Length of rigid wheel base (6.1 m)
Derived Characteristics
 • Derived parameters from speed (V), flow (q), density (k) 
are: 
 1. Time headway (Ht): It is the time gap between two 
successive vehicles crossing a section of a road, 
H
t
 = 
1
q
 = h/veh
 • Minimum time headway, H
t
 = 
1
q
max
 = 
1
c
h/ 
veh 
 • Maximum theoretical capacity, q
max
 = C = 
3600
H
t
 
s…(H
t
 in seconds) 
 2. Space/distance headway (S): It is the average 
distance between two successive vehicles on the road 
or it is reciprocal of traffic density,
S = 
1
K
 km/veh
 • Maximum space headway = 
1
K
max
 (At Jam 
density)
Traffic Stream Models
Green Shield’s macroscopic stream model: He assumed a 
linear speed–density relationship. 
 V = V
f
 1-
?
?
?
?
?
?
?
?
K
K
j
 Q = VK = V
f
 
1-
?
?
?
?
?
?
?
?
K
K
j
K
q
V
K
V = 0
q
max
V
max
V
q
?
?
?
?
?
? 2
V
max
V
max
 = Free flow speed/free mean speed
q
max
K
max
K
q
?
?
?
?
?
? 2
K
max
K
max
 = Maximum Jam density
K
max
K
V
V
max
q
max
 = 
V
max
2
?
?
?
?
?
?
 
K
max
2
?
?
?
?
?
?
SOLVED EXAMPLES
Example 1
The free mean speed on a roadway is found to be 70 km/h. 
Under stopped condition the average spacing between vehi-
cles is 6.9 m. Determine the capacity of flow (in vehicles/
km).
(A) 120 (B) 145 
(C) 160 (D) 175
Solution
Spacing between vehicles S = 6.9 m
 Jam density k
max
 = 
1000
s
 = 
1000
69 .
 k
max
 = 145 vehicle/km
Hence, the correct answer is option (B).
Example 2
Estimate the theoretical capacity of a traffic lane with 
one way traffic flow at a stream speed of 50 km/h. If the 
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