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CE 415Highway and Traffic Engineering

Lecture 3

Traffic Stream Characteristics

Engr. Amjad Khan

MS Transportation Engineering (NUST)

Traffic Streams

Traffic Streams Composition

Individual drivers (behavior)

Individual vehicles (characteristics)

Physical elements of the roadway/ environment

Individual vehicles within the traffic stream do not behave in

exactly the same manner

Human behavior being non-uniform – traffic stream is also non-

uniform

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Traffic Streams

No two traffic streams will behave in exactly the same way, even in

similar circumstances, because driver behavior varies with local

characteristics and driving habits

Dealing with traffic – Involves an element of variability

Flow of water through channel and pipes of defined characteristics

behave in entirely predictable fashion.

A given flow of traffic through streets and highway of defined

characteristics will vary both in time and location.

Traffic Streams

While exact characteristics vary – but consistent range of driver and

traffic stream behavior can be established

Example: 60 mph design speed, driver move at 45 – 65 mph (range)

Quantitative Description of Traffic Stream

Understanding inherent variability in traffic stream characteristics

Define normal range of behavior

Traffic stream has some parameters – which must be

measured/defined

Knowledge of traffic stream parameters help in

understanding/predicting traffic stream characteristics

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Types of Facilities

Uninterrupted Flow

No external interruptions to traffic stream

Pure uninterrupted flow – primarily on freeways (having full

control of access., no intersection at grade, traffic signal, STOP

or YIELD signs)

It can also exists on highways (mostly rural highways) – longer

distance between fixed interruptions- at least 2 miles

It is type of facility not the quality of operations on that facility.

Peak hours break down on freeways due to internal interactions

of traffic stream – still uninterrupted flow – cause is internal

Types of Facilities Interrupted Flow

Fixed external interruptions into design and operations.

Traffic signal is an interruption.

Traffic signs, (STOP, YIELD), unsignalized at-grade

intersections, driveways, curb parking maneuvers, other land

access operations.

Major difference between two facilities is impact of time.

Uninterrupted flow - Physical facility is available all time

Interrupted flow - Movement is barred by “red” signals.

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Counting of Traffic Volume

Mechanical Counters Manual Counts

These may be either fixed (Permanent) type or portable type. The mechanical counter can automatically record the total number of vehicles crossing a section of road in a desired period. The working may be by the effect of impulses or stimuli caused by the traffic movements on a pneumatic hose placed across the roadway or by using any other type of sensor.

Main advantage of mechanical counter is that it can work throughout the day and night, which may not be practicable in manual counting.

Main drawback is that it is not possible to get volumes of various classes of traffic in the stream.

Counting of Traffic Volume

Mechanical Counters

Pneumatic road tubes

inductive loop

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Counting of Traffic Volume

Manual Counts

This method employs a field team to record traffic volume on the prescribed record sheets. By this method it is possible to obtain data which can not be collected by mechanical counters, such as vehicle classification, turning movements etc.

It is not possible to count for all the 24 hours of the day and all the days round the year.

Mechanical Counters

Traffic Stream Parameters Macroscopic - Describes traffic stream a whole:

Volume or Flow rate

Speed

Density

Microscopic - Describes behavior of individual vehicle or pairs:

Speed of individual vehicle

Headway

Spacing

Fundamental Parameters - speed, flow and density

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Traffic Stream Parameters

Traffic volume:

Traffic volume is the number of vehicles that pass a given point on the highway in a given period of time. Period of time may be year, month, day, hour or sub-hour.

Daily volumes are used to

establish trends over time and for

general planning purposes.

Detailed design or control decisions

require knowledge of hourly

volumes for the peak hour(s) of

the day.

Traffic Stream Parameters

Flow Rate or Rate of Flow:

Flow rate is the equivalent hourly rate at which vehicles pass over a given point or section of a lane or roadway during a given time interval of less than 1 hour, usually 15 min.

Flow Rate = Peak Volume × no of intervals per 1 hour

A volume of 200 vehicles observed over a 15 minute period may be expressed as a rate of 200×4 = 800 vehicles/hour, even though 800 vehicles would not be observed if the full hour was counted. The 800 vehicles/hour becomes a rate of flow that exist for a 15 minute interval.

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Example for Volume and FlowTraffic volume data has been collected for 15 min time interval as shown, find total hourly volume, max flow rate?

Solution:

Volume = 250+350+300+200 = 1100 vehicles or veh/hr

Flow rate = peak volume × number of intervals per 1 hour

= 350 * 4 = 1400 veh/hr

Example for Volume and Flow

Time Interval Volume for Time Interval Rate of Flow for Time Interval5:00 - 5:15 PM 1000 40005:15 - 5:30 PM 1100 44005:30 - 5:45 PM 1200 48005:45 - 6:00 PM 900 36005:00 - 6:00 PM 4200

The hourly volume = 4200 vehicles

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Traffic VolumesDaily Volumes:Volumes can be expressed in terms of annual, daily, hourly, or sub-hourly periods. Traffic volume could be expressed as:1- Average Annual Daily Traffic (AADT) is the average of 24-hour counts collected every day of the year.

Consider Seasonal Variation of daily traffic within a year 2- Average Daily Traffic (ADT) is the average of 24-hour counts collected over a number of days greater than one but less than a year.

Consider day to day variation of traffic (say, within a week)

Traffic VolumesDaily Volumes:3. Average annual weekday traffic (AAWT) The average 24-hour volume occurring on weekdays over a full 365-day year (usually 260).

4. Average weekday traffic (AWT) The average 24-hour weekday volume at a given location over a defined time period less than one year; a common application is to measure an AWT for each month of the year.

260

year ain on weekday vehilcesofNumber AAWT

weekdaysofNumber

year) a than (less son weekday vehilcesofNumber AWT

Units: Vehicle/day Not differentiated by direction or lane but are total for entire

facility at point of interest

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Traffic Volumes Example for AADT

Traffic Volumes

Hourly Volumes

Daily volumes, while useful for planning purposes, cannot be

used alone for design or operational analysis purposes.

Volume varies considerably over the 24 hours of the day, with

periods of maximum flow occurring during morning or evening

rush hours.

Peak Hour Volume - The single hour of the day that has the

highest hourly volume is referred to as “Peak Hour volume”.

The peak-hour volume is generally stated as a directional

volume (i.e., each direction of flow is counted separately).

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Traffic Volumes

Hourly Volumes

Highways must be designed to adequately serve the peak-hour

traffic volume in the peak direction of flow.

In case of significant directional disparity, the concept of

reversible lanes is sometimes useful

In design, peak-hour volumes are sometimes estimated from

projections of the AADT (documented trend/forecasting

models)

Reversible lanes

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Traffic Volumes

Hourly Volumes

Directional Design Hourly Volumes - AADTs are converted to

a peak-hour volume in the peak direction of flow

)(*)(* flowPeakdirDPeakhrKAADTDDHV

K = proportion of daily traffic occurring during peak hour

D = proportion of peak hour traffic travelling in peak direction of flow

For design, the K factor often represents the proportion of AADT occurring during the 30th peak hour of the year

Traffic Volumes Example Consider the case of a rural highway that has a 20-year

forecast of AADT of 30,000 veh/day, for given highway, the Kfactor ranges from 0.15 to 0.25, and the D factor ranges from0.65-0.80.

)(*)(* flowPeakdirDPeakhrKAADTDDHV

hvehhighDDHV /000,680.*25.*000,30)(

hvehlowDDHV /925,265.*15.*000,30)(

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Traffic Volumes Subhourly Volumes

Problem: A facility may have sufficient capacity to serve the peak-

hour demand, but short term peaks of flow within the hour may exceed

capacity and create a breakdown

Suppose Capacity of facility is 4200 vehicles per hour

Time Interval Volume for Time Interval Departing Vehicles Queue Size5:00 - 5:15 PM 1000 1050 05:15 - 5:30 PM 1100 1050 0+1100-1050 = 505:30 - 5:45 PM 1200 1050 50+1200-1050 = 2005:45 - 6:00 PM 900 1050 200+900-1050 = 50

Queue exist for 3 out of 4 -15 minutes within the peak hour.

Due to such negative impacts – the design is done for max. rate of flow period within peak hour

Traffic Volumes……Subhourly Volume

Peak Hour Factor (PHF) - The relationship between the hourly volume and the maximum rate of flow within the hour is defined by the peak hour factor, as follows

hour within min volume 15 maximum x 4

umeHourly vol

Flow of rate Max.

umeHourly vol PHF OR

Time Interval Volume for Time Interval Rate of Flow for Time Interval

5:00 - 5:15 PM 1000 40005:15 - 5:30 PM 1100 44005:30 - 5:45 PM 1200 48005:45 - 6:00 PM 900 36005:00 - 6:00 PM 4200

875.012004

4200 PHF

875.04800

4200 PHF

OR

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Traffic Volumes……Subhourly Volume

Peak Hour Factor (PHF)

Value of PHF varies from 0.25 – 1.0

General range – 0.70 - 0.98

Maximum rate of flow within an hour can be estimated knowing, PHF and hourly volume

PHF

V(h) v

Where

v=Maximum rate of flow within an hour

V (h) = Hourly volume

Knowing the PHF and hourly volume max. rate of flowwithin the hour (veh/h) can be estimated

Speed: It is considered as a quality measurement of travel

It is defined as the rate of motion in distance per unit of time.

where, v is the speed of the vehicle in m/s, d is distance traveled in meters and time t seconds.

Speed of different vehicles will vary with respect to time and space.

To represent these variation, several types of speed can be defined.

(1) spot speed, (2) Free flow speed (3) 85th Percentile speed

(4) time mean speed (5) space mean speed

Traffic Stream Parameters……..Speed

t

d v

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Spot Speed:

Spot speed is the instantaneous speed of a vehicle at a specified location.Spot speed can be used

to design the geometry of road (horizontal and vertical curves, superelevation etc.)

Location and size of signs, design of signals, safe speed, and speed zonedetermination and accident analysis

Free flow speed:

Free-flow-speed of a vehicle is defined as: The desired speed of driversin low volume conditions and in the absence of traffic control devices.The mean speed of passenger cars that can be maintained in low tomoderate flow rates on a uniform freeway segment under prevailingroadway and traffic conditions

The average speed of vehicles over an urban street segment w/osignalized intersections, under conditions of low volume

The average speed of passenger cars over a basic freeway or multilanehighway segment under conditions of low volume

Traffic Stream Parameters……..Speed

85th percentile speed:

Speed at which 85% of the traffic is travelling

Speed limit is commonly set at or below the 85th percentile

speed.

Traffic Stream Parameters……..Speed

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Time Mean Speed Arithmetic mean of the speeds of the vehicles passing a point

on a highway during an interval of time. (Point measure) The time mean speed is found by

Traffic Stream Parameters……..Speed

Space Mean Speed is the harmonic mean of the speeds of vehicles passing a point on

a highway during an interval of time. It is obtained by dividing the total distance traveled by two or more vehicles on a section of highway by the total time required by these vehicles to travel that distance. The space mean speed is found by

Traffic Stream Parameters……..Speed

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Traffic Stream Parameters……..Speed

Calculate TMS & SMS, Is the space-mean speed equal to the time-mean speed?

sftSMS

t

nx

nt

xSMS

ii

i

i

/4.50119

10006

sftTMS

n

tx

TMS i i

/6.506

0.500.506.525.450.506.55

Vehicle # Distance, d (ft) Travel Time, t (s) Speed (ft/s)

1 1,000 18 55.6

2 1,000 20 50.0

3 1,000 22 45.5

4 1,000 19 52.6

5 1,000 20 50.0

6 1,000 20 50.0

Total 6,000 119 303.6

Average 6000/6 = 1000 119/6 = 19.8 303.6/6 = 50.6

TMS = 50.6 ft/s

SMS = 1,000/19.8 = 50.4 ft/s

x

Traffic Stream Parameters…….Density Density

The number of vehicles occupying a given length of a lane or roadway at a particular instant

Unit of density is vehicles/mile (vpm, vpmpl, v/km, v/km/lane)

x = road length, N=Numbers of Vehs, D= density

x

N D

Unit length (1 mile or 1 km)

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Traffic Stream Parameters…….Density

Density

Density measures the proximity of vehicles in the stream which

in turn affects the freedom to maneuver and comfortable

driving.

Difficult to measure directly

Often computed with the help of speed and rate of flow

Also a measure of quality of traffic flow – measure of close

proximity

Traffic Stream Parameters……. Occupancy

Occupancy

Defined as the proportion of time that a detector (normally a

magnetic loop detector) is occupied or covered by a vehicle in

a defined time period.

Density is difficult to measure directly, modern detectors can

measure occupancy.

Occupancy is measured for a specific detector in a specific

lane.

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Traffic Stream Parameters…… Occupancy

Occupancy

Proportion of time that a detector is occupied or covered by a vehicle in a defined time period

Occupancy O

(ft)detector ofLength L

(ft) vehicleofLength Average L

(vpmpl)Density D

Where

5280

d

V

dV LL

OD Detector

Vehicle

Lv Ld

Traffic Stream Parameters…… Headway Headway or Time Headway

Time, in seconds (or in minutes), between two successivevehicles as they pass a point on the roadway, measured fromthe same common feature of both vehicles (e.g. the front axleor front bumper)

Time, usually expressed in seconds, between the passing ofthe front ends of successive vehicles/ transit units (movingalong the same lane in the same direction)

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Traffic Stream Parameters……Headway

Headway or Time Headway….. it is related to flow If all headways “h” in time period, “t”, over which flow has

been measured are added then,

�ℎ�

�

�

= �

But the flow is number of vehicles, N, measured in time “t”

v= �

�=

�

∑ ����

=�

����

�=3600

ℎ���

Where : v = Rate of flow (veh/hr/lane)h(avg) = average headway in lane, seconds

Traffic Stream Parameters…… Spacing Spacing or Distance Headway

The distance, in m (or in ft), between two successivevehicles in a traffic lane, measured from the samecommon feature of the vehicles (e.g. rear or front end/bumper)

distance (front-bumper to front-bumper) between twoconsecutive vehicles

Spacing

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Traffic Stream Parameters……Spacing

Spacing or Distance Headway….. it is related to density If all space headways in distance, “x”, over which density has

been measured are added then,

���

�

�

= �

But the density is number of vehicles, N, occupying distance “x”

D= �

�=

�

∑ ����

=�

����

� =5280

����

Where : D = density (veh/mile/lane)d(avg) = average spacing between vehicles in the lane, ft

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Traffic Stream Parameters……

Travel Time: Travel time is defined as the time taken to complete a journey.

As the speed increases, travel time required to reach thedestination also decreases and vice versa. Thus travel time isinversely proportional to the speed.

However, in practice, the speed of a vehicle fluctuates over timeand the travel time represents an average measure.

Travel Speed: The avg speed in m/hr (Km/hr), of a traffic steam,computed as the length of a highway segment divided by the avgtravel time of the vehicles traversing the segment.

Time Space Diagram

Time space diagram is a convenient tool in understanding the movement of vehicles

It shows the trajectory of vehicles in the form of a two dimensional plot.

Time space diagram can be plotted for a single vehicle as well as for multiple vehicles.

Time space diagram can be used to estimate different traffic parameters ( e.g. headway, spacing)

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Time Space Diagram

Dis

tan

ce, x

Time, t

First Vehicle Trajectory

Second Vehicle Trajectory

Space Headway s

Time Headway h

Spot Speed U

Figure shows a time-space diagram for six vehicles with distance plotted on the vertical axis and time on the horizontal axis. At time zero, vehicles 1, 2, 3, and 4 are at respective distances d1, d2, d3, and d4 from a reference point whereas vehicles 5 and 6 cross the reference point later at times t5 and t6, respectively.

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NumericalFigure shows vehicles traveling at constant speeds on a two-lane highway between sections X and Y with their positions and speeds obtained at an instant of time by photography. An observer located at point X observes the four vehicles passing point X during a period of T sec. The velocities of the vehicles are measured as 45, 45, 40, and 30 mi/h, respectively. Calculate the flow, density, time mean speed, and space mean speed

Fundamental Relationship

Flow (v), Density (D) and Speed (S) Relationship

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Flow (v), Density (D) and Speed (S) Relationship