design traffic volume

8/6/2019 Design Traffic Volume

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Design Traffic Volume


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Types:Uninterrupted flow facilities:- These facilities are those on which no external factors

cause periodic interruption to the traffic stream.- Example: freeways, limited-access facilities, where

there are no traffic signal, stop or yield signs, orsurface intersections.

- It may also exist in long sections of rural highwaybetween signalized intersections

Interrupted flow facilities:

- These facilities have external devices that periodicallyinterrupt traffic flow (the principal device creatinginterrupted flow is the traffic signal)


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Traffic volume is defined as the number of vehicles thatpass a point on a highway, or a given lane or direction ofa highway, during a specified time interval.

A. Daily volumes:-Average Annual Daily Traffic: (AADT):

- Average Annual Weekday Traffic (AAWT):- Average Daily Traffic (ADT):- Average Weekday Traffic (AWT):

Note: The unit is vehicles per day (vpd).

` Daily volumes are used to establish trends over time and

for planning purposes.` Daily volumes generally are not differentiated by

direction or lane but are totals for an entire facility at thespecified location.


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- Average Annual Daily Traffic (AADT): is the average 24 hourtraffic volume at a given location over a full 365-day year

- that is the total number of vehicles passing the site in a yeardivided by 365

- Average Annual Weekday Traffic (AAWT): is the average 24-hour traffic volume occurring on weekdays over a fullyear.

- AAWT is computed by dividing the total weekday trafficvolume for the year by 260.

- This volume is of considerable interest where weekend

traffic is light, so that averaging higher weekday volumesover 365 days would mask the impact of weekday traffic.


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- Average Daily Traffic (ADT): is an average 24-hour trafficvolume at a given location for some period of time less

than a year.

- While an AADT is for a full year, an ADT may be measured for

six months, a season, a month, a week, or as little as two

day. an ADT is a valid number only for the period over whichit was measure.

- Average Weekday Traffic (AWT): is an average 24-hour

traffic volume occurring on weekdays for some period of

time less than one year, such as for a month or a season.The relationship between AAWT and AWT is analogous to

that between AADT and ADT


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` AADT and AAWT are used for several transportation analyses:

Computation of accident rates in terms of 100 million vehicles miles

Establishment of traffic volume trends

Evaluation of the economic feasibility of highway projects

Development of freeway and major arterial street systems

Development of improvement and maintenance programs

- ADT and AWT are used for several transportation analyses:

- Measurement of current demand

- Evaluation of existing traffic flow


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B. Hourly Volumes: Season Change p Daily Volumes p Planning

Hourly p Design and operational analysis

Peak-Hour: The single hour of the day that has thehighest volume.

This volume is not a constant value from day to day,season to season

The traffic volume within the peak hour is of greatestinterest to traffic engineers in design or operationalanalysis.

Note: - The peak-hour volume is generally a directionalvolume that is, a volume in which the directions of flow are

separated.


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Source: www.ecn.purdue.edu/~darcy


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` Used for several transportation analyses: Functional classification of roads

Design of geometric characteristics of highways (number oflanes)

Capacity analysis

Development of programs related to traffic operations

Development of parking regulations


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C. Sub hourly volumes and Rates of Flow

Flow (q) is the equivalent hourly rate at which vehiclespass a point on a highway during a time period less than1 hour.

Minimum Time Interval = 15 minutes or900 seconds(2000 Highway Capacity Manual)

The quality of traffic flow is often related to short-termfluctuations in traffic demand

It is often necessary to design facilities and analyzetraffic conditions for a period of maximum rate offlow within the peak hour

veh/hn timeobservatio

nobservatioduringvehiclesofnumber=qflowofrate !


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Example of volumes and rate of flow

Time Volume Rate of flowinterval (vehicles) (vehicles/h)

5:00-5:15 PM 950 950*4=3800

5:15-5:30 PM 1150 1150*4=46005:30-5:45 PM 1250 1250*4=50005:45-6:00 PM 1000 1000*4=4000For the hour5:00-6:00 PM 4350 (veh/h)

A facility may have capacity adequate to serve the peak-hourdemand, but short-term peaks of flow within the peakhour may exceed capacity, thereby creating abreakdown.


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` Peak-Hour Factor (PHF): is the ratio of the volumeoccurring during the peak hour to a maximum rate offlow during a given time period within the peak hour

Usually, 15 minute periods of flow are used (2000Highway Capacity Manual)

Where,HV Hourly Volume

V15 Maximum 15 minute volume within the hour

flowofratemaximum

umehourly vol=PHF

15V4

HV

15V(60/15)

HV

=PHF


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Example of PHF:HV= 4350 vehiclesV15 = 1250 vehicles

= 0.87

NOTE:

0.25 e PHFe 1.00, normal between 0.70 and 0.98Lower PHF indicates a greater degree of variation inflow during the peak-hour.

154PHF

01254

4350PHF


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DHV = Peak-Hour VolumePHF

ExamplePeak hour volume from previous = 375 vphPHF = 0.625

DHV = 375 = 600 vph0.625Note: the traffic you design for is the busiest 15

minutes during the peak hour another way tothink of it is 150 vehicles per 15 minutes = 600

vehicles per 60 minutes

design traffic volume

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