Design Traffic Volume

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, or surface intersections. - It may also exist in long sections of rural highway between signalized intersectionsInterrupted flow facilities:- These facilities have external devices that periodically interrupt traffic flow (the principal device creating interrupted flow is the traffic signal)

Traffic volume is defined as the number of vehicles that pass a point on a highway, or a given lane or direction of a 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 the specified location.

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

- – that is the total number of vehicles passing the site in a year divided by 365

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

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

- This volume is of considerable interest where weekend traffic is light, so that averaging higher weekday volumes over 365 days would mask the impact of weekday traffic.

- Average Daily Traffic (ADT): is an average 24-hour traffic volume 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 which it 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

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

B. Hourly Volumes:◦ Season Change Daily Volumes Planning ◦ Hourly Design and operational analysis◦ Peak-Hour: The single hour of the day that has

the highest volume. ◦ This volume is not a constant value from day to

day, season to seasonThe traffic volume within the peak hour is of greatest

interest to traffic engineers in design or operational analysis.

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

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

- Highways must be designed to adequately serve the peak-hour traffic volume in the peak direction of flow.- Most operational analysis must address conditions existing during periods of peak traffic volume.

Peak hour volumes are sometimes estimated from projections of the AADT. It is referred to as the “directional design hour volume” (DDHV)

DDHV = AADT * K * D (* T)Where:

K – proportion of daily traffic occurring during the peak hourD – proportion of peak hour traffic traveling in the peak direction of flow

Used for several transportation analyses:◦ Functional classification of roads◦ Design of geometric characteristics of highways

(number of lanes)◦ Capacity analysis◦ Development of programs related to traffic operations ◦ Development of parking regulations

C. Sub hourly volumes and Rates of Flow Flow (q) is the equivalent hourly rate at which

vehicles pass a point on a highway during a time period less than 1 hour.

Minimum Time Interval = 15 minutes or 900 seconds (2000 Highway Capacity Manual)

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

It is often necessary to design facilities and analyze traffic conditions for a period of maximum rate of flow within the peak hour

veh/hn timeobservatio

nobservatio during vehiclesofnumber =qflow of rate

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=4600 5:30-5:45 PM 1250 1250*4=5000 5:45-6:00 PM 1000 1000*4=4000 For the hour 5:00-6:00 PM 4350 (veh/h)

A facility may have capacity adequate to serve the peak-hour demand, but short-term peaks of flow within the peak hour may exceed capacity, thereby creating a breakdown.

Peak-Hour Factor (PHF): is the ratio of the volume occurring during the peak hour to a maximum rate of flow during a given time period within the peak hour

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

Where,HV – Hourly VolumeV15 – Maximum 15 minute volume within

the hour

flow of rate maximumumehourly vol = PHF

15V4

HV

15V(60/15)

HV = PHF

Example of PHF:HV= 4350 vehiclesV15 = 1250 vehicles

= 0.87

NOTE:0.25 PHF 1.00, normal between 0.70 and 0.98Lower PHF indicates a greater degree of variation in flow during the peak-hour.

15V4

HV = PHF

012544350

= PHF

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DHV = Peak-Hour Volume PHFExamplePeak hour volume from previous = 375 vphPHF = 0.625DHV = 375 = 600 vph 0.625Note: the traffic you design for is the busiest

15 minutes during the peak hour … another way to think of it is 150 vehicles per 15 minutes = 600 vehicles per 60 minutes