Traffic Stream Characteristics

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Traffic Stream Characteristics
Traffic facilities are broadly separated into two principal categories. * Uninterrupted flow
* Interrupted flow
The major difference between the uninterrupted and interrupted flow facilities is the impact of time. On uninterrupted facilities, the physical facility is available to drivers and vehicles all at a time. On a given interrupted flow facility, the movement is periodically barred by “signal systems”. The signal timing, therefore, limits access to particular segments of the facility in time. The interrupted flow is more complex than the uninterrupted flow. Traffic Stream Parameters

Traffic stream parameters fall into two broad categories.
Microscopic parameters: describe traffic stream as a whole. Major macroscopic parameters are * Volume (rate of flow)
* Speed
* Density
Microscopic parameters: describe the behavior of individual vehicles within the traffic stream. Major microscopic parameters are * Speed of individual vehicles
* Headway
* Spacing
1. Flow or Volume (Q)
There are practically two ways of defining flow or volume on a road. One 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 specific time interval. In second method the measurement is carried out by counting the number of vehicles, nt, passing a particular point in one lane in a defined time period T. The flow Q expressed in vehicles/hour is given by Q=ntT

Flow is expressed in planning and design field taking a day as the measurement of time. Volume, Demand and Capacity

In practical terms, volume is what is, demand is what motorists would like to be, and capacity is the physical limit of what is possible. Volume: would be the number of vehicles counted passing the study location in the hour. Demand: would be the volume plus the vehicles of motorists wishing to pass the site during the study hour who were prevented from doing so by congestion. The latter would include motorists in queue waiting to reach the study location, motorists using alternative routes to avoid the congestion around the study location, and motorists deciding not to travel at all due to the existing congestion. Capacity: would be the maximum volume that could be accommodated by the highway at the study location. Types of volume measurements

Since there is considerable variation in the volume of traffic, several types of measurements of volume are commonly adopted which will average these variations into a single volume count to be used in many design purposes. 1) Average Annual Daily Traffic (AADT): The average 24-hour traffic volume at a given location over a full 365-day year, i.e. the total number of vehicles passing the site in a year divided by 365. 2) Average Annual Weekday Traffic (AAWT): The average 24-hour traffic volume occurring on weekdays over a full year. It is computed by dividing the total weekday traffic volume for the year by 260. 3) Average Daily Traffic (ADT): An average 24-hour traffic volume at a given location for some period of time less than a year. It may be measured for six months, a season, a month, a week, or as little as two days. An ADT is a valid number only for the period over which it was measured. 4) Average Weekday Traffic (AWT): 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. 5) Peak Hour Volume: The single hour of the day that has the highest hourly volume is referred to as the peak hour. The peak hour volume is generally referred as the directional volume (each direction of the flow is counted separately). 6) Peak Hour Factor (PHF): Peak flow rates and hourly volumes produce the peak-hour factor (PHF), the ratio of total hourly volume to the peak flow rate within the hour, can be computed as: PHF=hourly volumemax.rate of flow

If 15-min periods are used, the PHF may be computed as:
PHF=V4 ×V15...
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