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Encyclopedia > Annual Average Daily Traffic

Annual average daily traffic, abbrevated AADT, is a term used primarily in transportation planning and transportation engineering. It is the total volume of vehicle traffic in both directions of a highway or road for a year divided by 365 days. AADT is a useful and simple measurement of how busy the road is. It is also sometimes reported as "average annual daily traffic". Transportation planning is the field involved with the siting of transportation facilities (generally streets and highways and public transport lines). ... The engineering of this roundabout attempts to make traffic flow as efficient as possible. ...

Uses

It is commonly used in pavement design to calculate the design traffic loading for a road. The design traffic loading is a calculated figure which represents the expected number of repetitions of a standard vehicle load throughout a design period (commonly 20 or 40 years). Other factors used in the calculations are; the percentage of heavy vehicles, traffic growth rate, and load factors for each heavy vehicle. For the purposes calculating the design traffic loading for a road, the number of light vehicles (cars, motorbikes etc...) is of little importance as they cause many times less damage than a single heavy vehicle. It has been put forward that a single two axle truck with dual rear tyres does as much damage to a pavement as 10,000 cars. This theory is often referred to as the 4th power rule.

As well as uses in pavement design AADT is a useful road network management tool. It can allow road authorities to assess the current usage of the road and allow it to be assigned to a particular road heirachy (major highway, arterial, distributor, collector, or local road), which is important in determining the funding needs for a road network. It also allows a road authority to assess whether or not the majority of the vehicular traffic is using the intended route through a particular area and serves as a check to see if drivers are "rat running" through local streets to aviod conjestion on major roads.

Data Collection

Many road authorities conduct traffic counts on road networks to assess the current AADT for a particular road. The counts can last up to one year using permanent data collection equipment however it is also common practice to use portable or removable equipment to conduct the survey for periods of 7-14 days. An AADT value is then extrapolated from the collected data. This can be more cost effective due to reduced maintainence needs on a particular site, or if the road is part of a lower heirachy on a road network and day to day knowledge of traffic conditions is not needed.

The methods used for collecting data and wide and varied and are somewhat dependent on the level of information required. Many traffic counts do not only record the number of vehicles using a particular road but also record speed, axle groups, length, and weight. This data is also useful for pavement design purposes as well as traffic management purposes.

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