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Methods for calculating AADT are generally based on data information resulting from two types of counts: permanent automatic traffic counts and short-period traffic counts. A combination of these two measurements is generally employed to obtain an AADT estimate over a larger road network as described below: 

• First, permanent automatic traffic recording stations provide continuous counting of the traffic on selected roads (mostly on highways) for the entire year. The advantage is to offer traffic counts that are typically recorded in 15 minute or hourly intervals, 7 days a week and 365 days a year intervals. It thus enables a finer level of analysis and a more accurate annual average than short-term counts. Permanent automatic traffic recorder is the only way to provide exact AADT values (when used under perfect conditions). 

• Secondly, short-term traffic counts (also called seasonal, portable or coverage counts) provide roadway segment-specific traffic count information on a cyclical basis for a large number of road segments. The collection data period typically ranges from 1 to 7 days where data are recorded in 15 min or hourly intervals. Due to differences in day-to-day variation in the traffic flow, the count duration is dependant on the road on which it is located e.g. rural or urban. In order to minimize this variation, the minimum requirements could be fixed for instance at 48-hours of continuous data for rural counts and 24-hours of continuous data for urban counts. Furthermore, special attention must be paid to count sites locations so as to improve the data accuracy.

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In Europe, the UNECE [UNECE05] conducts road traffic censuses every five years. The methodology recommended for the 2005 census is available on the UNECE website
They recommend three types of methods for providing AADT (for the year 2005): 

• Continuous traffic counts for the entire year;
• Short-term traffic counts;
• A combination of both measurements (sampling methods).

It was also mentioned that "in certain exceptional cases, AADT may be determined without
counting, based on previous counts or on counts on adjoining sections of the same road."

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2 Road traffic data collection methods: an overview 
2.1 Conventional "in-situ" technologies http://www.youtube.com/watch?v=j3CfU-f0JpI

Broadly speaking, "in-situ" technologies refer to traffic data measured by the means of detectors located along the roadside. Generally, traffic count technologies can be split into two categories: the intrusive and non-intrusive methods. The intrusive methods basically consist of a data recorder and a sensor placing on or in the road. They have been employed for many years and the most important ones are briefly described hereafter: 

• Pneumatic road tubes: rubber tubes are placed across the road lanes to detect vehicles from pressure changes that are produced when a vehicle tyre passes over the tube. The pulse of air that is created is recorded and processed by a counter located on the side of the road. The main drawback of this technology is that it has limited lane coverage and its efficiency is subject to weather, temperature and traffic conditions. This system may also not be efficient in measuring low speed flows. 

• Piezoelectric sensors: the sensors are placed in a groove along roadway surface of the lane(s) monitored. The principle is to convert mechanical energy into electrical energy. Indeed, mechanical deformation of the piezoelectric material modifies the surface charge density of the material so that a potential difference appears between the electrodes. The amplitude and frequency of the signal is directly proportional to the degree of deformation. This system can be used to measure weight and speed. 

• Magnetic loops: it is the most conventional technology used to collect traffic data. The loops are embedded in roadways in a square formation that generates a magnetic field. The information is then transmitted to a counting device placed on the side of the road. This has a generally short life expectancy because it can be damaged by heavy vehicles, but is not affected by bad weather conditions. This technology has been widely deployed in Europe (and elsewhere) over the last decades. However, the implementation and maintenance costs can be expensive. 

• Manual counts: it is the most traditional method. In this case trained observers gather traffic data that cannot be efficiently obtained through automated counts e.g. vehicle occupancy rate, pedestrians and vehicle classifications. The most common equipments used are tally sheet, mechanical count boards and electronic count board systems. 

• Passive and active infra-red: the presence, speed and type of vehicles are detected based on the infrared energy radiating from the detection area. The main drawbacks are the performance during bad weather, and limited lane coverage.  

• Passive magnetic: magnetic sensors are fixed under or on top of the roadbed. They count the number of vehicles, their type and speed. However, in operating conditions the sensors have difficulty differentiating between closely spaced vehicles.  

• Microwave radar: this technology can detect moving vehicles and speed (Doppler radar). It records count data, speed and simple vehicle classification and is not affected by weather conditions.

• Ultrasonic and passive acoustic: these devices emit sound waves to detect vehicles by measuring the time for the signal to return to the device. The ultrasonic sensors are placed over the lane and can be affected by temperature or bad weather. The passive acoustic devices are placed alongside the road and can collect vehicle counts, speed and classification data. They can also be affected by bad weather conditions (e.g. low temperatures, snow). 

• Video image detection: video cameras record vehicle numbers, type and speed by means of different video techniques e.g. trip line and tracking. The system can be sensitive to meteorological conditions. 

http://ftp.jrc.es/EURdoc/JRC47967.TN.pdf