From Roader's Digest: The SABRE Wiki
AADT stands for Annual Average Daily Traffic flow, and is the generally accepted method of comparing traffic flows on roads. In its simplest form, it is the total traffic flow over a year in both directions divided by 365. More complex forms used for traffic modelling concentrate on the busiest 12, 16 or 18 hours of the day, and are able to break down the traffic flow by hour, direction and vehicle type.
AADT figures can be viewed at the Department for Transport's Traffic Counts website.