Average Annual Daily Traffic (AADT)

"Raw" traffic counts taken with rubber tubes stretched across the road must be adjusted to compensate for overcounting by multi-axled vehicles. Classifying traffic allows for the development of axle correction factors, which are applied to any raw, tube-based counts. Axle factoring isn't needed for counts obtained by magnetic induction loops that are permanently imbedded beneath a roadway surface. Like wires installed in the street at signalized intersections to activate signal changes, electronic traffic counters connected to "loops" detect the presence of a vehicle via a magnetic field, and will not overcount vehicles with a lot of axles, such as multi-tailored ("18-wheeler") trucks.

Once the field crews bring in these raw counts to the office, the data are downloaded and stored in a computer for future processing and ultimate conversion to average annual daily traffic volumes (AADTs). Raw traffic counts taken by either tubes or loops require seasonal adjustments to compensate for monthly and daily fluctuations of vehicular traffic. These adjustments are done before any traffic volume information is published or quoted. The basic intent of this adjusting procedure is to provide a traffic volume which best approximates the use of a given highway section for a typical 24-hour day of the year. The seasonal adjustment factors are developed from a network of continuous traffic recorder stations, also known as automatic traffic recorders (or just ATR's for short). Currently, ADOT's Data Section operates 69 ATR stations statewide, which monitor vehicular traffic twenty-four hours day each day of the year. These ATR stations are "polled" daily via telemetry and computer software to report the previous day's travel activity. Traffic data polled from ATRs are stored and processed in both monthly and annual cycles, which are subsequently applied to raw counts taken on all highway segments that are assigned to a particular set of ATR stations.

Assembling and processing raw field data (including traffic data furnished by ADOT's Freeway Management System), monitoring the operation of the ATR network, and producing the final output of AADT volumes is the second and most complex aspect of monitoring vehicular traffic on Arizona's State Highway System. Fortunately, software has been developed to perform thousands of calculations, manage the collected data, produce final AADT volumes, and generate a number of useful ATR station reports. In addition to providing seasonal adjustment factors, the ATRs can generate reports that offer highly reliable insights about highway traffic behavior patterns over long periods of time. Depending on demand, future editions of this web site may contain more information about ATR's - including a station list, location map, and station-specific data reports. The third function under traffic monitoring primarily involves the maintenance and repair of both portable and permanent electronic traffic recorders by technicians of the Data Section's electronic shop. They are also responsible for overseeing that the ATR stations are in working order, and are dispatched into the field to troubleshoot them and make repairs when needed. ATR stations can be difficult to keep operating. They can be rendered unserviceable by random lightning strikes, traffic counter failures, telemetry and power outages, road construction projects, and acts of vandalism. It is an ongoing job to monitor the operation of this equipment, and keep it all in good working order. The shop crew also inspects new and existing loop installations, constructs new ATR stations, and may occasionally participate in traffic monitoring projects that involve specialized hardware.

A number of other functions under traffic monitoring are also undertaken. These mostly involve preparing a variety of traffic volume reports and general maintenance of collected data over the years. Currently, a project is under way to automate the assembly of these reports as well as produce traffic forecasts. At the present time, AADT forecasts are not available for 100 percent of the highway sections on the State System. Additionally, these forecasts may or may not be available for a specific year. Most interstate routes and those highways identified as primary transportation corridors should have some forecasts prepared for them. Historical information back to 1974 is also available.

Traffic Count Resectioning Project Nearing Completion

While many existing traffic count sections have been split up during this procedure, a number of them have been combined. Basic criteria for dissolving multiple sections into one primarily focus on the volume of traffic carried by intersecting roads and streets, and to what extent that may cause traffic volumes on the route in question to change on either side of them. Existing sections have been combined if the change in such traffic is less than 10 percent. These actions have been undertaken in the more rural, remote areas of Arizona where development is unlikely, or at least many years away from occurring. In these situations, current traffic monitoring levels will decrease.

Additionally, traffic section termini for a State Highway have been relocated regardless of rural or urban circumstances if more suitable breaks are determined. Factors considered to influence such a decision include intersecting roads of a higher functional classification, roads with more restrictive traffic control, or roads that generate more vehicular traffic than an existing terminus. Traffic count sections will also be created for the newer segments of the urban freeway system in Phoenix. Conversely, a number of sections will be permanently deleted where the route in question is no longer part of the State Highway System.

Currently, there are approximately 1,070 traffic count sections defined for the Arizona State Highway System. It is expected that this resegmentation effort will result in a net change of about 250 additional sections. Traffic counting stations (TCS) will be preserved where possible. But new ones will have to be established for highway segments that have been subdivided. Traffic monitoring stations on dissolved segments will be either moved to a new location or retained at one existing TCS site or the other. These issues will be addressed after the new set of traffic count sections has been finalized in early 2001. Transitioning from the old to new set of segments, and subsequent traffic data collection by our field crews will probably be introduced in phases over the next two calendar years.

• ADOT Data Team
  
• HPMS
  
• Highway Log
  
• Traffic Counts