The Process of Traffic Counting

The Iowa DOT uses manual traffic counters, portable tube counters and Continuous Count Stations (CCS) throughout the state to collect traffic data used in the calculation of Annual Average Daily Traffic (AADT). Traffic volume is aggregated based on the FHWA's 13 vehicle category classification for the primary highway system or as a summed total for non-primary and estimated roads.

Traffic counts are recorded at a specific point on the roadway. This point is referred to as a count station or site. The point often represents the characteristics of a road segment. For example, a count location is often assigned to a segment of road. The definition of a segment varies by State; here it refers to a section of roadway defined by the State. Since collecting traffic data is not feasible on every possible point within a segment, traffic data collected and representing a point on a segment is extrapolated to represent the entire segment.

Continuous Count Stations
County traffic maps
City traffic maps

Annual and Monthly Reports

In addition to collecting traffic volume data, the Iowa DOT also compiles by month and by year and submits these reports to the Federal Highway Administration. These reports provide insight on any changes that may take place over time with the usage of the roadways. The information contained in the reports is useful in understanding trends as it relates to growing, consistent or declining volumes of traffic.

Automatic traffic recorder reports

The Iowa Department of Transportation’s Office of Systems Planning, in cooperation with the Federal Highway Administration, also prepares traffic reports for the volume of traffic on the primary road system in the form of an annual traffic book.

Annual Traffic Book

How to View the Traffic Information

Traffic information is contained within popups in the web maps. To view a pop-up, click on any roadway segment. If viewing a segment on the primary highway system, a breakdown of each individual vehicle class can be viewed within the popups by scrolling to the left and right by clicking on the side arrows. To view more information, hover your mouse pointer over one of the graphs.

The non-primary and estimated roadway segments will only contain the overall AADT value, street name and the entity that has jurisdiction of the road. Some local jurisdictions can include counties and cities but some may also have another institution like a college or university that is responsible for it.

Vehicle Classification

For the primary highway system, the FHWA vehicle classification system separates vehicles into categories depending on whether they carry passengers or commodities.

  • Passenger vehicles (classes 1 - 3)
  • Single unit trucks (classes 4 - 7)
  • Combination trucks (classes 8 - 13)
Note: For the purposes of this web map, classes 11 - 13 are combined into a single category named "Multiple Trailers."


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