Montana Department of Transportation

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Traffic Data Collection and Analysis

Traffic Adjustment Factors

TDCA is responsible for collecting traffic data on approximately 3,800 active short-term traffic count sites statewide. These counts, also known as coverage counts, make up the bulk of the Department's traffic count program.

TDCA's policy is to collect a minimum of 36-hours of traffic count data during each short-term count setting. This allows the field staff that's dealing with an extensive roadway network, to make the most of Montana's limited inventory season.

Unlike permanent counters which collect traffic year-round, short-term counts can be taken only when weather permits, usually April through September. As a result, these counts reflect a ‘snapshot’ of the traffic conditions during that particular 36-hour period. Because of this, short-term counts must be seasonally adjusted in order to provide a better representation of traffic conditions on an average day (i.e. Average Annual Daily Traffic - AADT).

All roadways are categorized into Traffic Factor Groupings based on their operational characteristics. Traffic data from permanent counters in each grouping are used to develop adjustment factors to be applied to the short-term counts in that same grouping.

Traffic Factor Groupings
RI Rural Interstate
UI Urban Interstate
RPA Rural Principal Arterial
UPA Urban Principal Arterial
RMA Rural Minor Arterial
UMA Urban Minor Arterial
RMC Rural Major Collector
UC Urban Collector
REC Recreational

The type of adjustment factor applied is based on the type of count being performed.

For Volume Counts, which is a single hose setup, a combination Axle and Seasonal factor is applied.

Vehicle Classification Counts, only require a Seasonal Adjustment Factor because the two-hose setup accounts for the number of axles per vehicle.

The adjustment factors, which are three-year averages, are developed for Weekdays (Monday - Thursday), Saturday and Sunday by Traffic Factor Group for each month. Because Friday travel is considered to be non-typical, TDCA's policy is to not perform short-term counts on Fridays and to exclude Friday travel when developing adjustment factors.