The 2016 TMG Updates and an Alternative Method of Calculating AADT

The need for traffic data at the Federal and State levels requires that States develop a well-designed traffic monitoring program to support all business areas. In late 2016 the Federal Highway Administration (FHWA) updated the Traffic Monitoring Guide (TMG), which presents recommendations to be used by State departments of transportation (DOTs). These recommendations are intended to improve and advance the future of traffic monitoring while considering recent Moving Ahead for Progress in the 21st Century Act (MAP-21) legislation responsible for governing the funding and authorization of surface transportation funding.  

One important addition to the TMG is a new alternative method of computing Average Annual Daily Traffic volume (AADT). In previous versions of the TMG, AADTs were determined by using one of two traditional procedures. These include:

1. A simple average of traffic from all days.
2. An average of averages method using the America Association of State Highway and Transportation Officials (AASHTO) recommended method.

Sometimes over the course of a year traffic data collection may not provide continuous data due to construction changes, data equipment maintenance down time or loss of data. The effects inherent in missing or incomplete traffic data tend to bias the traditional methods, thus reducing the potential accuracy of the resulting AADT estimate.

The new alternative method for AADT computation takes into account hourly data, including partial days of data that would otherwise have been ignored under the two traditional methods. The calculation involves a two-step process:

1. Calculate the monthly average daily traffic from the available hourly count records.
2. Calculate the AADT from the twelve resulting monthly values.

The new alternative method reduces the effect of bias from using data that is not continuous. FHWA offers this new method, along with the two traditional methods, as options to state DOTs for calculating AADT estimates. 

At MS2, our collaborations with FHWA and our state DOT clients have provided us with the opportunity to study this new alternative method. We plan to update our Highway Performance Monitoring System calculations to incorporate the alternative method in time for the 2017 reporting period. This will provide our clients with all potential computation methods needed to successfully provide AADT estimates in compliance with MAP-21 requirements.