Why Two Days of Traffic Study Data Is Better than One

As everyone should acknowledge, existing traffic data is the basis of our modern Traffic Impact Study. Almost all studies will start with collecting data and then building upon it. At the start of my career, we had to sit at an intersection using a count board or a simple pad of paper to record turning movements at an intersection. It was boring, expensive, and hard to get much more than peak period counts.

Spack Consulting has always been at the forefront of improving data collection and traffic impact studies, at least in Minnesota. First with our move to video and the CountCam system. Then, we pushed our industry to collect turning movement counts over a 13-hour period instead of the just the peaks. (pat on the back here). This change has allowed to us to better determine the true time of peak hour traffic, whether traffic around lunch is another peak, and accurately examine traffic signal warrants, to name a few benefits.

Despite this improvement, we still felt that our Traffic Impact Studies were built on essentially a snapshot in time. Our counts were from one day of traffic data, which could have been impacted by any number of items: an office takes an afternoon off, a restaurant is closed for renovations, the local chapter of Hell’s Angels decides to ride by the park, etc. With our studies guiding roadway improvements, a five or ten percent adjustment in the base traffic data, either high or low, can greatly influence thousands or millions of dollars in infrastructure decisions.

After much discussion and debate, we at Spack Consulting started 2016 with a permanent change in our approach – 48-hour turning movement counts. The two counts will be added together for a raw two-day count. Depending upon the project, calendar time of count, location, and other factors, we will either use the average of the counts or apply a seasonal factor to determine the adjusted turning movement count. The use of the seasonal factor is similar to the MnDOT State Aid process.

Why two days?

-Smooths out variations in volumes – we’ve seen differences of 25 percent or more between days on a ‘typical’ week

-Matches how we determine our Average Annual Daily Volumes (AADTs) – if the process works for daily volumes on a road, it should work for turning movement counts at an intersection

-Minor increase in cost – leaving our counting equipment out for another day costs nothing while the extra counting is a small piece of the overall project budget

-Able to throw out a time period if necessary – in case of an event (crash, snowstorm, police stand-off), we at least have one other day of data to examine

-Harder to challenge the base counts – we’ve never had to defend our process in court, but saying we used two days of counts for our base seems obviously more defendable if needed

This process should be the new ‘normal’ to make our Traffic Studies better targeted to determining needed improvements. Our existing data is now on a solid foundation.

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