R. Moses Answers:
The first thing I need to ask is: What kind of traffic are we talking about? Cars, trucks, busses, bicycles, pedestrians? Each will have a different impact on the other, and counting only one of them won’t tell you anything about the others. Unfortunately, few bike or pedestrian counts are regularly done so I'm going to assume you're talking about vehicles.
Let me break your question down into two answers.
On traffic counts: Most states have standard procedures for traffic counts, and expect towns, cities, or private entities to follow these procedures in their traffic studies. Whether any particular count was conducted following the standard procedures will really depend on who did the count and why.
That said, there is a huge amount of variability of traffic counts depending on the day, season, weather, proximity to a holiday or other special event, and the list goes on. Traffic engineers also have standard procedures to adjust a count conducted on a single day, or over the course of a week, to reflect average conditions. These adjustments are sometimes made using data from continuous count stations, which operate 24/7, and provide a basis to compare a count from a specific day to the average traffic volumes in the area. One point to note is that a count that has been adjusted as needed to truly be average over the course of a year is typically called "AADT" or Annual Average Daily Traffic.
One thing to also keep in mind is that, due to the length of time between the traffic count and implementation of a plan, there can be many changes in the actual amount of traffic. New businesses, schools, or housing, or the loss of those things, can greatly change traffic patterns and counts in a very short time.
On volume thresholds for road diets: There is an ever-growing body of research and experience with road diets across the US. In brief, the results show that road diets can work very well with volumes much higher than was commonly believed 5 or 10 years ago. Various sources suggest road diets can readily be implemented and successful at ADT volumes of 23,000 to 25,000.
Actual observations in Seattle have shown road diets working well at ADT as high as 30,000. The success of a road diet at volumes over 20,000 will depend on how high the peak hour is relative to the daily volume, and the directional characteristics of the traffic during peak hours. These are all things that can be studied with common traffic engineering methods. The DOT official who made that statement may not be up to date on current research. This research paper from the Pedestrian and Bicycle Information Center and these guidelines from the Kentucky Transportation Center provide excellent additional information on road diet application, safety and guidelines.
A traffic count is not destiny. If our street networks change, with some streets narrowed and/or others expanded, drivers will change their routes and adapt accordingly. So just because a road has a certain amount of traffic now, that doesn't mean that it will always be that way.
How would you respond to this question? Jump in with your answers in the comments!
Note: R. Moses is not meant to be professional engineering advice nor should be relied upon as such. Consult your own technical professional before proceeding with your own project.