By Mike Spack, PE, PTOE
Just talked to a client of ours who asked for a quote to get a.m. and p.m. 2-hour turning movement counts at two intersections that are each controlled with stop signs (we do have a standard price list for our Minneapolis, Minnesota area – Download our most recent TDI Price List). Because we’re counting with the COUNTcam video system now, we’ve changed our pricing structure quite a bit so it’s not much more to collect counts from 6 a.m. to 7 p.m. vs. just doing the traditional 2 hour morning/evening counts.
A 13-hour turning movement count from 6 a.m. to 7 p.m. is so much more useful than 2 hour peak period counts – (1) you know you’ve captured the a.m. and p.m. peak hours and (2) you have data you can go back and do a traffic signal warrant analysis with (you can even choose to factor out some or all of your right turns in this analysis).
Let’s expand our thinking to a 24 hour turning movement count – the above advantages plus you get daily traffic volumes on each leg of your intersection.
Or why not expand to a 48 hour turning movement count – the above advantages plus you can average your study periods to control for daily variation.
Or why not expand to a 120 hour turning movement count – the above advantages plus you can look at weekend data which may be crucial in a big shopping district or if you’re near a large church.
So why are we still doing 2 hour turning movement counts? It’s a tradition that goes back to at least the first Highway Capacity Manual published in 1950. When you’re counting with people who are sitting at the corner, it gets very expensive to do turning movement counts for much more than a few hours. Being practical engineers – our industry decided the cost/benefit means a few hours of data is typically good enough. But its crazy to think we’re making six or seven figure decisions based on one data point.
Now that we’re in the 21st century – 2 hours isn’t good enough anymore. Shameless plug….
The turnkey COUNTkit system we’ve developed and sell at SpackSolutions.com changes our 60-year old cost/benefit calculation. 13 hour turning movement counts should be the minimum amount of data traffic engineers use for their analyses and 48 hour turning movement counts are justifiable.
I’m big on big data and I like the 13 hour TMC. As a signal timing guy near military bases and shopping malls often you are running coordination from 600am to 1000pm. What are some uses you have seen for TMC data for the the other 8 hours?
Like the blog!
Chris – adding on the extra 8 hours so you have 24 hours of count data gives you Daily Traffic Volumes which are useful in Traffic Impact Studies where you’re doing planning level analyses.
“Now that we’re in the 21st century – 2 hours isn’t good enough anymore.”
I would go further and state that in perfect conditions this kind of data should be obtained divided in a minimum of 2 year periods – winter and summer – counts should be made both weekend and week; vacations and non-vacations period. Should be taken at least during 2 days, preferable not consecutives so it is possible to compare the data and notice uncommon values.
My experience:
As a university work it was asked us to calculate the delays in an intersection either using a light system to control the intersection or with the common rule of priority.
We made a 2 hour turning movement counts in the morning and in the afternoon peaks, at one chosen day of the week.
At the same time there was a contracted traffic company doing the same kind of research in some intersection of the city. This company was working directly with our professor so at the end we could compare results. Conclusion, somehow our results had a substantial minor amount of total vehicles counted. Why?
For what we known it was a normal work day, we (university group) did not made miscalculations or counts since we had multiple teams in multiple intersections (we could compare results from two consecutives intersections and see if the data matches, which was the case)… With deeper investigation we found that a nearby polytechnic school was having special exams day to which only few students attended. This was enough to blow away our data although it was very accurate for that particular day of the year.
Thanks Mike. But what about guidelines? Why would I pay for a 24 hour TMC study when the guidelines is asking for a minimum of only using AM and PM peak hours?
24 hours of data allows for better anlayses (per the post). Hopefully, the engineer setting the “guidelines” recognizes that and changes the guidelines.