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 www.CountingCars.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.