When I was an intern (quite a while ago) I was put in charge of inspecting and measuring quantities on several street lighting projects. I measured about four miles of installed cable. The contractor questioned my quantity saying it was low and that I was shorting him. I met him early the next day expecting a long day of spot checking. The first thing I did was lay out my 25 foot tape measure and check to make sure my measuring wheel was accurate. I asked him to do the same. His wheel said the 25 foot tape measure was 28 eight feet long. We did the math and his quantities were off by the 28/25 ratio. Case closed.
Are you sure the traffic data you are getting is accurate? We test all of our counters every three months with a Jamar traffic counter tester in addition to visually verifying our counts every time we setup in the field. The traffic counter tester blows a prescribed number of air pulses so we can test the accuracy of our air switches (volumes as well as speed and classification data). We also do a manual test on our Jamar manual count boards – punching in a prescribed number on each button. It seems logical to me that you need some way to ensure your counters are working properly.
One of our public sector clients has twenty tube counters of their own. They just had us go through our test regimen for them. Seven of their twenty counters have at least one bad air switch. I am guessing the intern putting the counters out last year didn't think to question the accuracy of the counters. I wonder how much bad data they collected.