Traffic Count Estimates for Orchard Hills Villas

Not often do city council meetings dive into hardcore statistics and mathematics, but they did Wednesday evening.  I love it when this happens.  I also am likely one of few that do.  We were presented information that stated that there was a better way to estimate traffic counts by a neighbor opposed to the Orchard Hills Villas.

I think that Michelle Kallmes was absolutely 100% correct in her mathematics, however I believe incorrect in what the answer means.  The city uses a method that relies on the weighted average of all studies relating housing and number of vehicle trips generated.  The author of a study showed a different way of analyzing the existing data and came up with a natural logarithmic equation that better fit the data.  For those of you that are not nerds that is the “Ln” button on you calculator.  Michelle was also 100% correct when she said “just because we have always done something one way we should continue to do it that way.”

There is a technique in statistics that provides a measure for how accurately a line approximates data.  That is called the coefficient of determination (often shown as R squared).  Without getting too much into the details, this is a number between 0 and 1.  The closer it gets to 1 the better the fit.  The authors work yielded a result of .96 is outstanding and shows that this is a very good method of estimating traffic count.  As an aside, I used a similar technique when I discovered that the Shoppes on Maine retail has resulted in no net increase to our sales tax collections.

The math is correct and we could consider using it in the future.  That said; we are not in the business of coming up with the best math equations for traffic modeling, we are in the business of public safety and making sure that our vehicles have sufficient access.

We set a limit of 300 trips per day because at that level we know that our safety goals are met.  We came up with this number understanding that there is some error in a weighted average approximation.  As is the case with many engineering standards, a safety factor is built in.  If we were to use a new method that more accurately provided data, we could then decrease the size of the factor and increase the daily limit.

Further Fox Chase Road was engineered to be a through street.  How do I know that?  It is called a “road” that name is given to through streets.  Fox Chase Lane for example is dead end.  As such traffic loads could actually be in the thousands given the road design.  The decision to turn it into a dead end affords better environmental protection for the Decorah Edge.

Finally, we have had this standard for some time, and it has worked.  This count limitation or method are not new.

While the math was good, it was used in an attempt to distort policy and prevent a project that the city council found to be a reasonable proposal.  Had we changed a part of our established and consistent methodology while ignoring the other part, on the spot, to stop a project, I fear that would have been arbitrary and capricious.  It certainly would have been unfair for a project most of us thought was very good.

One comment

  1. From City Engineering:

    RCO 64.127 (9) sets the 300 a trip limit on culdesacs.

    However, RCO 64.127 (10) sets a limit of 500 trips on streets with no secondary access. In other words we allow development to occur up to 500 trips where secondary access is not provided as part of the initial phase of the development. Herein is the “buffer” you refer to that provides an additional level of protection on culdesac streets.

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