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Traffic engineering terms in Hansville speed tables study explained

The first installment of a two-day traffic-calming series looked at a $57,000 traffic study completed by Kirkland’s Transpo Group.
There were two traffic engineering terms — 85th percentile speed and collision rates — in Sunday’s story that deserve further explanation because, as Jon Pascal from Transpo Group, put it, “those are engineering terms (that) are hard to describe.”
The first term, 85th percentile speed, refers to the number of drivers who traveled at or below the recorded speeds, Pascal said.
It’s one of the ways traffic engineers determine what the average traveling speeds of drivers are. The 85th percentile model presumes that the remaining percentage of travelers will always speed excessively, regardless of road engineering.
On Hood Canal Drive, the study said, “The 85th percentile speed ranges between 42 to 46 mph prior to speed tables and 35 to 46 mph after the tables were installed.”

What this means is that 85 percent of travelers on Hood Canal drove at 42 to 46 mph or below before the tables were installed, Pascal said. The speed range was given to account for the three speed study locations placed on Hood Canal Drive for the study. So hypothetically, of the 85 percent of drivers, a number of them could have been going under the 42 to 46 mph speed range. The same logic would be applied to the 85th percentile speed  for the 35 to 46 mph speed range after table installation.

Collision rates presented in the study were also perplexing.

Engineers came up with numbers such as 0.3, 0.7 and 1.7 collisions by dividing the number of annual collisions by the number of years the study looked at, which in this case was three years before the tables were installed and three years after installation.

For instance, on Twin Spits Road, the study said there were 0.3 collisions per year from 2007 to 2010. So, within that three year period, there was about one collision a year after the tables were installed. On Hood Canal Drive, there was about 2 collisions per year from 2004 to 2007 ( 0.7 collisions annually) and about five collisions from 2007 to 2010 (1.7 collisions annually.)

Pascal said Transpo Group engineers rounded the numbers for the sake of table presentation.

Here’s a link to the report: Hansville traffic calming study