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| | Dear T.D.: On March 22, 2009, you wondered why a residential street like yours – Owyhee – was posted for 30 mph, the same speed as many busy streets like Capitol Boulevard. It took awhile, but ACHD has updated its speed evaluation for Owyhee Street.
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| | The study covered the stretch between Overland Road and Alpine Street. It considered crash history, actual free-flow speeds, land use, street geometry, parking, and pedestrian and cyclist presence.
The analysis found that the eight accidents reported in the last three years were caused by somebody, drunk or sober, failing to yield at a STOP sign.
How fast do people actually drive Owyhee? ACHD measured speeds on three stretches. Average speed was 29 mph. Eighty-five percent of all drivers went 33 mph or less. The "pace" – the 10 mph bracket that holds the most drivers – was 24-33 mph between Overland and Kootenai; it included 92 percent of all drivers.
The "85th percentile" is deemed to be what most prudent drivers will travel given the road conditions. It's only one variable, but can highlight big discrepancies between posted speeds and actual behavior.
Big discrepancies didn't make much showing in this study. After considering everything, the analyst recommended that the current 30 mph limit makes a pretty good fit with driver behavior and other conditions, and should not change.
A word about the whole idea of speed limits: Speed limits ALWAYS represent tradeoffs among risk, mobility, and community concerns. Those who drive at speeds more than what 85 percent of drivers deem prudent are willing to accept a higher risk of crash and injury in exchange for a shorter trip time (mobility). Unfortunately, they impose that risk on others.
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