ACHD's Road Wizard Sunday, September 29, 2019

This section of Bogus Basin comes up with some frequency with regards to speeding. Unfortunately, speed humps are not an option, as Bogus Basin is classified as an arterial, which by ACHD policy means raised elements are not permitted for use as traffic calming measures.

The most recent speed study from last fall found average speeds in the 35-36 mph range. Only 15 vehicles out of a sample of over 200 vehicles were traveling faster than 40 mph. It's uncommon to see motorists travelling at speeds of 45 mph or more on that stretch of roadway, though there are a few who do.

ACHD performs hundreds of speed studies throughout the county each year. With a data set that large, you start to see patterns and trends emerge. When ACHD establishes speed limits, it is based on two general principles: reasonable driver expectation and an engineering study. Posting a speed limit too low can result in more tailgating and other types of unwanted aggressive driving behavior. Posting a speed limit too high can also result in greater risk, as some drivers may be encouraged to drive at a speed that is too high for the conditions on the street.

Though it may not always seem to be the case, most drivers are reasonable, law-abiding citizens that drive at appropriate speeds given prevailing conditions on a roadway. Speed limits should be set at levels that drivers find appropriate unless there is evidence from the crash history or special physical characteristics that suggest a lower speed limit is needed. This usually falls very close to the 85th percentile speed of the roadway, which is the speed 85 percent of drivers are traveling at or below in non-peak time periods. Reducing a speed limit without justification virtually never results in a corresponding reduction in actual speeds.

ACHD has made changes in posted speed limits on several streets in Ada County in recent years, with changes usually in the 5-10 mph range. One common theme--speed limit changes without any other roadway modifications have little effect on speeds. Studies comparing before and after speed limit changes have shown speeds changed by two mph or less, except when increased enforcement was present. In cases where the speed limit was lowered without justification but with intensive enforcement, speeds went down for a while, but later returned to the original speeds after the initial enforcement period ended.