Message Monday - Dec. 21, 2015 - Only Reindeer Can Fly, Watch Your Speed
In order to reach roughly 2 billion children (discounting those on the naughty list) each Dec. 24, Santa’s sleigh has to travel really fast. Luckily speed limits don’t apply to nine reindeer pulling a sleigh driven by a rotund man in a red suit. The rest have speed limits in place to keep us all safer on the road.
If you have ever wondered how speed limits are set, here is your answer.
Initially, speed limits are set by Iowa Code Chapter 321.285 depending on the roadway type and engineering factors such as location and type of access points and traffic volume. This is why rural, two-lane roads are 55 mph and interstates are 70 mph. Some lesser known values they have set are:
- 20 mph in business districts
- 25 mph in residential and school districts
- 45 mph in suburban districts
- 65 mph on selected multi-lane highways
Whenever these statutory speed limits are not the appropriate speed limit for a specific section of a highway, the Iowa DOT is responsible for posting of other regulatory speed limits. This is done by a performing a speed study. Data is collected using a laser speed gun at selected locations on the roadway under ideal driving conditions. A technical analysis is done on the results to determine the 85th percentile. This is the value indicating the speed which most (85 percent) drivers are traveling at or below. Experience has shown that a posted speed limit near this value is the maximum safe and reasonable speed. Studies have shown that traveling much faster or slower than this value can increase your chance of being in a crash. To see more detail on this process, check out the Traffic & Safety Manual on setting Speed Limits. http://www.iowadot.gov/traffic/manuals/pdf/05a-01.pdf
Here is a good video that Maine DOT put together to help explain the process all states follow. http://media.maine.gov/cgi-bin/vid?id=WpS3JHzpraiGFFf
The fatality count of 305 is an increase of seven from last Monday. To see statistics published daily by the Office of Driver Services, go to http://www.iowadot.gov/mvd/stats/daily.pdf.