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Traffic Studies

When a complaint about speeding traffic is received, the Traffic Engineering Department will schedule a spot speed study at the location.  Traffic counters, which are capable of recording vehicle speeds by time of day, are set out for several days to collect data on traffic volume, speed, and direction.  This information provides an accurate picture of traffic conditions including the number of speed limit violations, the severity of the violations, and the times of day when violations are highest.  A report is prepared and forwarded to the Ponca City Police Department.  The Police Department will schedule enforcement efforts as it deems necessary.  A common suggestion to reduce speeding traffic is to install stop signs at intersections.  Stop signs are not intended to be used as speed control devices.  In fact, the Manual on Uniform Traffic Control Devices, the national standard on traffic control published by the Federal Highway Administration, specifically states the stop signs should not be used for speed control.  Stop signs are intended to assign right-of-way at intersections.  As such, they are not used for speed control in Ponca City.

In order for a stop sign to be installed, the Traffic Engineering Division will review the accident history of the intersection and check the existing sight distance.  If the accident history indicates a problem that could be corrected by the installation of a stop or yield sign or the sight distance is too low for an uncontrolled intersection, a recommendation will be presented to the Ponca City Traffic Commission for the installation of appropriate control.  The Traffic Commission’s recommendation is then taken to the City Commission for final approval.  Setting an appropriate speed limit is based on the premise that most drivers are concerned for their own safety and will not drive in a manner that feels threatening.  Admittedly, there are variations in driver comfort levels.  There are extremes on both ends of the scale.  Some drivers are very aggressive, some are overly cautious.  Most fall somewhere in between, and select safe and proper speeds based on roadway and traffic conditions.  The maximum safe speed at any location will vary as traffic, road, weather, and light conditions change.  So, based on this premise, the process that is most generally used and preferred by traffic engineers is to determine the prevailing speed on a given roadway and set a speed limit that is close to that speed.  The required data are gathered by conducting a spot speed study over several days to get an accurate sampling of the existing traffic. 

There are two things that are provided by these studies that help determine the proper speed limit.  One is the 85th percentile speed, the speed that 85% of drivers are traveling at or below.  The other is the 10 mph pace speed.  That is the 10 mph range with the highest percentage of observations.  Speed limits should be consistent with speeds that drivers feel are safe and proper.  The 85th percentile speed has come to be accepted as a good gauge of that number.  It recognizes the fact that most drivers are reasonable drivers.  Speed limits set around the 85th percentile speed will be voluntarily obeyed by most drivers and reduce the enforcement burden to a reasonable level.  The 10 mph pace is important because it measures the degree of diversity of speeds in the traffic stream.  Faster speeds are not necessarily the biggest contributor to accidents.  Speed differential is also an important factor in safety and becomes more critical as volume increases.  A wide range of speeds produces many more conflicts and increases the potential for accidents.  One of the objectives of speed control is to produce a uniformly moving traffic stream.  Artificially low limits produce the opposite effect

516 E. Grand, Ponca City, OK 74601
PO Box 1450, Ponca City, OK 74602
Ph. (580) 767-0339
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