045-I Wyo. Code R. § 1.5
(1) Speed limits and speed zoning can be controversial. Engineers, public safety officials, and others involved in setting and enforcing speed limits may disagree on the appropriate balance between safety and road-user convenience that should prevail on particular road segments, considering conditions of topography, weather, adjacent activities, and traffic. Motorists, other road users, and roadway neighbors have their own perspectives on this balance and may not abide by the professionals' judgments.
(2) The criteria to a set a speed limit generally consider such factors as the physical features of the roadway, crash experience, traffic characteristics and control (for example, signals and other control devices), and the length of the roadway segment under consideration (speed-limit changes should not be too frequent or applied to very short road segments).
(3) If the regulation of speed is to be effective, the posted limit must be generally consistent with speeds that drivers feel are safe and proper. Enforcement is widely recognized to be crucial to the success of speed limits as a means for making roads safer. If law enforcement officers and the courts are confident that speed limits have been developed on a reasonable basis, their enforcement of the limits will be more effective. Generally, speed limits should be set at levels that are self-enforcing so that law enforcement officials can concentrate their efforts on the worst offenders.
(4) One of the factors considered very important for setting a speed limit is the prevailing vehicle speed. The Manual on Uniform Traffic Control Devices (MUTCD) is quite explicit, stating that when a speed limit within a speed zone is posted, it should be within 5 mph of the 85th-percentile speed of free-flowing traffic. The Speed Study output sheets show the recommended speed limit based on this criterion.
(5) The MUTCD indicates other factors may also influence the appropriate speed limit, including roadway characteristics such as shoulder condition, grade, alignment, and sight distance; the pace; roadside development and environment; parking practices and pedestrian activity; and reported crash experience for at least a 12-month period.
(6) These other factors may be used to justify a speed limit somewhat lower than the 85th-percentile speed, but in no circumstance should the speed limit be posted below the 50th-percentile speed or lower limit of the 10-mph pace.
(7) In cases where the speed limit is posted below the 85th-percentile speed based on the other factors, the speed limit sign serves to remind motorists that conditions in the area are such that the speed reduction is reasonable. Proper use of speed limit signs instills confidence in the minds of drivers that the information on the speed limit sign is accurate and not simply a desire on the part of a policy maker to reduce speed arbitrarily for emotional or political reasons.
045-I Wyo. Code R. § 1.5