There are two types of speed studies – Operational and Design.
Operational speed studies are based on the actual speed of traffic along the roadway. A statistical analysis is conducted of a sample size (of speed data) to evaluate the 85th percentile speed. The 85th percentile speed typically dictates the posted speed limit.
Design speed studies are based on the review of the plans and profiles to evaluate actual design features. The ‘effective’ speed is extracted based on the design features shown on the plan. Design speed studies can be performed prior to the construction of the roadway, and any tweaks required to adjust the effective speed on the road can be made at the plan level. Hence, for those reasons, a design speed study is recommended over an operational speed study.
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