Chapter 3: Speed Zone Studies

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Section 1: Overview

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Engineering and Traffic Investigation

This chapter includes information concerning interim speed limits for a new or reconstructed highway and a description of how to conduct an engineering and traffic investigation as the basis for establishing a regulatory speed zone along a roadway. This investigation is commonly called a “speed zone study.”

Sound and generally accepted engineering practices are an integral part of such speed zone studies and are discussed in Section 4, Speed Zone Design of this chapter.

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Interim Speed Limits for New or Reconstructed Highways

An interim speed limit for a new or reconstructed highway shall be determined and posted before the highway is opened to traffic.

To set an interim speed limit, a traffic and engineering investigation will be conducted.

The traffic and engineering investigation will include a review of:

  • the statuatory prima facie speed applicable to the highway
  • the design speed applicable to the highway
  • a trial run speed study for the highway.

To set an interim speed limit at a speed that is less than the prima facie speed applicable to the highway, a commission minute order or a city ordinance setting the interim speed limit is required.

Warning signs and advisory speed signs may be used on a highway with an interim speed limit to alert drivers to any hazards.

When traffic speeds have stabilized on a highway, an 85th percentile speed study will be conducted as discussed under Section 2 of this chapter. The interim speed limit will be used on the highway until the maximum speed for the highway is determined following the conclusion of that investigation.

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Scope of Study

The speed zone study should cover the entire length of a potential zone, even though an analysis of the data may later indicate that the actual limits of the area that requires zoning are less than the limits of the potential zone.

A speed zone study consists of the following principle areas:

  • determining the 85th percentile speed
  • crash study
  • developing of strip maps
  • speed zone design
  • rechecks of speed zones.

This chapter contains sections describing each of these areas.

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