Chapter 2: Regulatory and Advisory Speeds
Section 1: Application of Regulatory and Advisory Speeds
Introduction
When an engineering and traffic investigation shows that the statutory speed limits are no longer applicable for the existing conditions, the
prima facie
maximum speed limits should be altered accordingly with a speed zone.The types of speed zones are as follows:
- regulatory
- construction
- school
- private road.
Each type of zone and its use is discussed later in this chapter.
Advisory speeds may be posted within speed zones to advise drivers of a safe operating speed to negotiate roadway features. This section discusses regulatory speeds and advisory speeds and explains their differences.
Regulatory Speeds
Regulatory speed zones should be applied only to those locations
and sections of highways which are not dealt with adequately by
the general statewide speed limits, and they should be indicators
of the speed limitations imposed by physical and traffic conditions
at such locations. Speed limits are determined by specific roadway
and traffic conditions. Speed limits should not be lowered to the extent
necessary for a driver to avoid a collision with a pedestrian or
other motorist who is entering or crossing the highway in violation
of an existing traffic regulation.
Roadway safety is an important consideration in establishing
speed limits. The following factors affect roadway safety and,
therefore, should be considered when establishing speed limits:
- horizontal and vertical curves
- hidden driveways and other roadside developments
- high driveway density
- crash history along the location
- rural residential or developed areas
- lack of striped, improved shoulders.
Advisory Speeds
Advisory speeds are the desirable speeds for curves, intersections, or other locations where design standards or physical conditions of the roadway restrict safe operating speeds to values less than the maximum legal speeds or posted regulatory speed limit. Figure 2-1 illustrates the use and application of warning signs with advisory speeds. For additional information on determining advisory speeds, see
.

Figure 2-1. Typical applications of warning signs with advisory speeds. Note that distances shown are from the point of curvature.
Advisory Speed Sections in Regulatory Zones
If an advisory speed is located within a regulatory speed
zone, it is not necessary to lower the zone speed to conform with
the advisory speed. In erecting the signs, however, care should
be taken not to erect a regulatory speed limit sign so near the
advisory speed sign that drivers may become confused by two different
speed values.
An advisory speed within a regulatory speed zone should not
be posted for a value higher than the posted speed of the regulatory
speed zone. Care should also be taken not to place a regulatory speed
sign between an advisory speed sign and the location to which the
advisory speed applies.
Regulatory Versus Advisory Speeds
Advisory speeds are determined primarily by physical and design characteristics of the roadway. The setting of regulatory speeds, while also affected by physical and design factors, is determined in large part by existing free flow traffic speeds.
A Transportation Commission minute order, city ordinance, or county ordinance is not required for advisory speed zones but is required for regulatory speed zones. Therefore, advisory speed zones are more flexible in that they can be changed without revising existing Commission minute orders or city or county ordinances.
From the standpoint of enforcement, when a regulatory speed zone has been established and signs are posted, the speed values shown on the signs are the maximum lawful
prima facie
speeds. An advisory speed sign serves to advise drivers of safe speeds that are recommended for certain roadway conditions, such as horizontal curves. It does aid the enforcement officer, however, in determining reasonable and prudent speeds. A driver might be cited for exceeding the posted value of an advisory speed zone on the grounds that they were driving at a speed that was not reasonable and prudent for the conditions existing at the time and location.