ESTABLISHING SPEED LIMITS
a case of "MAJORITY RULE"
Speed zoning is the establishment of reasonable and safe
speed limits based on an engineering study. This summary tells
what realistic speed limits will do, what they won’t do, and
how they are established.
WHY SPEED LIMITS?
Since most citizens can be relied upon to behave in a reasonable
manner as they go about their daily activities, many of our
laws reflect observations of the way reasonable people behave
under most circumstances.
Generally speaking, traffic laws that reflect the behavior
of the majority of motorists are found to be successful, while
laws that arbitrarily restrict the majority of motorists encourage
violations, lack public support, and usually fail to bring
about desirable changes in driving behavior. This is especially
true of speed zoning.
Speed zoning is based upon several fundamental concepts deeply
rooted in our American system of government and law:
- Driving behavior is an extension of social attitude,
and the majority of drivers respond in a safe and reasonable
manner as demonstrated by their consistently favorable driving
- The normally careful and competent actions of a reasonable
person should be considered legal.
- Laws are established for the protection of the public
and the regulation of unreasonable behavior on the part
- Laws cannot be effectively enforced without the consent
and voluntary compliance of the public majority.
The public normally accepts the concepts noted in the previous
section. However, the same public when emotionally aroused
in a specific instance, will often reject these fundamentals
and rely instead on more comfortable and widely held misconceptions,
- Reducing the speed limit will slow the speed of traffic.
- Reducing speed limits will decrease the number of accidents
and increase safety.
- Raising the posted speed limit will cause an increase
in the speed of traffic.
- Any posted speed limit must be safer than an unposted
speed limit, regardless of the traffic and roadway conditions
- Drivers will always go 5 m.p.h. over the posted speed
Contrary to popular belief, speed in itself is not a major
cause of accidents. In fact, accidents appear to depend less
on absolute speed and more on the variation of speeds in the
INTENT OF SPEED ZONING
The basic intent of speed zoning is to identify a safe and
reasonable limit for a given road section. The most widely
accepted method is to set the limit at the speed which 85
percent of the traffic is moving at or below. This reflects
the safe speed as determined by a large majority of the drivers.
Research has shown that the 85th percentile speed is the
speed where accident involvement is the lowest. Reducing the
speed limit below that which is warranted can actually be
detrimental to safety.
HOW SPEED LIMITS ARE ESTABLISHED:
Kansas Statutes, Section 8-1559 allows the establishment
of speed limits on the State Highway System “upon the basis
of an engineering and traffic investigation.”
Speed zoning in Kansas is based on the widely accepted principle
of setting speed limits as near as practicable to the speed
at or below which 85 percent of the drivers are traveling.
According to a Federal Highway Administration study, all states
and most local agencies use the 85th percentile speed of free
flowing traffic as the basic factor in establishing speed
This speed is subject to revision based upon such factors
as: crash experience, roadway geometries, parking, pedestrians,
curves, adjacent development, and engineering judgment. This
practice is in accordance with the Manual on Uniform Traffic
Control Devices which has been adopted by the State of Kansas.
HOW SPEED DATA IS COLLECTED
Radar is used to collect speed data from random vehicles
on a given roadway. Off peak hours are normally used in conducting
a spot speed study with the speed of approximately 50 free
flowing vehicles in each direction obtained. On low volume
roads where it would be difficult to obtain a sample of 100
vehicles, the study may be terminated after a study period
of one hour. Vehicles are selected at random from the free
flow of the traffic stream to avoid bias in the results.
Probably one of the most important factors in a speed study,
but the one most difficult to define, is engineering judgment.
No matter how complete policies and guidelines are, there
will always be speed studies with peculiarities requiring
Sometimes, the decision to raise or lower a speed limit in
a certain area may have to be based on the traffic investigator’s
own personal judgment. In some remote areas, where there is
insufficient traffic for a valid speed sample, traffic engineers
may have to base their decision on the impression obtained
from driving the speed study area.
In the final analysis, it is the judgment of the traffic
engineer that determines which, if any, of the factors in
the speed study warrant a downward adjustment of the 85th
After all variables are considered and a speed limit is established,
traffic should flow at a safe and efficient level.
WHAT REALISTIC SPEED LIMITS DO:
Realistic speed limits are of public importance for a variety
- They invite public compliance by conforming to the behavior
of the majority.
- They give a clear reminder of reasonable and prudent speeds
to non-conforming violators.
- They offer an effective enforcement tool to the police.
- They tend to minimize the public antagonism toward police
enforcement which results from obviously unreasonable regulations.
- They encourage drivers to travel at the speed where the
risk of accident involvement is the lowest.
WHAT UNREALISTIC SPEED LIMITS DO:
Unrealistic speed limits are also of public importance for
the following reasons:
- They do not invite voluntary compliance, since they do
not reflect the behavior of the majority.
- They make the behavior of the majority unlawful.
- They maximize public antagonism toward the police, since
the police are enforcing a “speed trap”.
- They create a bad image for a community in the eyes of
- They increase the potential for accidents
along a roadway.
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