Mesa speed limit map
Establishing Speed Limits...A Case of Majority Rule
Realistic speed limits are a traffic engineering tool used to
derive the best traffic service for a given set of roadway
conditions. 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. Traffic regulations are
invariably based upon observations of the behavior of groups of
motorists under various conditions.
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 drivers encourage
wholesale 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
- 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 of individuals.
- Laws cannot be effectively enforced without the consent
and voluntary compliance of the public majority.
Public acceptance of these concepts is normally instinctive.
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, such as:
- Speed limit signs will slow the speed of traffic.
- Speed limit signs will decrease the accident rate
and increase safety.
- Raising a 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
Before and after studies consistently demonstrate that there
are no significant changes in traffic speeds following the
posting of new or revised speed limits. Furthermore, no
published research findings have established any direct
relationship between posted speed limits and accident frequency,
although short-term reductions have resulted from saturation
enforcement efforts directed at speed and other traffic law
Police agencies necessarily rely on reasonable and well
recognized speed laws to control the unreasonable violator whose
behavior is clearly out of line with the normal flow of traffic.
Contrary to popular belief, speed in itself is not a major
cause of accidents. In fact, there is a consensus of
professional opinions that many speed-related accidents result
from both excessively low and high speeds.
It is accepted within the traffic engineering profession that
there is a demonstrated need to produce as much uniformity as
possible in the traffic flow and to eliminate the so-called
speed trap. A speed trap may be defined as a street or road
which is wide enough, straight and smooth enough, and
sufficiently free of visibility limiting obstructions to permit
driving a certain speed, but where the law nevertheless calls
for a much lower speed.
What Realistic Speed Limits Do
- Realistic speed limits are of public importance for a
variety of reasons:
- 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
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 tourists.
How Realistic Speed Limits are Established
Arizona Revised Statutes, Section 28-702 allows the
establishment of speed limits on the State Highway System "upon
the basis of an engineering and traffic investigation." Speed
zoning in Arizona 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. This speed
is subject, of course, to downward revision based upon such
factors as: accident experience, roadway geometrics, and
adjacent development. Some questions which need to be answered
prior to establishing a speed limit are:
- Is the section of roadway sufficiently long enough to
permit safe accelerating and decelerating for the 85th
- Is the alignment, both vertical and horizontal,
capable of safely accommodating vehicles traveling at the
85th percentile speed?
- Are the lane widths, traffic volumes, and surface
conditions compatible with this speed?
- Will a vehicle traveling at the 85th percentile speed be
capable of making a safe and smooth stop, if necessary?
- Has a pattern of accidents developed which would
indicate that the 85th percentile speed is not appropriate?
- Is a certain speed limit necessary to provide signal
- Is development adjacent to the roadway causing a
significant amount of turning maneuvers or congestion?
One of the most important factors in a speed study, but the
one most difficult to define, is engineering judgment based on
the experience of the traffic engineer. No matter how complete
policies and guidelines are, there will always be speed studies
with peculiarities requiring engineering judgment. 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, the traffic investigator may
have to base his decision on a driving impression of the speed
In the final analysis, it is the engineering judgment
of the investigator that determines which, if any, of the
factors in the speed study warrant a downward adjustment to the
85th percentile speeds. After all variables are considered and a
speed limit is established, traffic should flow at an optimum
safe and efficient level.
300 E. 6th St.
Mesa, AZ 85201
City of Mesa
PO Box 1466
Mesa, AZ 85211
Monday - Thursday
7:00 a.m. - 6:00 p.m.
Closed Fridays & Holidays
480-644-2262 (after hours)