Traffic

Frequently Asked Questions

Establishing Speed Limits

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. The content of this summary is based on a booklet called "Establishing Speed Limits: A Case of 'Majority Rule,'" published by the Arizona Department of Transportation.

Why do roads have speed limits?

Because most citizens can be relied upon to behave in a reasonable manner as they go about their daily activities, many of 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 travelers under various conditions.

Generally speaking, traffic laws that reflect the behavior of the majority of vehicle operators are found to be successful, while laws that arbitrarily restrict the majority of drivers encourage violations, lack public support and usually fail to bring about desirable changes in driving behavior. These effects are especially true of speed zoning.

Speed zoning is based upon several fundamental concepts deeply rooted in the 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 records.
  • 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:

  • Speed limit signs slow the speed of traffic.
  • Speed limit signs decrease the accident rate and increase safety.
  • Raising a posted speed limit causes 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 prevailing.

"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 violations.

Police agencies necessarily rely on reasonable and well-recognized speed laws to control the unreasonable violators 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 that 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 do 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 nonconforming violators.
  • They offer an effective enforcement tool to law-enforcement officers.
  • They tend to minimize the public antagonism toward law enforcement that results from obviously unreasonable regulations.

How are realistic speed limits 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. There are some questions that need to be answered prior to establishing a speed limit:

  • Is the section of roadway sufficiently long enough to permit safe accelerating and decelerating for the 85th percentile speed?
  • 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 that would indicate that the 85th percentile speed is not appropriate?
  • Is a certain speed limit necessary to provide signal progression?
  • Is development adjacent to the roadway causing a significant amount of turning maneuvers or congestion?

How does engineering judgment affect the establishment of a speed limit?

Probably 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 a decision on an impression of the speed study area.

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.