Speed Limits
Q: Can we lower the speed limit on our street?
A: An unrealistically low speed limit can actually lead to accidents. Here's why:
First, many studies conducted over several decades in all parts of the country have shown that a driver's speed is influenced more by the appearance of the roadway and the prevailing traffic conditions than it is by the posted speed limit. Tests have been conducted where the numbers on speed signs were arbitrarily raised or lowered to see what effect this would have on traffic. Checks where made before and after the changes, and the conclusive results were that traffic speeds remained very nearly the same regardless of the numbers on the sign (see figure below). These studies indicate the validity of the assumption that drivers select their speeds intuitively, based on the environment around them and the speeds with which they feel comfortable and safe.
Second, some drivers will obey the lower posted speed while others will feel it's unreasonable and simply ignore it. This disrupts the uniform traffic flow and increases accident potential between the faster and slower drivers. As the figure below shows, the possibility for an accident is lowest for speeds near the 85 percentile (that is, the speed at which 85 percent of the traffic is moving at or below).
Third, when traffic is traveling at different speeds, the number of breaks in traffic to permit safe crossing is reduced. Pedestrians also have greater difficulty in judging the speed of approaching vehicles. Local authorities may post intermediate speed limits - those between 25 and 55 miles per hour. But these speed limits should always be based on traffic engineering surveys that include and analysis of roadway conditions, accident records and the prevailing speed of prudent drivers.
Q: What factors are considered before installing traffic signals?
A: Properly designed and operated traffic signals are valuable devices for controlling the assignment of right-of-way at intersections justifying this type of control. Unfortunately, traffic signals are also viewed by many persons as a solution to any traffic problem and a panacea for vehicle or pedestrian accidents. Traffic signals do not always reduce or prevent accidents and are not always an asset to traffic control. In some instances, the number of accidents and injuries increase after signals are installed. Usually in such cases, the right angle collisions are reduced but the total number of collisions, especially rear-end or turning type accidents, may stay the same or increase.
Q: When can a traffic signal be an asset instead of a liability to safety?
A: In order to answer this, traffic
engineers have to ask and answer a series of questions:
1. Are there so many vehicles on both streets that signal controls are
necessary to assign the right-of-way
or relieve congestion?
2. Is the traffic on the main street
so consistently heavy that drivers on the side street must try to cross
when it is unsafe?
3. Are there so many pedestrians trying to cross a busy main street
that confusing, congested, or hazardous conditions result?
4. Are there so many school age children trying to cross the street at
the same times that they need special controls for their supervision or
protection? If so, is a
signal the best solution?
5. Are signals at this location going to help drivers maintain a
uniform pace along the major street without being stopped unnecessarily?
6. Does the collision history indicate that a signal will reduce the
probability of driver actions which cause a collision?
7. Is the character of the minor street such that additional traffic
attracted by the existence of a signal desirable to the adjacent
neighborhood?
8. Is there a combination of the above conditions, which indicated
that a traffic signal will result in an improvement rather than a
detriment?
To
aid in answering these questions, engineers compare the existing facts and
conditions to nationally accepted minimum guidelines.
These guidelines (called "warrants") were established
using observations throughout the country over many years.
Where the established guidelines are met, traffic signals generally
operate effectively and safety is improved.
When the guidelines are not met, additional hazards result.
Unnecessary traffic signals increase congestion, waste tax money,
and breed disrespect for other forms of traffic control.
More importantly, however, signals in the wrong locations fail to
provide safety and protection to anyone.
Q: How do I report a malfunctioning signal?
A: To report a traffic signal malfunction or bulb out, call 515-239-5535.
Children at Play
Q: Won't a Children At Play sign help protect our kids?
A: At first consideration, it might seem that this sign would provide protection for youngsters playing in a neighborhood. It doesn't. Studies made in cities where such signs were widely posted in residential areas show no evidence of having reduced pedestrian accidents, vehicle speed or legal liability. In fact, many types of signs which were installed to warn of normal conditions in residential areas fail to achieve the desired safety benefits. Further, if signs encourage parents with children to believe they have an added degree of protection - which the signs do not and cannot provide - a great disservice results. Obviously, children should not be encouraged to play in the roadway. The "children at play" sign is a direct and open suggestion that it is acceptable to do so. Federal standards discourage the use of "children at play" signs and they are not even recognized in traffic sign manuals. Specific warnings for schools, playgrounds, parks and other recreational facilities are available for use where clearly justified. Examples of these follow:
Q: Why don't they put up more stop signs?
A: A stop sign is one of our most valuable and effective control devices when used at the right place and under the right conditions. It is intended to help drivers and pedestrians at an intersection to decide who has the right-of-way. One common misuse of stop signs is to arbitrarily interrupt through traffic, either by causing it to stop, or by causing such an inconvenience as to force the traffic to use other routes. Where stop signs are installed as "nuisances" or "speed breakers," there is a high incidence of intentional violation. In those locations where vehicles do not stop, the speed reduction is effective only in the immediate vicinity of the stop sign, and frequently speeds are actually higher between intersections. For these reasons, it should not be used as a speed control device. Most drivers are reasonable and prudent with no intention of maliciously violating traffic regulations; however, when an unreasonable restriction is imposed, it may result in flagrant violations. In such cases, the stop sign can create a false sense of security in a pedestrian and an attitude of contempt in a motorist. These two attitudes can and often do conflict with tragic results. Well-developed, nationally recognized guidelines help to indicate when such controls become necessary. These guidelines take into consideration, among other things, the sight distance available to motorists, traffic volume and accident histories.
Q: Can the city have bushes trimmed that block sight distance?
A: The provisions found in Chapter 29 of the Municipal Code of Ames, Iowa 1985, Section 29.408, visibility at intersections, set forth certain restrictions concerning the planting and growth of foliage on corner lots. Section 29.408 states that no foliage plant is permitted to grow to a height of more than three (3) feet above the elevation of the established curb grade at the intersection of these streets on the part of any yard which is bounded by the property lines of the intersecting streets and a line connecting two (2) points on said property lines twenty (20) feet from their point of intersection.
Credits
Portions of this document have been taken from materials published by the
following organizations: