Signals offer the maximum degree of control at intersections. They relay messages of both what to do and what not to do. The primary function of any traffic signal is to assign right of way to conflicting movements of traffic at an intersection, and it does this by permitting conflicting streams of traffic to share the same intersection by means of time separation.

Guide signs require simplicity and clarity because drivers of moving vehicles are unable to read lengthy or complicated messages on signs. For this reason, sign messages should not exceed three lines.

On freeways, high traffic speeds demand that the number of signs be limited to those absolutely essential for the guidance of the motorist. Freeway exits are identified by the exit number, the route number or the name of the intersecting road. Certain additional messages may be provided where justified.

The principal purpose of guide signs is to direct travelers to their destinations by the best route. However, it is not feasible to install signs listing all the possible destinations that may be reached from the highway. Drivers must make reasonable preparation for locating their destinations and have information that is readily available on road maps or GPS systems.

Traffic signals should be installed when they will alleviate more problems than they will create. This determination must be based on an engineering study.

A warranted traffic signal that is properly located and operated may provide for more orderly movement of traffic and may reduce the occurrence of certain types of accidents. On the other hand, an unwarranted traffic signal can result in increased delay, congestion and accidents.

Posted speed limits have very little effect on actual traffic speeds. There is a common belief that the posting of speed limit signs will cause drivers to react accordingly. This is not true and is why posted speed limits must be realistic to achieve compliance.

Legal speed limits are established by law (in Arizona) and may be changed only when justified on the basis of an engineering study.

"Uniformity" means treating similar situations in the same way. Applied to traffic engineering, uniformity simplifies the drivers' tasks because it aids in instant recognition and understanding. Uniformity aids police, courts and road users by giving everyone the same interpretation. It aids public-highway officials through economy in manufacturing, installing, maintaining and administering the roads.

Traffic control devices are all signs, signals, markings and devices placed on, or adjacent to, a street or highway by a public body having authority to regulate, warn or guide traffic.

Traffic engineers promote safer traffic conditions by providing roadway conditions that contribute to smooth and efficient traffic flow.

Experience has shown that safety goes hand in hand with smooth traffic operation. Disrupting the smooth flow of traffic increases the probability of accidents.

Erratic traffic operation may be caused by vehicles stopping or slowing in the roadway, passing and weaving maneuvers, or other surprise elements. For example, unwarranted traffic signals, unreasonably low speed limits and too many signs may cause driver confusion and indecision.

The role of traffic engineers may be compared to that of the medical profession in protecting the public. As trained professionals, traffic engineers look at the symptoms of general traffic conditions, and to make a competent diagnosis, they take traffic counts, analyze accident statistics, study speed data, examine roadway conditions, conduct research and study what other professionals are doing and the results they have achieved.