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.

Traffic engineering is that phase of engineering that deals with the planning, geometric design and traffic operations of roads, streets and highways and their networks, terminals, abutting lands and relationships with other modes of transportation for the achievement of safe, efficient and convenient movement of people and goods.

Traffic engineering applies engineering principles that help solve transportation problems by considering the psychology and habits of the transportation system users.

Active studies, as well as those that are programmed (funded but not yet assigned to a consultant), are listed online in the Research in Progress database. To learn more about these studies:

The FHWA State Planning and Research (SPR) program funds most research projects following the requirements of 23 CFR 420. Those projects are designated as “SPR” plus a project number assigned by the Research Center, e.g, SPR-595.

The ADOT project manager, in cooperation with the sponsor and champion, leads the process of identifying TAC members, developing a scope of work, releasing a solicitation, and selecting a consultant to conduct the study.