This article presents five key lessons learned from more than 25 years of experience as a traffic engineer and planner. These lessons are intended to help young professionals avoid many of the pitfalls the author encountered during his career. The field of traffic engineering/planning is unique when compared to other engineering disciplines due to the involvement of the public and their perceptions. These lessons are: 1. Only traffic engineers/planners understand the concept of level of service. 2. Traffic engineers/planners know too much math. 3. The public often are the real local traffic experts. 4. Visit the study area several times during the project, especially during the mot relevant time periods and just before key meetings. 5. Understand each jurisdiction's traffic study requirements, design standards and hearing/meeting procedures.