Traffic control devices are intended to promote safe and uniform operation of motorized and nonmotorized traffic using the roadway. Motorists rely on traffic control devices to provide information about traffic laws and regulations, to identify potential roadway hazards, and to provide information to help them find their desired destinations. However, traffic control devices serve little purpose if they are not understood by a significant proportion of the driving population. The findings and recommendations of a 5-year research study conducted by the Texas Transportation Institute to assess and improve motorist understanding of traffic control devices are presented. Phase I of the project was devoted to several evaluations of 52 devices, administered to 2,414 Texas drivers. Phase II of the project was devoted to the development and evaluation of alternative designs for 10 traffic signs identified in Phase I that exhibited potential for driver misunderstanding. The Phase II evaluations included four focus groups, an initial statewide survey of 747 Texas drivers, and a follow-up survey of 212 drivers. The results of all evaluations were analyzed to distinguish significant comprehension difficulties. Recommendations for each device were based on these results and include retaining the current standard design because of adequate comprehension levels, modifying the design or use of the device to increase comprehension levels, or conducting further research to better understand driver comprehension difficulties.