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19 pose of deciding upon the applicability, design, operation, or OTHER RECOMMENDATION installation of a traffic control device. An engineering study shall be performed by an engineer, or by an individual work- In addition to the changes to the MUTCD noted above, a ing under the supervision of an engineer, through the appli- fourth related recommendation is provided: cation of procedures and criteria established by the engineer. An engineering study shall be documented. The NCHRP should consider funding a project to incor- porate these factors in an expert system similar to the U.S. It is argued here that the definition is too general for the pur- Limits system being developed to provide guidance on the poses of placing TCDs on horizontal curves; it is thus recom- speed limit to be posted in speed zones. mended that a more explicit list of factors to be considered be The bases for this last recommendation are as follows: included in the definition. The definition cited does little to help practicing engineers know what to look at when under- · The practitioner focus-group participants indicated a taking such a study. Indeed, there are other instances in the need for guidance in the use of supplemental TCDs. MUTCD where additional factors to be considered in other sit- · The literature review demonstrates that no single variable uations are explicitly listed. For example, in §2B.13 regarding is effective in defining "problem" curves, but a combi- speed limit signs on p. 2B-11, six factors are listed including nation of factors can be associated with problem curves. road characteristics, pace speed, roadside development and This is a scenario where expert systems are most useful. environment, parking practices and pedestrian activity, and crash experience. Signal and other warrants are also, in essence, In the next chapters, practitioner responses to these pro- lists of factors to be considered in engineering studies. posals are outlined and final recommendations are presented.