U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-97-152
Guidelines for the Use of Raised Pavement Markers
SECTION 3. LOOK–UP TABLES
A working set of user–friendly guidelines is essential for the traffic engineer to have clear, concise instruction for RPM use. To provide the traffic engineer with the most efficient tool to make decisions regarding roadway delineation, it is suggested that a form of look–up table be developed to present the information in a clear and concise format. Ideally, this could be presented in a software package; the cells of a first–level table would have a hypertext link to an anchored second–level table. This would allow the user to point and click on the cell that requires more detailed information about delineation.
Each level should specify the use of RPMs on the basis of the evaluation of several variables. For instance, in order to determine if and how to delineate horizontal curves with RPMs, variables such as ADT, degree of curvature, super elevation, radius of curvature, and length of curve need to be specified. If it is determined that the road segment of interest qualifies for RPM delineation, a second–level table will provide the specifications of placement and spacing of markers according to passing/no–passing zones, lane width, ambient lighting, etc. The following is a rough example of a first– and second–level look–up table for tangent sections of road. If the characteristics of the road to be delineated match the criteria set for a two–way, two–lane rural highway in table 1, the user can then refer to the table listed in that cell (table 2) to obtain more detailed information about how and where to use RPMs.
Table 1. Example of a first–level look–up table to determine when delineation by RPMs is required for tangent road sections.
Table 2. Example of a second–level look–up table to determine how to delineate using RPMs for a multilane tangent section of rural highway (two–way).
The variables used to determine RPM use are number of lanes, type of roadway, and ADT. Once it is determined that RPMs will be used, the engineer can look up placement and spacing information on the basis of striping and traffic zones. Matthias lists the practices of each State with regard to RPM use.(11) Although limited, the information provided by the State of Illinois can be used to demonstrate these look–up tables (refer to table 3). Boxes remain empty if no information is provided, and the "Not Specified" option is presented only to accommodate the information available by this source. Ideally, all boxes would have specific criteria for RPM use, or a statement such as "Do not delineate using RPMs" should be present. If one determined that the average daily traffic of a two–lane rural highway was greater than 15,000, the table would indicate that RPMs should be used to delineate the road. The user would then refer to the listed table (table 4) to determine the placement and spacing to be used.
Table 3. Example of a first–level look–up table to determine when delineation by RPMs is required for a tangent road section, using practices of Illinois.(11)
Table 4. Example of a second–level look–up table to determine how to delineate using RPMs for a two–way, multilane tangent section of rural highway, using practices of Illinois.(11)
There is currently no national (or even regional) standard from which these tables can be generated. States may use vastly different criteria. For instance, according to Matthias, Illinois will use RPMs on two–lane, two–way rural highways if the ADT is greater than or equal to 15,000 vehicles/day. Wisconsin, however, will use RPMs to delineate two–lane, two–way rural highways if the ADT is greater than 6,000 vehicles/day.(11) It would be useful to standardize RPM use across the country (or within regions, to accommodate weather differences) for both when and how they are used.