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45 On state highways at locations determined by the Bureau PRPMs (thus, the results of the engineering study of Traffic Engineering on the basis of accident data should be entered into the safety resource allocation (snowplowable PRPMs). process). The guidelines suggest that snowplowable PRPMs should not be installed on interchange ramps (5). The suggestions 6.3 PROPOSED GUIDELINES FOR PRPMS above should be considered cautiously because the authors ON FOUR-LANE FREEWAYS likely did not intend that freeways and Interstate highways in regions with no snowfall should have snowplowable markers. The following guidelines are proposed: AMFs shown in Table 6-2 should be used to guide deci- 6.2 PROPOSED GUIDELINES FOR PRPMS sions on where to install PRPMs (i.e., when an AMF is ON TWO-LANE ROADWAYS less than 1). An AMF is the ratio between the number of The following guidelines are proposed: crashes per unit of time expected after a measure is implemented and the number of crashes per unit of time AMFs shown in Table 6-1 should be used to guide deci- estimated if the implementation does not take place. An sions on where not to install PRPMs (i.e., when an AMF AMF less than 1 would indicate a positive safety effect is greater than 1). An AMF is the ratio between the num- (i.e., a reduction in crashes), while an AMF greater than ber of crashes per unit of time expected after a measure is 1 would indicate a negative safety effect (i.e., an implemented and the number of crashes per unit of time increase in crashes). If a cost-effectiveness study is required, the analytical estimated if the implementation does not take place. An AMF less than 1 would indicate a positive safety effect engineering procedure illustrated for two-lane roadways (i.e., a reduction in crashes), while an AMF greater than can be used in a similar manner. 1 would indicate a negative safety effect (i.e., an increase in crashes). 6.4 PROPOSED REVISIONS TO THE MUTCD Given the negative safety impacts that are demonstrated to be associated with curves with more than 3.5 degrees In light of the findings of the composite and disaggregate of curvature, and given the findings of speed increases analyses of PRPMs on two-lane roadways and four-lane in association with PRPMs, it would seem prudent to divided freeways, the following changes to the MUTCD are avoid placing PRPMs well in advance of roadway sec- proposed: tions with substandard geometry or where the feature is unexpected because of the character of the road previ- 1. Add the following paragraph after the initial paragraph ously encountered by the driver. under Support for Section 3B.11 on top of page 3B-29: An analytical engineering procedure should be under- taken at locations where an AMF is less than 1 to assess Retroreflective raised pavement markers enhance guid- the cost-effectiveness of the PRPM installation. ance for drivers by providing longer delineation of the The results of the analytical engineering procedure travel path during nighttime and wet pavement condi- should form part of the decision-making process for tions. They also provide auditory feedback when the motorist approaches the edge of the travel lane, whether to install PRPMs at a given location. Other although snowplowable raised markers do so to a much issues to be considered with this information are lesser extent. These positive effects can be offset some- Other measures for improving nighttime crashes that times by inducing higher speeds, which under certain may result in higher benefit-cost effectiveness and conditions, such as on sharp curves, can result in an Other locations that may result in a higher-than- overall negative safety benefit. expected cost-effectiveness from the installation of The purpose of the above paragraph is to recognize TABLE 6-1 AMFs (nighttime crashes) the positive and potential negative effects of snow- derived from Table 4-3 AADT (veh/day) AMF TABLE 6-2 AMFs (nighttime When When crashes) derived from Table 4-7 DOC 3.5 DOC > 3.5 AADT (veh/day) AMF 05000 1.16 1.43 500115000 0.99 1.26 20000 1.13 1500120000 0.76 1.03 2000160000 0.94 > 60000 0.67 DOC = degree of curvature.