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40 CHAPTER 5 DISCUSSION OF STUDY RESULTS This chapter discusses the results presented in Chapter 4 in support this report's hypothesis of snowplowable PRPMs' conjunction with related human factors issues. The purpose of effect on lane control and positioning. the discussion presented here is to link the statistical results of this research study with other findings of past research studies that have analyzed PRPM installations and observed effects in driver behavior. The discussions are presented for two-lane 5.1.1.2 Speed Control roadways and four-lane freeways separately. When the preview of the road ahead is reduced, as it is dur- ing nighttime with low-beam headlights, lane control becomes more difficult and driver workload increases, causing drivers 5.1 TWO-LANE ROADWAYS to compensate by reducing their speed. Conversely, when the preview of the road is improved through delineation, driver 5.1.1 Overview of Human Factors Issues workload decreases and drivers may compensate by The purpose of PRPMs is to provide improved delineation increasing speeds. Harms (43) investigated speed choice in at night. Studies have shown that drivers on approaches to fog and found that drivers tend to undercompensate (i.e., not curves need 3 to 5 seconds of preview distance in order to reduce speeds enough) in poor visibility conditions. Because feel comfortable with the changes in the road path (25). At of this and other studies of driver speed choice, Rumar and night, such long preview distances cannot be provided by Marsh (44) predict that drivers overcompensate (i.e., increase paint, but are possible using PRPMs, post-mounted delin- speeds too much) in improved visibility conditions. eators, and chevrons. It is expected that the improved visi- Studies have found that speed increases at night after the bility produced by PRPMs will affect crash rates by affecting implementation of PRPMs (21, 34). Improved delineation, in two types of driver behavior: the form of post-mounted delineators, was associated with nighttime speed increases and increased crash frequency on Lane control and positioning and roads with low design standards, but not on roads with high Speed control. design standards (42). A driver who increases speed, especially at night, is responding inappropriately. While PRPMs improve the vis- 5.1.1.1 Lane Control and Positioning ibility of changes in the road path, they do not improve the visibility of other hazards, such as pedestrians, bicyclists, Previous studies of conventional PRPMs have found that animals, and debris. Higher speeds lead to longer stopping dis- PRPMs on curves cause drivers to shift away from the cen- tances and greater crash potential. Higher speeds in curves will terline at night (21, 34, 42). However, the impact on lane result in an increase in lateral acceleration and a greater position during the day is not conclusive: one study (35) shows potential for run-off-road crashes. a shift toward the centerline and another study (21) shows a The issue of speed is likely to be more of a problem on shift away from the centerline. curves with small radii. On high-speed roads, such curves The conventional PRPMs analyzed in the above studies force drivers to make large speed reductions. However, stud- protrude much higher above the road surface than do the ies of driver lateral acceleration in curves show that drivers snowplowable PRPMs analyzed in this study. Because of this drive closer to the safety margin on tight curves than on gen- protrusion, conventional PRPMs provide an auditory warning tle curves (45). This suggests that drivers are reluctant to of lane crossing that is not found with snowplowable PRPMs. drop speed too much and trade off comfort for time savings. Therefore, any change in lane position at night with snow- Any small increase in speeds associated with PRPMs will plowable PRPMs is expected to result from improved delin- have a greater negative safety effect when drivers are closer eation. Kallberg (42) found that drivers move away from con- to the safety margin. This greater negative safety effect may spicuous post-mounted delineators. Kallberg's finding may be the reason underlying the Kallberg study's finding (42),