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3 Chevrons · RPMs tend to have more effect during the night than the day. The literature review pertaining to chevrons resulted in the · RPMs had more effect on lateral placement over and following summary comments: above freshly painted centerlines over time. · Drivers have been noted to shift away from chevrons as · The variability of speed and lateral placement was de- they negotiate curves--toward the centerline on curves creased with RPMs. to the left, away from it on curves to the right. · There was some evidence that RPMs had greater effect · Several studies noted that average (day and night) speeds than PMDs and chevrons. increased when chevrons were added. It is also worth noting that the recent NCHRP Report 518: · Crash reductions have been noted on curves marked with Safety Evaluation of Permanent Raised Pavement Markers chevrons where standard curve-related signs have not contains a definitive review of the effectiveness of perma- been effective. nent RPMs. While some results were mixed and/or inconclu- · While chevrons provide additional guidance for the sive, significant crash reductions were noted for wet-weather driver, perhaps not more than post-mounted delineators crashes and especially at night. or raised pavement markers. · Chevrons have more effect at night, on sharper (>7°) curves, and when used in conjunction with edgelines. Discussion and Comments Notwithstanding the impact of different TCDs in different Edgelines and Centerlines circumstances, improvements to road geometry are clearly preferred to warning signs or delineation at locations with The literature review pertaining to edge- and centerlines hazardous changes in horizontal curvature. However, where resulted in the following summary comments: right-of-way, economic, or other practical limitations pro- · Results regarding increase or decrease in vehicle speeds hibit or delay desirable improvements, standard TCDs can be when edge- and centerlines are used are varied: some used effectively to warn drivers and to guide them through studies show an increase, and others show a decrease. the potentially hazardous road segment. · Freshly painted lines tended to decrease lateral placement Identifying hazardous road segments is best accomplished variance. by considering curve operational and physical properties and · As above, edgelines seem to have more effect when used crash history. Posted, design, and operating speeds alone can- with chevrons. not predict overall safety on horizontal curves. It appears that aggregated crash rates are positively correlated with high degrees of curvature: curvature of approximately 5° or greater Post-Mounted Delineators poses an increased hazard of which the driver should be The literature review pertaining to post-mounted delin- warned. Other results showed that, in practice, the sharpest eators (PMDs) resulted in the following summary comments: curves already have extensive TCD treatment and that some modest curves were among the most problematic in terms of · Similar to chevrons, drivers tend to shift away from higher crash rates and less treatment. PMDs--that is, away from the edge of road and toward TCDs used on horizontal curves should be applied uni- the centerline. formly to foster desirable driver expectancies. In placing · Day- and nighttime speeds tend to increase when PMDs curve signs, engineers should consider preview sight distance, are used. driver perception of road geometry, and drivers' expectations · PMDs have been shown to reduce variance in lateral for curve warning. In addition to curve warning signs, delin- placement. eation will improve driver perception of alignment changes. · PMDs should only be placed on the outside of the curve Other roadside information is also an important information and could be confusing if placed otherwise. carrier--any practical measures that can be taken to improve · PMDs have more effect when used with freshly painted a driver's curve perception should take priority over warnings. centerlines. While most of the TCDs for curves have been shown to be effective in certain experiments, the testing on which the Raised Pavement Markers results are based was rarely comprehensive or conclusive across the spectrum of conditions encountered in the field. The literature review pertaining to raised pavement markers For example, some work has shown that chevrons, PMDs, (RPMs) resulted in the following summary comments: and RPMs result in higher speeds on curves. Given that most curve crashes occur because the motorist enters the curve at · RPMs are more effective at reducing crashes at high- too high a speed, would we want to increase motorist speeds crash locations than elsewhere although general reduction on all, or any, curves? Clearly, a definitive guideline is hard was not noted. to develop based on the results of existing work.