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2 CHAPTER 2 FINDINGS LITERATURE REVIEW Summary comments from the literature review pertaining to different types of devices follow. The literature review included examination of traditional sources and readily available reports. It should be noted from the outset that there appears to be a considerable body of lit- Curve and Turn Signs erature that is not documented in any traditional way and thus The literature review pertaining to curve and turn signs re- is not included. This includes work done by various agencies sulted in the following summary comments: that is either not documented in any usable way or only avail- able within the agency. An example of the former would in- · The messages conveyed by curve-related warning signs clude a TCD treatment that was implemented in the field but are likely more generic than traffic engineers might hope. never documented or studied in any methodical way; an exam- Messages are probably weakened by driver limitations ple of the latter would include an interoffice memo that might in perceiving and understanding nuanced curve/turn summarize an implementation and a brief study at a specific warning signs, which reaffirms the need for redundancy location. Such documents include unwritten guidelines for at highest-risk locations. placement of various devices. For example, a local jurisdic- · While there are varied results regarding whether curve- tion (in Michigan) has "rules" for when chevrons are used as related signs reduce crashes, run-off-the-road (ROR) and well as for choosing the number and placement locations-- single-vehicle crashes are, nonetheless, probably reduced these "rules" are not published but simply "known" within the when such signs are used (when compared with no signs). agency. · If conventional curve-related signs are ineffective at Such problems notwithstanding, some of the more inter- high-crash locations, there is some evidence that special esting overarching results from the literature review are sum- treatments such as oversized or traffic-actuated signs marized immediately below and then followed by comments with beacons are effective. regarding specific types of devices: · The longitudinal placement for curve signs is typically · The MUTCD provides only general guidance on the based on the 85th-percentile or posted speed. This is con- selection and application of TCDs used to inform drivers sistent with the sign placement table in the MUTCD. of a change in horizontal alignment. · Novelty effects should be carefully considered in any Advisory Speeds new TCD evaluation. · Currently, advisory speed signing appears to be largely The literature review pertaining to advisory speeds resulted ineffective if the goal is for drivers to actually travel at in the following summary comments: the posted advisory speed: drivers either fail to notice advisory speed plaques, or, more likely, they simply re- · Some studies indicate that advisory speed plaques are no ject the literal advisory speed recommendations, driving more effective than curve/turn signs alone. Conversely, at a reduced speed that they feel is appropriate. others found that there was some speed reduction asso- · Because raised pavement markers and post-mounted ciated with the placement of advisories although not to delineators provide both far and near guidance up to and the posted speed per se--that is, advisory speeds are rou- through a curve, delineation should be part of a com- tinely exceeded. prehensive curve-risk reduction program. Also see more · Drivers may underestimate their actual speeds on curves. specific comments below. · Surveyed practitioners feel that guidelines for assigning · Specific recommendations for curve TCDs on low- advisories are sufficient. volume rural roads are not provided. · Some results indicate that single-vehicle and ROR · Benefits and guidance for using new TCDs in the MUTCD crashes are reduced when advisories are posted and that (e.g., the combined alignment/advisory speed sign) are not "sections" (i.e., a segment of road with numerous curves) provided in the literature. with advisories posted experience fewer crashes.