Communicating Changes in Horizontal Alignment (2006) / Chapter Skim
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Pages 5-18
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From page 5... ...
• Specific recommendations for curve TCDs on lowvolume rural roads are not provided. • Benefits and guidance for using new TCDs in the MUTCD (e.g., the combined alignment/advisory speed sign)
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From page 6... ...
Identifying hazardous road segments is best accomplished by considering curve operational and physical properties and crash history. Posted, design, and operating speeds alone cannot predict overall safety on horizontal curves.
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Items that should likely be included in such a study for TCD selection and placement at horizontal curves include consideration of the following: • How other potentially limiting factors such as weather conditions would affect curve perception and vehicle performance; • Natural features of the road and roadside environment that provide delineation; • Alignment that drivers may find difficult to perceive or interpret; • Driver expectations; • Application of both the preview sight distance concept and 85th-percentile speed to longitudinal warning sign placement guidelines; • Degree of curvature and other parameters associated with high-risk alignments; • Guidelines for when redundant warning or warning along with delineation is recommended; and • Guidelines for when curve warning is not recommended based on curve or volume parameters apart from crash history. The second need is to recommend advisory speeds based upon driver comfort or an assessment of how fast drivers are already driving.
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From page 8... ...
The Michigan group indicated that this is partially due to the flexibility afforded by "engineering judgment" but mostly because of the differences in budgets allocated to TCD deployments. The latter resulted in practitioners in some jurisdictions being "comprehensive" when it came to signing and marking horizontal curves.
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From page 9... ...
For example, one of the primary responses to the question "Which horizontal curves are the most difficult to sign and mark appropriately? " was "intersection on curve." However, there is a standard intersectionon-a-curve sign (W1-10)
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From page 10... ...
These sessions were spent discussing TCDs in general, identifying actual problem curve situations encountered by the participants, and then reviewing the TCDs that had been deployed and how they might be modified to make the situations less problematic. There was also discussion about responses to advisory speed signs and the general topic of TCDs for horizontal curves.
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From page 11... ...
• Respondents generally stated that advance curve warning signs helped them anticipate and drive through a curve. • Respondents generally stated that large arrows and chevrons as well as pavement markings helped them anticipate and drive through a curve.
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From page 12... ...
These caveats notwithstanding, differences between the two groups included the following: • Crash groups in both states were more likely than their typical counterparts to disagree with a statement indicating that they were getting enough advance information about upcoming curves. • Although all respondents were generally positive about advisory speed signs, crash groups were more likely to disagree with a statement indicating that advisory speed signs were helpful.
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From page 13... ...
to a maximum safe speed on dry pavement. FIELD STUDY OF DRIVER BEHAVIOR USING DRIVER PERFORMANCE MONITORING TECHNIQUE The project also included a driver observation study using driver performance monitoring (DPM)
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From page 14... ...
An example of the latter is comparison between average subject speed and the posted regulatory and advisory speeds. Another basis for comparison of driver performance is a hierarchical ranking of "curve difficulty" or complexity based on the TCDs currently deployed.
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From page 15... ...
. Signed advisory speeds for the DPM sequences ranged from 20 to 40 mph.
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From page 16... ...
Given the numbers and types of driver errors with existing treatments, it seems appropriate that signing for curves with the greatest difference between the posted speed limit and the advisory speed should be comprehensive (e.g., advance warning signs, speed advisories, chevrons, or centerline markings)
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From page 17... ...
This phenomenon is also noted in the literature review. • Raising advisory speeds may well result in drivers who routinely exceed advisories and approach the maximum safe speeds, at least in the short term.
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From page 18... ...
vatively, negotiating them with ease. The other sequence was more problematic with a much more abrupt curve with lower visibility through it.
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