Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections (2008) / Chapter Skim
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Pages 29-61
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From page 29... ...
This section contains an overview of each treatment, as well as more detailed descriptions, summaries of applicability and pertinent considerations such as maintenance, discussions of potential layouts and designs, and summaries of the documented effectiveness of each treatment in reducing speeds and improving safety. The treatments discussed are • Dynamic warning signs, • Transverse pavement markings, • Transverse rumble strips, • Longitudinal rumble strips, • Wider longitudinal pavement markings, • Roundabouts, • Approach curvature, • Splitter islands, • Speed tables and plateaus, • Reduced lane width, • Visible shoulder treatments, and • Roadside design features.
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From page 30... ...
Dynamic warning signs have been used at curve approaches, in work zones, and in other locations that require reduced speeds. Dynamic warning signs have also been used to warn drivers of conflicting cross-traffic at intersections.
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From page 31... ...
If a radar unit is included as part of the dynamic sign treatment, it must be placed to detect drivers' speeds in advance of the sign. 4.2.4 Speed Effects At each NCHRP Project 3-74 test site in Washington and Texas, dynamic warning signs were tested at two approaches.
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From page 32... ...
Exhibit 4-3. Dynamic warning sign used as a collision avoidance system.
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Transverse pavement markings are commonly used in speed management to reinforce the need to reduce speed or to warn drivers of an approaching condition that may require vehicular maneuvers. Common applications of transverse pavement marking locations include approaches to traffic circles and intersections, horizontal curves, construction areas, bridges, and freeway off-ramps.
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From page 34... ...
Other appropriate locations for treatment installation may include the stopping sight distance for the approach speed, or a point where the roadway environment changes, such as at the point of 34 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Exhibit 4-4. Peripheral transverse pavement markings.
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. Studies of segment applications of transverse pavement markings have reported reduced mean and 85th-percentile speeds on the order of 20%-30%.
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In many cases, the objective of a transverse rumble strip is to call attention to a specific traffic control device, such as a Stop Ahead sign. Transverse rumble strips are placed perpendicular to the direction of travel, and are designed to reduce the deceleration rate and the potential for sudden braking, skidding, and loss of control.
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From page 37... ...
Exhibit 4-5. Transverse rumble strips in wheel path.
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From page 38... ...
The markings have potential to draw additional attention to those warning signs and to encourage drivers to reduce their speeds as they approach the intersection. Other appropriate locations for installation may include the stopping sight distance for the approach speed, or at a point where the roadway segment environment changes, such as at a point of tangency or at a driveway in advance of the intersection.
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From page 39... ...
On stop-controlled approaches, transverse rumble strips have repeatedly resulted in more gradual deceleration by drivers and increased the percentage of drivers making a full stop at the stop sign by about 30%. (Kermit and Hein, 1962; Owens, 1967; Zaidel et al., 1986; Harder et al., 2001)
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From page 40... ...
Their primary function is to use tactile and auditory stimulation to alert inattentive or drowsy drivers that their vehicles are encroaching on the opposing lane. Centerline rumble strips may also discourage drivers from cutting across the inside of horizontal curves.
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Centerline rumble strips on the centerline pavement markings. (Credit: Torbic et al., 2004)
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From page 42... ...
4.6 Wider Longitudinal Pavement Markings 4.6.1 Overview No test sites provided documented applications for the high-speed intersection treatments discussed in this section. Wider longitudinal pavement markings increase intersection visibility 42 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections
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From page 43... ...
Although no documentation for wider longitudinal pavement marking applications at intersection approaches was found, this treatment may be an effective speed reduction treatment for some high-speed intersections because it may increase driver awareness of the presence of an intersection and help reinforce the need for drivers to operate differently at the intersection than in the roadway segment. The most common reasons that jurisdictions apply wider longitudinal pavement markings are to improve visibility, assist older drivers, and reduce crashes.
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From page 44... ...
The markings have potential to draw additional attention to those warning signs and to encourage drivers to reduce their speeds as they approach the intersection. The standard width for longitudinal pavement markings is four inches.
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Roundabouts are appropriate for locations with a high crash frequency or severity, intersections where queues need to be minimized, intersections with irregular geometry, intersections that need to accommodate U-turns, and areas with a large amount of right-of-way available. Pedestrians are accommodated at a roundabout by crossings through splitter islands located around the perimeter of the roundabout.
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, research found a 72% reduction in all crashes and an 87% 46 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections (Credit: FHWA, 2000b) Exhibit 4-13 .
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(FHWA, 2000b) 4.8 Approach Curvature 4.8.1 Overview No test sites provided documented applications for the high-speed intersection treatments discussed in this section.
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It may be appropriate to introduce the curved geometry concurrently with additional treatments such as warning signs. The dominant feature of approach curvature treatments is the radius of the curve.
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(FHWA, 2000b) 4.9 Splitter Islands 4.9.1 Overview No test sites provided documented applications for the high-speed intersection treatments discussed in this section.
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FHWA has sponsored a study to determine the effectiveness of combining splitter islands on minor approaches with other types of speed reduction treatments, such as centerline rumble strips or pavement markings, on major approaches at two-way stop-controlled intersections on rural highways. 4.9.3 Treatment Layout/Design The length and geometry of splitter islands vary significantly.
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From page 51... ...
However, there is no information to indicate to what extent the safety effects of splitter islands are due to speed reduction and to what extent they are due to the separation between traffic moving in opposite directions. A study of 134 intersections in New Zealand found that installing throat or fishtail islands resulted in crash reductions of 30%-60% for fatal, injury, and pedestrian crashes during both Treatment Descriptions 51 Exhibit 4-18.
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(LTSA, 2001) 4.10 Speed Tables and Plateaus 4.10.1 Overview No test sites provided documented applications for the high-speed intersection treatments discussed in this section.
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4.10.3 Treatment Layout/Design Speed tables and plateaus should be placed at a location where vehicles will not abruptly encounter them at a high speed. A report from the Institute of Transportation Engineers (ITE, 1993)
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Reduced lane widths heighten driver attention by narrowing the available lane width. These treatments can be applied on intersection approaches.
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Reducing lane widths can negatively impact bicyclists if bicycle lanes or wide curb lanes are not maintained, but it can also improve bicyclist conditions if the additional space is used to provide or widen bicycle lanes. Reducing lane width can provide positive effects such as space for other roadway features (i.e., medians and curbside parking)
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From page 56... ...
, and • May increase capacity at signalized intersections due to decreases in pedestrian crossing times. Research that evaluates how effectively reduced lane widths reduce speeds in work zones found that the treatment was most effective on two-lane rural roads and found that a 7% reduction in speed was achieved by reducing lane widths to 11.5 and to 12.5 ft.
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Visible shoulder treatments can consist of shoulder pavement markings, pavement coloring, shoulder composition, and shoulder rumble strips. If introduced in advance of the intersection, these treatments may be used to alert drivers of the change from roadway segment to the intersection influence area.
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In cases where the shoulder exists or is planned, the special treatment area might be limited to the vicinity of the intersection influence area to help differentiate between the roadway segment and the impending intersection. 4.12.4 Speed Effects No documentation was found that describes the effect visible shoulder treatments have on speed or safety at high-speed intersection approaches.
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From page 59... ...
are prominent physical features that inform drivers they are transitioning into a new roadway environment and can include landscaping, lighting, signage, or physical structures. Maintaining landscaping treatments can be costly and difficult.
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No information has been found to describe the speed reduction benefits associated with roadway environment changes at intersection approaches. Cross-sectional changes can influence the way a driver perceives the road and can affect driver behavior.
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From page 61... ...
Appendix B provides scenario-based case studies to aid the user in applying the screening process. By understanding the fundamentals and considerations of speed and potential treatments and applications, the user can consider possible treatments.
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