Design Guidance for High-Speed to Low-Speed Transitions Zones for Rural Highways (2012) / Chapter Skim
Currently Skimming:
Pages 49-83
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 49... ...
49 S e c t i o n 4 This section provides design guidance for selecting geometric design, traffic control devices, pavement surface, and roadside treatments for transitioning from high- to low-speed roadways on rural highways. The design guidance identifies specific treatments for use in encouraging drivers to reduce their speeds, as intended by the designer, and where possible, quantifies the effectiveness of those treatments.
|
From page 50... ...
50 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways Section 4.5 explains the importance of evaluating the effectiveness of transition zone treatments after implementation. This section also provides general information for conducting before/after evaluations to assess the affects of the transition zone treatments on speed-limit compliance and safety (i.e., crashes)
|
From page 51... ...
Design Guidance 51 zones (rural, transition, and community) , while another identifies four zones (rural, approach, transition, and community)
|
From page 52... ...
52 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways that has similar characteristics to the rural zone. It may also include the edge of the developed community.
|
From page 53... ...
Design Guidance 53 step in the process, the transition zone assessment, the geographic limits can be refined based on more detailed planning and engineering information. This will include the use of existing crash and speed data, as well as a review of the current roadway design features.
|
From page 54... ...
54 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways • Land Use and Zoning: current and future; • Roadway Alignment: vertical and horizontal (and lines of sight) ; • Traffic Volumes: daily and peak hour; • Vehicle Types: cars, trucks, agricultural, and emergency response; • Non-Motorized Transportation: pedestrians and bicyclists; • Transit Design or Operational Features; • Signs, Striping, and Traffic Control; • Intersection Geometry; • Roadway Design Elements (cross-section elements and widths, etc.)
|
From page 55... ...
Figure 4-2. Straight-line diagram tool (P-R 5 perceptionreaction, Decel 5 deceleration, Vert.
|
From page 56... ...
56 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways project identification phase, it is perfectly acceptable to determine that no concerns exist or at least the concern is not of the magnitude of what was initially thought. On the other hand, results of the project identification phase may provide the necessary information to prompt further detailed investigation and possibly project development and prioritization.
|
From page 57... ...
Design Guidance 57 The recommended steps of the project identification phase are presented in Figure 4-3 and described below. Step 1: Define Study Area and Referencing System At the outset of the project identification phase, the engineer should define the geographic extents of the study area and the referencing system to be used to enter data.
|
From page 58... ...
58 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways speeds should be collected during low volume, free-flow conditions. The data should be collected following proper sampling methods and in accordance with agency guidelines and/or the proper procedures (e.g., see Chapter 5 of the Institute of Transportation Engineers (ITE)
|
From page 59... ...
Design Guidance 59 speed at the end of the transition zone (i.e., Location B) is acceptable, but the 85th percentile speed through the community (i.e., Location A)
|
From page 60... ...
60 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways Depending upon the lengths of the transition and community zones, sliding window and peak searching methods can be used to identify the location(s) within the transition zone and/or community zone which could most likely benefit from implementation of a safety treatment (AASHTO, 2010)
|
From page 61... ...
Design Guidance 61 munity threshold should be located such that drivers can clearly discern that the nature of the roadway changes beyond that point. Based on the 85th percentile speeds in the rural zone and the target speed in the community zone, the recommended minimum length of the transition zone can be approximated using Figure 4-5, thereby setting the upstream transition threshold.
|
From page 62... ...
62 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways Step 6: Assess Initial Results of Project Identification Phase The results of the speed study and crash analysis taken together will yield a first indication of whether improvements and/or further investigation are needed for the transition and community zones. If both studies do not raise questions or concerns, then the two zones may be functioning adequately.
|
From page 63... ...
Design Guidance 63 4.3.3.3 Detailed Speed and Crash Studies During the problem identification phase, both speed and crash data were examined. During the detailed assessment phase, it may be desirable to investigate these topics in more depth.
|
From page 64... ...
64 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways Given the importance of stakeholder input, it is useful to make a list of stakeholders at the very beginning of the study process. The stakeholders (including highway users)
|
From page 65... ...
Design Guidance 65 ments should be selected to alert drivers of changes ahead, and in the deceleration area, physical treatments to the roadway and roadside should be used to induce the intended driver response. This section provides several guiding principles to be followed in designing effective transition zones.
|
From page 66... ...
66 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways implemented within a transition zone; rather, it is a starting point for practitioners to begin and as more knowledge is gained this list can be modified as appropriate. This list is not meant to discourage creative approaches to transition zone design, but to provide information for engineers to develop informed decisions.
|
From page 67... ...
Design Guidance 67 Figure 4-7. Roundabout (adapted from Forbes, 2011)
|
From page 68... ...
68 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways Figure 4-8. Roadway narrowing (adapted from Forbes, 2011)
|
From page 69... ...
Design Guidance 69 Figure 4-10. Transverse pavement markings (adapted from Forbes, 2011)
|
From page 70... ...
70 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways Treatment: Rumble strips Category: Surface treatment Description: Rumble strips are placed in the travel lanes perpendicular to the direction of travel to alert drivers of a change in the environment. Milled rumble strips are currently the prevalent type among transportation agencies.
|
From page 71... ...
Design Guidance 71 Treatment: Layered landscaping Category: Roadside treatment Description: Roadside landscaping is provided to enhance the aesthetics of the roadside environment and to increase driver awareness of the environment. Plants are grouped according to height, with smaller plants (i.e., ground cover)
|
From page 72... ...
72 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways limit) through the community (Forbes, 2011; ECMT, 2006; NRA, 2005; LTSA, 2002; ETSC, 1995; ODOT, 1999)
|
From page 73... ...
Design Guidance 73 • Design of the gateway must consider potential impacts to trucks, agricultural vehicles, emergency response vehicles, and so forth. • Roadside features should be set back sufficiently to avoid vehicles coming into contact with these elements and the potential negative consequences that could be caused by such features.
|
From page 74... ...
74 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways It is important to note that the optical width of the road extends beyond the limits of the roadway (i.e., outside edge of shoulder) to features located along the roadside.
|
From page 75... ...
Design Guidance 75 diagram tool and is based upon an actual location and real data. The example also illustrates that the guidelines presented in this document should not be followed as a "cookbook"; rather, the analyst/designer must exercise engineering judgment, especially when data are limited or when field conditions fall outside the boundaries of recommended methodology.
|
From page 76... ...
Figure 4-18. Straight-line diagram for example study site.
|
From page 77... ...
Design Guidance 77 Step 3: Conduct Speed-Limit Compliance Study The third step is to assess compliance with the current posted speed limits. In this example, speed data were collected using traffic classifiers positioned at approximately 2,400 ft, 450 ft, and 20 ft within the study area (as recommended in Figure 4-4)
|
From page 78... ...
78 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways Next, the analyst defines the upstream boundary of the theoretical transition zone using the values in Figure 4-5. Based on the rural zone 85th percentile speed of 64 mph (posted speed limit of 65 mph)
|
From page 79... ...
Design Guidance 79 verse pavement markings in the travel way and/or in the shoulder areas may also be considered, starting at the speed reduction warning sign and continuing into the deceleration area. If these improvements are determined not to be sufficient to achieve speed compliance at the community threshold, the highway geometry is such that a number of other options could be considered.
|
From page 80... ...
80 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways To conduct a before/after study, it is critical that the evaluation process/methodology be considered prior to construction of the transition zone treatment. Ideally, the speed and crash data gathered during the project identification phase could be used as the before period data for the analysis; otherwise, the same type of information would have to be collected/gathered a second time for the evaluation process.
|
From page 81... ...
Design Guidance 81 treatment in reducing speeds. It is possible that speeds may be reduced in the short term following installation of a treatment, but as drivers become accustomed to the treatment over time, speeds may increase to the same levels as before installation of the treatment.
|
From page 82... ...
82 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways 4.5.3 Lessons Learned In addition to the science-based approaches to evaluating the effectiveness of a transition zone treatment/project in reducing speeds and crash frequency/severity, consideration should also be given to collecting additional knowledge and understanding about the effectiveness of a transition zone project by gathering input from those stakeholders most affected by the treatment. For example, interviews could be conducted with public citizens, law enforcement, emergency responders (i.e., fire and ambulance personnel)
|
From page 83... ...
Design Guidance 83 To minimize the potential for claims, as well as to maximize the potential for a successful cost-effective project, it is suggested that agencies follow a rational planning and implementation process. Some elements of such a process could include the following: • Use traffic, speed, crash, design, and other data to clearly demonstrate a transition zone concern that requires government action.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.