A Performance-Based Highway Geometric Design Process (2016) / Chapter Skim
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Pages 68-122
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From page 68... ...
68 This section of the report outlines a performance-based geometric highway design process. The highway design process is a major part of the overall transportation project development process.
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performance-Based highway Design process 69 cost-effective solutions, the geometric road design process should be strongly bound to and driven specifically by the problem or needs being addressed. Clearly defining the problem is the first step in an effective decision process.
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70 a performance-Based highway Geometric Design process Of great importance is the need for each project's problem statement to be objective. Problems with pavements may be rutting, lack of drainage, or cracking.
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performance-Based highway Design process 71 5.2.1 Internal Agency Stakeholders Within the agency that owns the project, the stakeholders include all different departments from which data, input, or assistance is needed. In addition to the road design department, other internal stakeholders will typically include traffic engineering, safety, maintenance, construction, public relations, and environmental planning.
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72 a performance-Based highway Geometric Design process 5.2.5 Stakeholder Chartering The concept of chartering refers to formal acknowledgement of each stakeholder's presence and reason for having an interest in the project. It includes agreement on the roles of stakeholders, which may range from advisory to decision making; timing and methods of communication; and, most importantly, validation of the problem being addressed.
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performance-Based highway Design process 73 specific definition of the problem should translate directly into the appropriate geometric design process, as discussed in detail below. Stakeholder discussions should be facilitated in a manner that generates objective and wellunderstood problems expressed in neutral ways (i.e., that do not bias or presuppose a solution)
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74 a performance-Based highway Geometric Design process • With an existing road, there is a clear and well-understood operational and safety performance history; none exists for a road on new alignment. • The costs of construction and constructability of each project type are based on significantly different factors.
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performance-Based highway Design process 75 relatively small strip taking of 5 feet will often produce adverse impacts beyond the direct cost of the right-of-way itself. Such impacts may include: • Elimination of building setbacks required by local ordinance, thus rendering the property as non-conforming to such ordinance; • Loss of mature landscaping and plantings serving as visual buffers between the road and development; • Loss of site parking, again rendering the property as non-conforming and potentially damaging the economic viability of the development; • Newly created conflicts with existing utilities (both subsurface and aboveground)
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76 a performance-Based highway Geometric Design process 5.4.2.5 Transportation Performance Is Known and Measurable Finally and most importantly, roads to be reconstructed have a known, observable traffic operational and safety profile. Indeed, it is the measurable performance of the road combined with agency policies and performance benchmarks that should have been used to frame the problem statement and define the need.
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performance-Based highway Design process 77 additional problems suggested, the scope of the project, applicable type, and design approach may significantly change. An effective and responsive highway design process resolves problem definition and designates the appropriate project type early in the process.
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78 a performance-Based highway Geometric Design process 5.4.7 Establishment of Planning Level Implementation Budget With the project type and limits established, a project construction or implementation budget and schedule should be readily set based on historic data. The budget should reflect the reasoned judgments of the type and nature of the constructed project elements.
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performance-Based highway Design process 79 considers both transportation function and value, as well as costs and cost-effective principles. Figure 25 shows the format of a matrix for the context framework for geometric design criteria.
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80 a performance-Based highway Geometric Design process national and state parks, national forests, and lands proximate to such lands with similar environmental features. For all road types and functions, geometric design elements are purposely minimized dimensionally to reinforce the primacy of protecting the character of the zone.
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performance-Based highway Design process 81 • Special Purpose Roads serve unique, designated road functions or users. These may be transitonly corridors, resource recovery roads, and agricultural roads.
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Context Zones 1 & 2 -- Natural and General Rural Zones Context Zone 3 -- Suburban Zone Context Zone 4 -- General Urban Zone Context Zone 5 -- Urban Center Context Zone 6 -- Urban Core Characteristics of Fatal and Injury Crashes by Context Zone -- A Demonstration Ped/Bike 17% SV 34% MV 49% K and A-Injury Crashes for Road Segments (2007-2009) 2,659 Severe Crashes on 17,563.5 Lane-Miles Ped/Bike 16% SV 7% MV 77% 2,735 Severe Crashes on 47,008 Intersections K and A-Injury Crashes for Intersections (2007-2009)
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performance-Based highway Design process 83 This analysis of city of Chicago and Cook County data could be confirmed and further refined using data from other cities and more rigor or objective descriptors of zone definitions. However, it clearly demonstrates the importance and value of a more robust land use definition to apply to the geometric design control task.
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84 a performance-Based highway Geometric Design process Roadway Type Rural Natural Zone Rural Zone SuburbanZone General Urban Zone Urban Center Zone Urban Core Zone Local Collector Arterial Freeway Accessibility to adjacent land uses with minimal cost and environmental disruption Access to land uses for motor vehicles and vulnerable users Access to land uses by pedestrians, transit users, and bicyclists; access for freight and goods delivery.Mobility and reliability of traffic service (travel me and travel me variance) for reasonable range of vehicle types Mobility for full range of road users including motor vehicles, bicycles, and pedestrians Travel me reliability for transit buses and taxis; mobility for pedestrians Minimizaon and reliability of minimizaon of total costs of motor vehicle trips of all types (including especially freight)
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performance-Based highway Design process 85 Transportation Design Priorities Typical Speed Regime Context Influence on the Approach to Roadway Design Special Purpose Generally Not Applicable Local Access to adjacent properties (employers, residents, delivery vehicles) ; Safety of pedestrians 20 to 50 mph Maintain character of the natural environment.
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86 a performance-Based highway Geometric Design process Transportation Design Priorities Typical Speed Regime Context Influence on the Approach to Roadway Design Special Purpose Generally Not Applicable Local Access for motor vehicles to designated locations; accessibility and mobility for pedestrians and bicyclists 20 to 35 mph Limit to 2 lanes; provide on-street parking per land use and local policies; employ minimum lane widths of 10 feet to 11 feet. Provide all-stop, 2-way stop or mini-roundabout intersections with minimal corner radii for speed control.
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performance-Based highway Design process 87 Transportation Design Priorities Typical Speed Regime Context Influence on the Approach to Roadway Design Special Purpose Mobility and access for special vehicle types (transit buses, pedestrians, bicyclists) 20 to 30 mph Special purpose defines spatial needs (transit, pedestrian mall, etc.)
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88 a performance-Based highway Geometric Design process Transportation Design Priorities Typical Speed Regime Context Influence on the Approach to Roadway Design Special Purpose Mobility and access for special vehicle types (transit buses, pedestrians, bicyclists, truck deliveries) 10 to 25 mph Special purpose defines spatial needs (transit, pedestrian mall, etc.)
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performance-Based highway Design process 89 Transportation Design Priorities Typical Speed Regime Context Influence on the Approach to Roadway Design Special Purpose Mobility and access for special vehicle types (transit buses, pedestrians) 10 to 25 mph Special purpose defines spatial needs (transit, pedestrian mall, etc.)
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From page 90... ...
90 a performance-Based highway Geometric Design process should identify, acknowledge, and address changes in access, travel time, or location of public health and safety facilities either permanently or during construction periods. Protected lands such as environmentally sensitive wetlands, forests, cemeteries, historically significant properties, and unique local features should be noted and identified as external controls.
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performance-Based highway Design process 91 • Open median or continuous two-way left-turn lane to provide access to properties between intersections vs. raised median to control access.
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92 a performance-Based highway Geometric Design process project (e.g., an intersection or curve)
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performance-Based highway Design process 93 was typically associated with an 85th percentile observed free speed. It has become common practice now for posted speeds to be set by governmental policy or even legislation, outside of an engineering analysis of speed behavior or roadway context.
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94 a performance-Based highway Geometric Design process 5.5.4.1 Design or Target Speed and Road Geometry Speed currently directly influences horizontal alignment design, vertical curvature (through application of SSD, which is based on speed) , intersection sight distance (ISD)
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performance-Based highway Design process 95 5.5.4.1.2 Design or Target Speed Regime -- Rural Nonfreeway Roads. The use of a speed metric for rural roads appears necessary.
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96 a performance-Based highway Geometric Design process Transportation agency planning policies will generally establish both a timeframe and basis for design year traffic, with certain parameters not subject to judgment or choice. These would include average daily traffic and percentages of heavy vehicles, which are used in pavement design.
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performance-Based highway Design process 97 vehicles (and persons) using the roadway.
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98 a performance-Based highway Geometric Design process by the facility increases. The agency costs for maintenance increase as the facility gets older.
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performance-Based highway Design process 99 5.5.6 Determine Basic Design Controls -- Design LOS (or Operating Condition) Current design practice applies the concept of levels of service to the roadway design.
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100 a performance-Based highway Geometric Design process LOS is a design control that is the choice of the designer or agency. Current design policy in the AASHTO Green Book provides guidance, but the text and intent of the policy is for this to be a choice.
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performance-Based highway Design process 101 Both the knowledge base and traffic operational technology now in practice allow for the highway designer to adjust a design, or to evaluate the operation of a design based on a range of potential driver behavior scenarios. Agency design standards may be based on less aggressive or more "conservative" driver assumptions, but different driver profiles in practice may produce different operations.
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102 a performance-Based highway Geometric Design process • City bus or single unit truck assumed for intersection design, • Small corner radii to slow turning traffic and increase queuing space for pedestrians at intersections, • Raised medians for streets with more than six lanes (three each direction) to facilitate pedestrian crossings, and • Space for transit stops (near side or far side)
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performance-Based highway Design process 103 differ based on the applicable approach. (The possibility that multiple problems may exist is acknowledged.
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104 a performance-Based highway Geometric Design process the existing road serving as the baseline for alternative development. The actual crash history is directly relevant and applicable to the expected safety performance, from which crash cost benefits are computed.
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performance-Based highway Design process 105 enable efficient and consistent application of the analysis approaches briefly summarized as follows: Step 1: Develop annual costs for base alternative.* • Base alternative will typically be the existing condition or no-build.
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106 a performance-Based highway Geometric Design process a project type or types. The lower the discount rate, the more future projected costs and benefits will represent meaningful values in the analysis.
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performance-Based highway Design process 107 Agencies would need to establish policies for minimally acceptable B/C ratios based on their total available resources and priorities. The quantitative benefits will generally accrue to agency customers -- the road users.
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108 a performance-Based highway Geometric Design process budget priorities, and policies, but the underlying approaches would be consistent assuming the agencies are referencing the same body of technical knowledge on the geometric effects on safety and operational performance. Regardless of the problem(s)
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performance-Based highway Design process 109 expertise is needed. In many cases the optimal solution will include stakeholder engagement activities, and combined geometric design and operational components.
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110 a performance-Based highway Geometric Design process of transportation service provided for all modes, the inclusion or exclusion of specific features (lanes by type and usage, medians by type, on-street parking) , treatments (intersections versus roundabouts, types of intersections, types of interchanges)
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performance-Based highway Design process 111 roadway type (both segments and intersections, including the extent of access control and number of lanes)
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112 a performance-Based highway Geometric Design process The concept of using proper scales in design is important regardless of the ability for the designers to print or plot their plans at any scale. Working at too large a scale (1 inch equals 20 feet)
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performance-Based highway Design process 113 alternatives is narrowed, the level of effort and detail is increased. The third dimension may be developed over digital terrain mapping (vertical alignment)
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114 a performance-Based highway Geometric Design process Advances in the knowledge base of both traffic operations and safety are such that the nominal safety mindset of Figure 15 is demonstrably not correct. The measurable performance of a design can and does vary as dimensions and elements are tested and iterated.
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performance-Based highway Design process 115 audits (RSAs) , cost-estimating validation processes (CEVPs)
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116 a performance-Based highway Geometric Design process • For reconstruction projects that may include safety performance, operational performance, or both, the RSA may best be performed following Step 8, with the focus being a review of the preferred plan. This will necessarily involve a virtual review using design visualization of the three-dimensional geometry.
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performance-Based highway Design process 117 ATCs can include substantive changes to the original concept. For example, they may include reconfiguration of an interchange, realignment (horizontal, vertical, or both)
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118 a performance-Based highway Geometric Design process signing and ITS elements, traffic control devices, roadside barriers, sound walls, striping, and landscaping. The functional design (type and location)
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performance-Based highway Design process 119 revised. Each building model element can carry attributes for selecting and ordering the objects automatically, providing construction quantities for cost estimates, and material tracking and ordering.
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120 a performance-Based highway Geometric Design process Figure 39. BIM output to evaluate vertical alignment design for drainage.
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performance-Based highway Design process 121 • Placement of navigational signing and confirmation of adequate sight lines (conflicts with overhead structures and retaining walls on curved alignment) ; • Placement of roadside objects to avoid conflicts (signs, light poles, and traffic signals)
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122 a performance-Based highway Geometric Design process to constantly revise safety-effective design policies and solutions, assuming the positive trends continue. • The advent of connected vehicles, driverless cars, and other new technology is unknown.
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