A Performance-Based Highway Geometric Design Process (2016) / Chapter Skim
Currently Skimming:
Pages 123-153
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 123... ...
123 The approach to design criteria and technical content in the Green Book needs to be updated. The following is a review of the key findings: • Context matters -- and it varies, particularly with respect to the transportation service for vulnerable road users; • AASHTO dimensional criteria should be based on proven, known measurable performance effects; • Speed is an essential input to the determination of design values and dimensions; • Some AASHTO criteria are not sensitive to key context attributes that are proven influencers of performance and cost effectiveness, specifically traffic volume and road type; • Some AASHTO criteria are overly simplistic in their formulation, or are based on rational models lacking a proven basis in science; • AASHTO design criteria produce inconsistent outcomes with respect to performance; • AASHTO criteria should reflect known interactive safety and operational effects of geometry; and • Dimensional guidance should be replaced with direct performance guidance (i.e., dimensions derived from performance metrics)
|
From page 124... ...
124 a performance-Based highway Geometric Design process Geometric design guidance is broadly classified as being based on either empirical research or rational engineering models. In the case of the former, the research basis for a design dimension or value can be further classified as relating to any of the following: • Spatial or operational characteristics of motor vehicles, • Human factors (drivers, pedestrians, or cyclists)
|
From page 125... ...
Updating the technical Guidance on Geometric Design in the aaShtO policies 125 restrict the ability of designers to implement geometric solutions. As a minimum, should AASHTO retain these, it should be made clear that the guidance has no meaningful safety or operational basis.
|
From page 126... ...
126 a performance-Based highway Geometric Design process associated with lane-width requirements and interactive effects of lane widths given incorporation of bicycle and transit lanes with general purpose lanes. This specific topic is being addressed in ongoing NCHRP research.
|
From page 127... ...
Updating the technical Guidance on Geometric Design in the aaShtO policies 127 To summarize, the application of less than 12-foot lane widths has become commonplace for high-volume urban freeways, despite the stated criteria in the AASHTO Policy. There is an adequate knowledge base on both safety and operations that would enable the development of a performance-based process and criteria with more flexibility to address limitations in right-of-way and high marginal costs of widening that are typical on urban and suburban freeways.
|
From page 128... ...
128 a performance-Based highway Geometric Design process and incidents is greatly facilitated by full shoulders. Road user benefits include reductions in non-recurring congestion, and survivability of crash victims by access to and egress from the site by fire, police, and ambulances.
|
From page 129... ...
Updating the technical Guidance on Geometric Design in the aaShtO policies 129 6.3.1 Framework for Design Criteria Development -- Lane Width and Shoulders Design values for lane width increased over the years as vehicle dimensions increased and speeds increased. For many agencies a 12-foot lane width is considered a standard minimum.
|
From page 130... ...
130 a performance-Based highway Geometric Design process 6.3.2 Local Roads in Rural Contexts Local roads in rural contexts are low-volume in nature and lower speed. Lane widths of 9 to 10 feet are sufficient.
|
From page 131... ...
Updating the technical Guidance on Geometric Design in the aaShtO policies 131 Study 5 in Chapter 7 is an example of a freeway where using narrow lanes and shoulders to allow an additional lane provides better performance. Urban freeway corridor reconstruction projects can have initial capital costs on the order of $50 million to $100 million per mile.
|
From page 132... ...
132 a performance-Based highway Geometric Design process 6.5.1 Horizontal Curvature Horizontal alignments consist of intersecting tangents (the point of intersection or PI) , with simple circular curvature providing a smooth pathway connecting the two tangents.
|
From page 133... ...
Updating the technical Guidance on Geometric Design in the aaShtO policies 133 terms. This behavior is inherently low speed.
|
From page 134... ...
134 a performance-Based highway Geometric Design process Additional research performed in the 1990s characterized horizontal alignment in terms of risk associated with the number of curves and the sum of the central angles of the curves over a given length of roadway (Glennon et al.
|
From page 135... ...
Updating the technical Guidance on Geometric Design in the aaShtO policies 135 The manner in which the AASHTO model is applied to curve design policy produces inconsistent outcomes. This effect is based on the variance in maximum superelevation rates that are allowed by AASHTO.
|
From page 136... ...
136 a performance-Based highway Geometric Design process superelevation, the overall risk in setting of design policy seems to be in providing not enough rather than too much superelevation. 6.5.1.2 Context Insensitivity of AASHTO Curve Design Perhaps the most significant concern regarding the AASHTO curve model is its context insensitivity.
|
From page 137... ...
Updating the technical Guidance on Geometric Design in the aaShtO policies 137 spirals are used)
|
From page 138... ...
138 a performance-Based highway Geometric Design process • Basic Problem Is Safety -- The policy should direct a safety performance analysis potentially employing the same models or methods used above in derivation of new alignment criteria. The quantitative safety models would be adjusted to incorporate site-specific data through Empirical Bayes or other methods.
|
From page 139... ...
Updating the technical Guidance on Geometric Design in the aaShtO policies 139 6.5.1.4.1 Alternative Design Vehicles. For some road types or conditions, a larger vehicle may be more appropriate as the basis for design.
|
From page 140... ...
140 a performance-Based highway Geometric Design process This overshoot behavior was found to be independent of the speed. Krammes and Otteson (2000)
|
From page 141... ...
Updating the technical Guidance on Geometric Design in the aaShtO policies 141 outcomes for lower-volume than higher-volume roads, all other factors being equal. The outcome by policy is one that reflects the known attributes of system performance and agency policies.
|
From page 142... ...
142 a performance-Based highway Geometric Design process 6.6 Sight Distance The concept of sight lines providing sufficient distance to drivers to perceive and react to conditions ahead is a central requirement of geometric design. The following summarizes the current approach to design for sight distance in the AASHTO Green Book: • There are four distinct types of sight distance -- stopping, passing, intersection, and decision sight distance.
|
From page 143... ...
Updating the technical Guidance on Geometric Design in the aaShtO policies 143 that included road type, traffic volume, and road context features. The model was described in theoretical terms; at the time the data and knowledge base for its development were lacking (The model was actually adopted by the authors of TRB Special Report 214 on 3R design criteria)
|
From page 144... ...
144 a performance-Based highway Geometric Design process with such lane changing occurring over greater lengths of roadway. High-volume freeways (those with average daily traffic in excess of 150,000 vpd)
|
From page 145... ...
Updating the technical Guidance on Geometric Design in the aaShtO policies 145 intersections and mid-block) may be the best performance basis for geometric design of urban roads and streets.
|
From page 146... ...
146 a performance-Based highway Geometric Design process potential conflict (intersection, driveway, horizontal curve) away from the zone of limited sight distance, allowing the use of a basic crest vertical curve.
|
From page 147... ...
Updating the technical Guidance on Geometric Design in the aaShtO policies 147 6.7 Vertical Alignment Vertical alignment is composed of tangent grades and parabolic vertical curvature. Grades are expressed as a percent, and vertical curves as crest (an upgrade followed by a downgrade)
|
From page 148... ...
148 a performance-Based highway Geometric Design process performance of freeways and interchanges is suspected but not known, as this geometric variable was excluded in the research that developed the safety prediction models for freeways and interchanges for the HSM. The grade and length of grade has a substantial effect on the ability of bicyclists to use the road.
|
From page 149... ...
Updating the technical Guidance on Geometric Design in the aaShtO policies 149 Reconstruction projects have less flexibility in design of grades than roads on new alignment. In most cases, the reconstruction design, which may include widening to add lanes, will closely follow the existing grades.
|
From page 150... ...
150 a performance-Based highway Geometric Design process horizontal curves milder than minimum controlling curves are used) , the practice is unusual.
|
From page 151... ...
Updating the technical Guidance on Geometric Design in the aaShtO policies 151 rural, two-lane highways with SSD above and below the AASHTO SSD criteria and found that the presence of SSD below AASHTO criteria was not sufficient to produce elevated crash frequencies. However, when SSD limited the approaching driver's view of a critical feature, such as a horizontal curve, intersection, or driveway, elevated crash frequencies were observed.
|
From page 152... ...
152 a performance-Based highway Geometric Design process 6.8 Vertical Clearance Vertical clearance is the dimension between the roadway surface and overcrossing structure. Sufficient clearance is necessary to enable legal-height vehicles to pass beneath the structure.
|
From page 153... ...
Updating the technical Guidance on Geometric Design in the aaShtO policies 153 There is substantial research on the safety performance of medians by their type and relationship to driveway and intersection frequency and level of traffic (AASHTO 2010)
|
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.