Design and Management of Historic Roads (2012) / Chapter Skim
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Pages 65-99
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From page 65... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-1 5.0 Highway Design; Past, Present and Balanced 5.1 Background Understanding why streets and roads look like they do and how they function plays a critical role in developing successful strategies for preserving them since it is likely that the basic design principles that shaped the initial construction of a historic road will in some fashion inform their improvement. The geometric design policies that underlay current criteria are founded on the distillation of over 100 years of road-building practice and analysis, but the basic design criteria that still control highway design were compiled into the American Association of State Highway Officials (AASHO)
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From page 66... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-2 design needs. Whether engineered or evolved, roads are a geometric design, which is defined as the combination of the fundamental three-dimensional features of the road that are visible and affect their operational quality and safety.
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From page 67... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-3 highways, including those in urban areas, freeways and interstates. In 1984, the previously compiled and published policy by AASHTO for urban roadways (Blue Book)
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From page 68... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-4 Book as its design standards, and compliance with its policies is required for all highways on the National Highway System (NHS)
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From page 69... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-5 5.4 Balancing Design Criteria with Preservation of Historic Significance Making historic roads safer and operationally more efficient or in compliance with current design criteria does not mean that historic significance has to be or will be lost. Transportation agencies all across the country maintain and rehabilitate historic roads while preserving what makes them historic in the first place.
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From page 70... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-6 a historic district. And while potential treatments to achieve balanced solutions are not exclusive to historic roads, what is different is the thoughtfulness that needs to be used by engineers and preservationists, among others, to evaluate their appropriateness based on the particular historic significance of the road and the transportation problem(s)
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From page 71... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-7 Thresholds are used to define the design controls that in turn inform the values for the 13 controlling design criteria. Design speed serves as both a controlling criteria and as a design control to establish the range of values for the other controlling criteria, and it is the only one of the 13 criteria that is not a specific physical attribute of the roadway.
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From page 72... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-8 Design speed is a selected speed used to determine the various geometric features of the roadway. The assumed design speed should be a logical one with respect to the topography, anticipated operating speed, the adjacent land use, and the functional classification of the highway.
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From page 73... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-9 Consider basing design speed on the research-proven principle that safety is improved most by speed consistency (not higher speeds)
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From page 74... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-10 Green Book criteria specify a minimum curve radius for a given design speed, and that value is calculated from the maximum rate of superelevation. The AASHTO horizontal curve design model is based on providing a level of comfort to drivers, and that data is derived from empirical research on what drivers are willing to accept in cornering.
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From page 75... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-11 placement in support of preservation does not adversely affect historic significance on unrestricted usage streets and roads (Figure 5.4)
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From page 76... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-12 To improve drivers‘ ability to stay within the lane or ability to recover if they leave the lane, use enhanced pavement striping, delineation, rumble strips, and safety edges. This can include wide pavement marking in curves and roadside delineators.
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From page 77... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-13 steep grades. Grade-affected speed differential can also cause problems as cars climb faster than trucks, and a horizontal curve at the base of a steep grade can contribute to run-off-the-road crashes.
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From page 78... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-14 To increase night-time driver comfort in a sag vertical curve, install lighting to improve stopping sight distance and driver comfort. This is a non-construction approach that will maintain existing geometry.
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From page 79... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-15 highways are slightly higher, but the difference between pre- and post-2000 standards is minimal. Appropriate Treatments for Stopping and Intersection Sight Distance Use the Green Book guidance recommending looking beyond its operational model to assess the risk of limited stopping sight distance or criteria below current values as an alternative to a construction solution that changes historical geometry.
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From page 80... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-16 In order to tailor improvements to their context, consider match improvements in historic districts to scale and the basis for significance of the district. This includes customizing details like intersection design to conform to existing treatments rather than standard details when those treatments are adequate.
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From page 81... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-17 generally greater opportunities using flexibility in urban environments, where the range of values offers more latitude to maintain existing widths due to lower speeds and less traffic volume. Design policy provides some flexibility on how lane widths can be tailored to fit the particular environment in which the roadway functions.
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From page 82... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-18 They are not usually considered a pedestrian facility. Shoulders can also improve stopping sight distance at horizontal curves and provide an offset to objects like traffic barriers and bridge substructure units.
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From page 83... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-19 significance. For instance, when travel lanes are widened, reestablishing the historic shoulder treatment can minimize the visual impact of the lane widening.
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From page 84... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-20 Where lanes are narrow, incrementally widen at sharp horizontal curves and/or rumble strips to improve safety performance concerns instead of full reconstruction of segments of highway. This approach is most useful when the reason for the project (purpose and need statement)
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From page 85... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-21 Unimproved shoulders and treatments that convey that character are often a significant feature of historic roads. They serve as the visual transition from the pavement to the setting and features beyond the pavement.
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From page 86... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-22 To provide ability to recover if a driver leaves the traveled way, move the drop off farther from the travel lane when the side slope is not a feature that makes the road historic, or construct a safety edge that provides a beveled edge pavement instead of a near-vertical edge. The safety edge is particularly useful for limited cross sectional width and local roads and will have minimal impact on the character of the road.
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From page 87... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-23 Consider using soil bio-engineering to stabilize slopes. Landscape storm water management facilities (retention ponds)
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From page 88... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-24 Use of conjectural or contemporary pavement treatments, like stamped or formed pattern concrete, should be avoided. They are contemporary treatments that are not historic or appropriate as a means for preserving historic significance.
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From page 89... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-25 Appropriate Treatments for Vertical Clearance Sign an alternative route when the overhead obstruction is significant to maintaining historic significance. Provide advance and on-obstruction signing.
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From page 90... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-26 5.5.4.2 Lateral Offset (To Obstruction) Lateral offset to obstruction is the distance from edge of pavement or designated point to a vertical roadside element, like a utility pole, bridge substructure, or tree.
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From page 91... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-27 A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. Clear Zone and Horizontal Clearance, Frequently Asked Questions, FHWA, 2005.
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From page 92... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-28 The design of intersections on existing roads will be significantly influenced by site-specific features or constraints as well as its potential users, from pedestrians to oversized vehicles (Figure 5.16)
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From page 93... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-29 Appropriate Treatments for Intersections When desired turning lane arrangements cannot be developed, use different movementcontrol designs, including turn prohibitions, special signal phasing, or other measures. See Appendix A
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From page 94... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-30 phases may permit additional traffic to travel through the intersection without additional lanes. See Appendix A
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From page 95... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-31 Decisions regarding roadside design for existing facilities involve engineering judgment to determine whether a feature can remain or function even though it does not conform to current guidance. Crash histories of existing facilities are a very important factor in selection of a clear zone value and deficiencies that warrant addressing.
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From page 96... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-32 variety of appropriate railings as well as the ability to custom design one like Oregon‘s steelbacked wood railings provides the opportunity to use a railing design that meets current safety criteria and is compatible with the setting (Figure 3.2)
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From page 97... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-33 Appropriate Treatments for the Roadside Run-off-the-road crashes are generally a response to a geometric design deficiency, not a deficiency in the roadside. Whenever possible, appropriately define the project‘s purpose and need and consider improving the geometric deficiency first when the roadside obstruction is very important to historic significance.
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From page 98... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-34 When a decision comes down to demolition, relocate historic features away from the roadway. When such features are appropriately reconstructed in their historical configuration, the effect may not be adverse.
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From page 99... ...
Chapter 5: Highway Design; Past, Present and Balanced 5-35 LOS A indicates that free-flow speeds prevail, maneuverability is optimal, and the effects of incidents are easily absorbed. LOS F is characterized by breakdowns in vehicular flow, slow speeds, heavy congestion, and the inability to recover from incidents.
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