Design and Management of Historic Roads (2012)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

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) guidelines (precursor to the Green Book) during the late 1930s and early 1940s. Since then, the principles that underlie design criteria and the values therein have been constantly questioned, studied, reconsidered, and refined by the profession in order to provide the most cost-effective safe and efficient highway designs. The 1960s were a watershed in highway design because non-engineering perspectives were given greater standing in shaping the outcome of projects. Over the ensuing decades the project development process has also evolved to include the flexibility needed to accommodate a variety of goals and objectives, from protecting habitat to tailoring projects to fit with communities and the natural environment. Much of the flexibility to achieve results that balance sound engineering with preservation is provided by the highway design community itself through its policies and manuals, as well as through legislation, administrative action, and professional judgment. Key to developing that judgment is understanding the purpose and reasoning behind geometric design criteria and values. Knowing the intent of a criteria and how it factors into the integrated design of a roadway provides the skill set needed to develop alternative ways to achieve outcomes that meet transportation goals while accommodating issues important to others, like preserving historic properties or scenic vistas. Successful balanced designs are often nuanced designs, and knowledge is the basis of a nuanced approach. Knowing what underlies the design criteria also facilitates working toward a balanced solution from the outset of the planning and project design process rather than in reaction to a predetermined design later in the process. And since most roads in service today are engineered to work as an integrated system of features, the same skill set supports looking holistically at each of the controlling criteria to determine where there is flexibility to address deficiencies in a manner that includes considerations beyond the transportation need. 5.2 Evolution of Geometric Roadway Design Policy and Criteria Highways are complex designs affected by many factors and generally subject to unrestricted usage. They typically extend for many miles and have a construction history that has evolved over decades or even centuries. Their inherent complexity is compounded by the many different types of roads (local, collector, arterial, freeway, etc.), each with their particular usage and

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. Whether a road is old or new, a completely designed facility or one that evolved, its design is largely governed by the principles of balanced design. Balanced design means that all roadway elements — curve radius, lane width, shoulder width, sight distance, superelevation, grade, etc. — are determined by and based on consistent speed (the design speed) so that drivers can easily anticipate road conditions and do not encounter surprises. The integrated principle of balanced design matured during the years between the world wars when this country was transforming wagon roads to highways capable of meeting the needs of motorized vehicles. The understanding of how important stopping sight distance and superelevating (banking) horizontal curves were to safety came to the fore in the 1910s, as did the importance of lane width and pavement type to operations. In 1914, the federal Bureau of Public Roads (BPR) and the states established the American Association of State Highway Officials (AASHO) that in 1973 became the American Association of State Highway Transportation Officials, or AASHTO, to address all modes of transportation as a means to more effectively disseminate knowledge about research results and practical, effective road-building practices. The cooperative federal-state partnership represented by AASHO worked through a committee structure where the federal government assumed the lead for research and the states approved nationally applicable geometric design criteria. Problems, like horizontal curves or length of sight lines, were researched. Data was synthesized to inform draft policy that was refined and approved by the states. By 1942, a national policy was in place for primary highway design based on seven policies linking design values to safety and driver comfort. The breakthrough for a nationally applicable highway design policy came in the mid-1930s when research proved the correlation of safety to speed and superelevation in curves. In 1937, the BPR completed a manual of design standards for curves. The standards were founded on research that linked speed, curve radii, and superelevation with driver comfort. It included a practical set of design tables for spiral curves (curves that transition from superelevation to normal cross section) that are still used today. The manual calculated in 10 mph increments all curve features with the maximum permissible design speed not exceeding a useful tire side friction coefficient of 0.30 in order to counteract centrifugal force. The data resulted in a new design concept of using curve radii, superelevation, and side friction to define design speed and then using the design speed for coordination of all alignment and geometric design values. The principles of balanced design quickly gained currency and were used as the basis for the seven design policies. Though refined over the decades, the principles of balanced design continue to underlie geometric design policy to this day. After World War II, balanced design, as well as better understanding of the relationship between traffic-carrying ability and roadway characteristics (Highway Capacity Manual published by the Highway Research Board in 1950), were used to develop design policy for different types of

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) and for rural roadways (Red Book) were combined into one publication – A Policy on Geometric Design of Highways and Streets (Green Book). It has been revised periodically to remain current with research findings. Two recent Green Book policy revisions with positive effects for historic roads include the 2001 policy on low volume local roads (ADT ≤2000) and on very low volume local roads (ADT ≤ 400). Since local roads primarily serve local or repeat drivers familiar with the facility, the revisions allow for less restrictive design criteria than used on higher volume roads. To that end, widening of lanes and shoulders, changes in horizontal and vertical alignment, and roadside improvements are discouraged except when such improvements are likely to provide a substantial safety benefit (cost benefit). This provides flexibility to retain the existing roadway widths, including bridges, and roadside design when the existing features are performing satisfactorily (no proven safety problems). In addition to the standards and policy provided by the Green Book, there is other guidance in the form of manuals that are based on best practices and good engineering. With the exception of Federal Highway Administration‘s (FHWA) Manual on Uniform Traffic Control Devices (MUTCD), which sets the mandated standards for highway signs and markings and traffic signals and the 13 controlling criteria, the guidance in the Green Book and the supporting manuals is advisory. And like the Green Book, all of the manuals are updated periodically to reflect new technology and research. AASHTO‘s Roadside Design Guide addresses the safety of the roadside beyond the pavement. Elements for which guidance is provided include slopes of the right-of-way, ditches, and barriers/railings. AASHTO‘s Highway Safety Manual (HSM) is a guide used to quantify the number and severity of crashes that may be reduced by making certain improvements to a highway. Transportation Research Board‘s (TRB) Highway Capacity Manual provides a methodology for determining the number of highway lanes required to accommodate a given volume of traffic. History of Geometric Design Criteria Sources Seely, Bruce E. Building the American Highway System Engineers as Policy Makers. Philadelphia: Temple University Press, 1987. U. S. Department of Transportation Federal Highway Administration. America’s Highways 1770-1976. Washington, DC: Government Printing Office, 1976. 5.3 Green Book Applicability to Existing Streets and Highways The Green Book criteria do not apply to all projects on existing streets and highways. The Green Book is intended by AASHTO and FHWA as the policy for new construction (built on a new alignment) and full-depth reconstruction (rebuilt along the existing alignment with the complete replacement of the roadway). In response to a Congressional mandate, FHWA uses the Green

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). Additionally, states, in cooperation with FHWA, can develop and adopt their own design criteria for all roads except those on the NHS. Some states do not want multiple standards for the same functional classification of roadways, so they either adopt the AASHTO Green Book as their design standard or have design standards based on it. Other states set their design criteria to exceed or be less than Green Book values. The Green Book is not intended by AASHTO as the policy for the engineering definition of resurfacing, restoration, or rehabilitation (3R) projects on existing roads. For projects where major realignment is not needed, existing design values may be retained. 3R projects typically involve rehabilitating short segments of pavement with partial-depth repairs and targeted safety improvements, and states, in cooperation with FHWA, can and generally do develop their own 3R design criteria to meet the needs of their jurisdiction for all types of highways, except those on NHS. 3R standards may have values lower than Green Book values. Additionally, many states may have standards for bridges to remain in place (rehabilitated rather than replaced), and these too have lesser values. The purpose of 3R standards is to maintain the investment in an existing roadway that is operating satisfactory and its overall condition does not require complete replacement. The cost of 3R repairs for operational or safety reasons are generally small compared to the cost of reconstructing the entire roadway. Since 3R projects involve retention of existing three-dimensional alignment, they represent a category of work commonly associated with existing streets and highways. Types of work that can be advanced using 3R design criteria include widening pavement where it is limited to less than a lane width, improving or widening shoulders, improvements to horizontal alignment, and other work that improves safety, like adding rumble strips on the shoulders or down the center line. 3R projects are not intended to add capacity by the addition of lanes, including lanes adjacent to an existing alignment or turning lanes, changing the fundamental character of the highway, or reconfiguring intersections and interchanges. Figure 5.1. Depressed Roadway in Danville, PA Historic District. The depressed roadway takes the new river crossing approach road under, rather than through, the Danville, PA Historic District. Although the new road is a modern element introduced into the historic district, it represents an innovative, balanced solution that is scaled and detailed to be compatible while meeting the transportation need and purpose of the project. Source:contextsensitivesolutions.org

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. For example, New Jersey‘s seminal Route One Extension is both National Register listed because of its national significance as America‘s first superhighway and an arterial highway in the most congested part of the state. It is just one of the countless examples of how roads can be made safe and efficient while preserving what it is that makes them historically significant. Likewise, new roads can be constructed through historic districts without ruining historic character as demonstrated by the new dualized Paris Pike in Fayette County, Kentucky or the 320 foot-long depressed approach roadway to the 1991 Susquehanna River Bridge that passes under rather than through the heart of the Danville, Pennsylvania Historic District (Figure 5.1). Modern features can also be introduced, like the Natchez Trace Bridge outside of Nashville (Figure 5.2). In this guidance, each of the Green Book's 13 controlling design criteria is described as to its purpose and how it is integrated into the overall balanced design of a road. Alternative or non- traditional ways to meet the purpose applying inherent flexibility are described. Often, balanced solutions are dependent on addressing more than one criteria or using more than one treatment, like improving a bypass route that can accommodate oversized vehicles rather than making dramatic changes to scale and character of the existing route along a main street that is located in Figure 5.2. Natchez Trace Parkway The award-winning bridge carrying the National Park Service‘s Natchez Trace parkway over Birdsong Hollow near Franklin, TN illustrates the compatibility of contemporary design with historic districts and designed landscapes. The elegant, straightforward design where form follows function is timeless and enhances rather than competes with its setting. Compatible contemporary design is generally the correct choice for balanced designs in historic districts. The Trace was started in 1938 but not completed until the mid 1990s. Source: Byways.org.

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) to be solved. In many ways, this approach is not unlike that used for any other transportation project involving constraints, like steep topography or a densely developed urban site. Similarly, treatments are neither universally applicable nor all-inclusive. They need to be developed and considered on a case-by-case basis. Conditions vary from site to site, as does what makes a particular road historic. An approach that works in one location, like the selected removal of trees to improve roadside safety or stopping sight distance, may not be appropriate for another, like along a landscaped parkway where the plantings are integral to its historic significance. Additionally, circumstances stimulate an innovative solution. What is a constant is that designs need to be tailored to fit their location, the reasons the road or setting is historic and the understanding of the transportation problem to be solved. 5.5 The Thirteen Design Criteria that Control Roadway Design The Green Book and the related design criteria that states have adopted cover a wide range of design considerations. In order to focus on the elements deemed most important to safety and operations, FHWA has identified 13 controlling design criteria as having substantial importance to the safety and operational performance of any highway (Figure 5.3). When conditions prevent meeting any of the controlling criteria, a design exception justifying why a specific criterion cannot be met must be secured. Many features associated with the design of highways, like roadside features, intersections, signage, etc. are not controlling design criteria. Selection of design elements beyond the 13 controlling criteria, like treatment of the roadside, offer opportunities for flexibility based on engineering judgment, with the exception of traffic control devices, that are governed by the MUTCD. The 13 controlling criteria are based on five inputs known as design controls: (1) design speed, (2) traffic volume, (3) functional classification of the roadway, (4) terrain, and (5) locale. Source: TranSystems Figure 5.3 Design Criteria and Controls

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. The other 12 controlling design criteria can be placed in one of three broad categories: (1) elements of design that are related to the plan and horizontal and vertical profiles of the road; (2) cross sectional elements; and (3) clearances and bridges. Horizontal and vertical elements of design include stopping sight distance, horizontal alignment (curves), superelevation, vertical alignment, and grade. Cross sectional elements include lane width, shoulder width, and pavement cross slope. Clearances and bridges include bridge width and bridge structural capacity, vertical clearance, and lateral offset to obstruction. 5.5.1 Design Speed Design speed has more effect on the design of a roadway than any other criteria. It serves as the controlling criterion that establishes the range of design values for the geometric features that affect or are affected by driver speed, like lane width, horizontal curve radius, superelevation, and stopping sight distance. Since speed is used as both a design criteria and a performance measure, there is desire to achieve a harmonious relationship among design speed, operating speed, and the posted speed limit, but historically design speed is often higher than the posted speed. Factors that influence determining an appropriate design speed include those over which the designer has no control, like terrain, location and climate, as well as those associated with the nature and characteristics of the roadway, like its functional classification as either urban or rural arterial, collector, or local street, and the volume and composition of traffic. Every state has a method for selecting design speed. One of the two most common is to add five mph to the speed limits for road types set by state statute (e.g., rural roads posted for 55 mph or urban streets posted for 25 mph). Typically, speed limits represent the 85th percentile speed of all drivers (i.e., 85% of all drivers obey the speed limit), and the design speed represents the 95th percentile speed (i.e., 95% of all drivers will not exceed the speed limit by more than five mph). In states that do not have statutory speed limits, the speed limit is typically determined after the highway is constructed and in use by measuring the 85th percentile speed of vehicles actually using the highway. In these states, the design speed is determined by the anticipated speed limit. The traditional thinking behind selecting design speed has been to select as high a value as practical because higher speed designs are generally safer, but recent trends indicate a modification in this thinking. Many states are using operating speed or the anticipated posted speed as the design value. As a rule, the lowest design speed that can theoretically be selected is the anticipated speed limit of the roadway. AASHTO‘s new definition for design speed supports this practice:

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. Green Book policy provides a wide range of speed values matched to highway type and terrain to facilitate using engineering judgment in selecting design speed, not necessarily the highest value. It recommends that the selected design speed for new and full reconstruction should accommodate most drivers and be consistent with driver expectations. Importantly for historic roads, the policy also states that where significant constraints are encountered, other appropriate values may be used. The wide range of appropriate speed values combined with the general guidance on using significant constraints represents flexibility for designers in states without statutory speed limits. It is the intent of AASHTO that designers exercise judgment in the selection of an appropriate design speed for the particular circumstances and conditions. This provides some measure of flexibility when addressing historic roads where lower design speeds will result in less dramatic changes to the geometry. Research shows that most drivers (85th percentile) will not significantly alter what they consider to be a safe operating speed regardless of the posted speed. Designs based on artificially low operating speeds, instead of the anticipated operating speeds, can result in inappropriate geometric features that violate driver expectations and thus degrade the safety of the facility. The Green Book, which is founded on the principle of balanced design, recommends that designers not propose a different design speed for a segment of highway or seek a design exception for design speed. The recommended approach is to consider each geometric feature and address design exceptions, including mitigation, on a feature-by-feature basis. Higher speeds generally mean greater change to historic roads – wider lane widths and shoulders, straighter and lower profile alignments, and greater stopping sight distances. Greater opportunities for preservation are often associated with lower design speeds and are more common in urban and suburban areas and in park settings. Urban areas can present opportunities for using the inherent flexibility in the Green Book to create a safe roadway environment in which the driver is encouraged by geometric values and treatment of the roadside to operate at low speeds. There is less inherent flexibility with design speed for arterial highways and freeways that are intended for higher speed and through traffic. Considerations for Determining Design Speed That Favor Balanced Solutions Green Book policy permits considering other-than-recommended ranges of design speed when significant constraints, like historic properties, are encountered. This includes applying AASHTO‘s 2004 definition of design speed. That definition supports considering the historic roads as a constraining factor. A lower design speed, or a consistent one, may provide more opportunities to retain historical geometric features.

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). This may support using a lower design speed and its associated lower design criteria values that in turn may provide more opportunities to retain historical geometric features. Consider using the Highway Safety Manual to assess the effect of different potential design speeds on the expected safety performance. The analysis will enable direct comparison of higher and lower values and also demonstrate the long-term safety performance benefit. The analysis will make the purpose and need for the transportation project stronger and may provide more opportunities to retain historical geometric features. Consider using cross-sectional elements, like more enclosed urban cross section, to manage speed. This gives drivers the cue to slow down and it contributes to discomfort when going too fast. This approach may provide more opportunities to retain historical geometric features. Design Speed Sources Design Speed, Operating Speed and Posted Speed Practices. NCHRP Report No. 504, 2003. 5.5.2 Horizontal and Vertical Elements 5.5.2.1 Horizontal Alignment (Curves) and Superelevation Horizontal alignment and superelevation are combined because the two criteria are interrelated in terms of their effect on geometric design. The horizontal alignment of a highway is composed of tangents (straight segments), simple circular curves, and spiral curves used at the ends of a curve section to transition from superelevation to normal pavement crown (cross section). Of the design controls that affect the physical appearance of the highway, none is more important than horizontal alignment. Curvilinear horizontal alignment is based on a design speed that uses the combination of superelevation and the curve radius to provide an acceptable level of driver comfort. Horizontal alignment also affects another design control – stopping sight distance. Superelevation, the banking of the pavement on the approach to and through a curve, along with tire side friction helps the driver steer through the curve. Insufficient superelevation can cause a vehicle to skid, resulting in run-off-the-road events. Superelevation is the rotation of pavement on the approach to and through a horizontal curve. It is intended to assist the driver by counteracting the lateral acceleration produced by tracking the curve. Design speed for horizontal curves is based on the combination of superelevation and curve radius. In order to preserve a tighter curve (smaller radius), consider if it is possible to increase the superelevation.

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. The most common problems associated with insufficient horizontal alignment on any road are curve radius and resulting run- off-the-road accidents. Appropriate Treatments for Horizontal Alignment and Superelevation When possible, advance incremental work as a 3R project to increase opportunity for favorable preservation outcomes. Much of the work to existing roads, including selected safety improvements, are site-specific and may not require full-depth reconstruction to achieve the desired increase in safety performance. This approach will maintain more of the historical roadway geometry. Ensure that the inherent flexibility in the Green Book and engineering judgment have been used to select an appropriate design speed since it will control horizontal curve values. Design speed controls geometry values, like lane and shoulder width, curve radii and stopping-sight distance, and these often are important features of historic roads, especially in historic districts where it is largely about scale and the proportions of historical development. Since design speed for horizontal curves is based on the combination of superelevation and curve radius, consider if it is possible to increase the superelevation in order to achieve the safety improvement and preserve a tighter curve (smaller radius) without full reconstruction. This approach would be applicable when curve radii and cross sectional geometry are reasons why the road or historic district is significant. Use the IHSDM or HSM to characterize risk associated with curves and superelevation on existing roads and to quantify the effect of changes to geometry in terms of expected long-term safety performance. The analysis can serve to verify that the predicted effect on safety performance will be achieved and support the project purpose and need statement, or it can validate that the road is currently performing satisfactorily or has specific locations with safety problems. Use an approach of incremental improvements for curves with a crash history rather than full depth reconstruction if the road is otherwise performing satisfactorily. This can result in less change to the historic cross section and alignment when those features are important to historic significance. Place warning and advisory signs or pavement markings in advance of sharp curves as an alternative to construction or as mitigation if a design exception is warranted. This includes dynamic signs reporting real-time conditions. In addition to generally being a non-construction treatment or mitigation, the signs are reversible. Their placement will still need to be considered for effect on historic significance, but in many instances, sign

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). To keep vehicles in lanes, place delineation in curves, e.g., chevron signs, rather than a construction solution. This treatment is particularly appropriate when there are constraints to realigning the facility. (Figure 5.5) Install skid-resistant or grooved pavement if paving is not what makes the road historic. This can improve safety without changing the geometry. Address the roadside to mitigate substandard horizontal alignments. The importance of a forgiving roadside generally increases as the horizontal alignment becomes more severe due to the increased likelihood of errant vehicles and run-off-the road crashes. Figure 5.4. Ten Sleep-Buffalo Highway The Ten Sleep-Buffalo Highway (US 16) illustrates how to blend safer and more efficient roads into environmentally sensitive settings. The scenic rural highway leads into Big Horn National Forest in north-central Wyoming in a spectacular setting with challenging topography. It has a variety of users from tourists to logging and heavy trucks, school buses, and bicyclists. Because of a higher-than- state-average crash rate, a 9-mile long segment was rebuilt in 2004 using a variety of treatments to address site constraints, including environmental considerations. For many reasons, the balanced solution was to rebuild on the existing alignment using mitigation for design exceptions rather than strict adherence to Green Book design criteria. Meeting all design criteria was neither feasible nor prudent. Treatments used to improve safety and operations include advance signing, truck brake-check and turnout areas, climbing lanes, and guardrail that blend with the rugged natural setting.

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. Install lighting to improve all-weather visibility at curves with a crash history or selected segments of high-speed rural roadways with narrow lane or shoulder widths. Horizontal Alignment and Superelevation Sources Mitigation Strategies for Design Exceptions, FHWA, 2007. Flexibility in Highway Design, FHWA, 1997. A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. Low-Cost Treatments for Horizontal Curve Safety, FHWA, 2006. A Guide for Reducing Collisions on Horizontal Curves. NCHRP Report 500, Volume 7, TRB 2004. 5.5.2.2 Grade and Vertical Alignment Vertical alignment includes vertical tangents (straight segments) and vertical curvature, both crest (top of vertical curve) and sag (bottom of vertical curve) of a highway. The design of the vertical alignment is linked to meeting requirements for safe stopping sight distance, comfortable operation, and a pleasing appearance. A vertical curve is used to provide a smooth transition between two vertical tangents of different slope rates. Crest vertical curves are designed to meet minimum stopping sight distance requirements. The design of sag vertical curves is based primarily on the ability of a vehicle‘s headlight to illuminate the roadway throughout a distance equal to the stopping sight distance for a specific design speed. Grade is the rate of change of the vertical alignment, and the criteria require grades to be within maximum and minimum values. Grade is related to terrain and functional classification of the highway. Adequate grade is needed to establish proper drainage for both safety and operational reasons. Grade affects vehicle speed and control, particularly for large and heavy trucks, where the safety concern is that drivers will lose control when descending Figure 5.5. Signs can be used to warn in advance of sharp curves when there are constraints that preclude realignment, like on this early-19th century road with its stone arch bridge in a historic district near Princeton, NJ. Flashing chevrons and advisory signs were selected as the non- construction solution for several deficiencies, including alignments and superelevation. Note how striping has been used to mark and maintain travel lane width over the bridge while the shoulders are not. To accommodate pedestrians, the county placed a separate bridge (left of road bridge). All of these treatments preserve historic features while making the crossing as adequate as possible given the site limitations and strong desire by the community to retain the historic character of the district. Photo M. McCahon.

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. Issues relating to operational effects of grade on heavy vehicles are a significant factor in developing balanced solutions. The Green Book policy on grades largely reflects design practices related to cost and operational efficiency, particularly regarding heavy vehicles, as opposed to safety. While designers are encouraged to stay within Green Book policy, flexibility may be acceptable to meet local conditions. Steeper grades may be acceptable, for example, if they are short and the operational effects can be proven to not be adverse, if there is a small percentage of heavy traffic, or if the total traffic volume is low. They may also be acceptable if the vertical curve is long enough to enable sufficient stopping sight distance. Less than minimum grade may be acceptable where the cross-slope can be designed to compensate for drainage and where alignment is primarily straight (tangent). Terrain can preclude ability to meet minimum values, in which case a design exception will be required. Appropriate Treatments for Vertical Alignment and Grade When improvements to drainage are necessary, make those improvements in keeping with original/period treatment if that is a feature that makes road or setting historic and it is effective. This can be an especially important consideration for early engineered highways and park roads and parkways. Otherwise, develop a system that is compatible in appearance and effective in operation. In areas with curbed cross sections, the profile of the gutter can be adjusted by slightly varying the cross slope of the lanes thereby creating the grade along the curb between the inlets. (Figure 5.6) Provide advance warning of steep grades, a proven effective treatment, instead of a construction solution (Figure 5.4). Provide incremental climbing lanes or downgrade lanes when added in a manner that maintains historic character. Figure 5.6. On the Historic Columbia River Highway, the removal of generations of asphaltic overlay brought back to the original Warrenite pavement (lighter color), an early 20th century proprietary bitulithic paving material. This has also allowed for the restoration of the concrete gutter system. Photo courtesy Robert Hadlow, Oregon DOT.

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. Treat the roadside, including providing sufficient recovery area and compatible barrier treatment, as a component of a balanced solution when these treatments do not adversely affect what makes the road historic. Vertical Alignment and Grade Sources Mitigation Strategies for Design Exceptions, FHWA, 2007. A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. Highway Drainage Guidelines, AASHTO, 2000. A Guide for Reducing Collisions Involving Heavy Trucks, NCHRP Report 500, Volume 13, TRB, 2004. Recommended Procedures for the Safety Performance Evaluation of Highway Features, NCHRP Report 350, TRB, 1993. 5.5.2.3 Stopping Sight Distance Stopping sight distance refers to that distance a driver needs to see an object of a given height in the road ahead with enough distance to avoid a crash by braking to a full stop. The range of stopping sight distance values varies in relationship to the design speed. The Green Book provides design guidance for other sight distances in addition to stopping sight distance. They are (1) intersection sight distance; (2) passing sight distance; and (3) decision sight distance, which is intended for avoidance maneuvers. Stopping sight distance is intended by Green Book policy to apply to the entire length of a highway, but the relative risk of limited sight distance can vary significantly over its length. Sight restrictions associated with vertical geometry require a geometric solution, but evaluations of locations with limited sight distance need to be well understood before defining a need and developing potential solutions. This includes evaluating the roadway or other conditions in the area of limited sight distance and how significant the deficiency is to safety and operations. Some conditions pose a greater safety risk than others, including high-volume intersections and steep grades. When considering flexibility in stopping sight distance, Green Book values are not directly derived from measures of safety performance, even though safety is why sight distance is important in balanced design. NCHRP research confirmed that the values for stopping sight distance and vertical curvature provide a substantial margin of safety against the actual risk of a crash attributable to insufficient stopping sight distance. The values were revised in 2001 using a higher object height and a lower driver eye level to reflect characteristics of the current vehicle fleet. The values for lower speed roads are slightly shorter while those for higher speed

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. The crash history should be used to inform the assessment, and it may support that the road is performing satisfactorily and does not warrant changing. In urban areas, use turn restrictions or traffic signals to eliminate higher risk maneuvers instead of reconstructing intersections. Refer to Appendix A. When not affecting what makes the road or its setting historic, remove or relocate physical obstructions that interfere with sight distance, especially at curves, interchanges, and intersections. Manmade obstructions, like overhead bridges, stone walls, and other edge-of-right-of-way treatments often contribute to historic significance, and the appropriateness of this treatment needs to be evaluated on a case-by-case basis. But when it is determined that relocating the feature or removing vegetation facilitates a mutually agreeable solution, it should be done in accordance with original orientation and using original construction techniques or generally accepted preservation practices. This balance accommodates needed safety and operational improvements while maintaining historic character. Selectively cut back vegetation and limit slope reductions to increase sight distance before developing a construction alternative. In many instances, vegetation is not what makes a road or setting historic. When plantings are a significant feature, consider appropriate pruning, which is also a sound maintenance practice. In many instances, sight lines can be improved and original geometry preserved. Additionally, diseased material may be considered for removal and/or in-kind replacement to improve safety (Figure 2.2). To better understand location-based risk of limited stopping sight distance, use the IHSDM to create stopping sight distance profiles for 2-lane rural roads. Findings can be used to justify and support changes to geometric features that are important to preserving historic significance. The modeling program permits safety to be quantified and the results used to demonstrate the increase in safety performance. As an alternative to a construction solution, increase driver awareness of intersections with advance warning signs or enhanced signs (e.g., larger signs, install flashing lights). This offers a non-construction option for improving safety performance.

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. Attention to details like these go a long way in preserving historic significance in historic districts. In urban settings use a narrower cross section to slow drivers instead of changing existing geometry to manage speed and improve stopping sight distance. For severe sight restrictions, some improvements can be effective even if minimum stopping sight distance is not provided. Gaining some increase may be beneficial, even if criteria are not met. Stopping Sight Distance Sources Mitigation Strategies for Design Exceptions, FHWA, 2007. A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. Determination of Stopping Sight Distances, NCHRP Report 400, TRB, 1997. Recommended Procedures for the Safety Performance Evaluation of Highway Features, NCHRP Report 350, TRB, 1993. 5.5.3 Cross Section Elements The roadway cross section includes the lanes and shoulders, any medians, border areas that include tree lawns and sidewalks, side slopes, and drainage. Lane width, shoulder width, and shoulder surface type play a significant role in roadway operations and safety. Design criteria values for these elements are greatly influenced by factors such as traffic volume and composition, including pedestrians, bicyclists, and the nature of the adjacent land use (urban, suburban, and rural). These factors become increasingly important as the alignment becomes more curvilinear. Shoulder treatments, in particular, are generally a prominent feature of historic roads, and the original treatment can be important to preserving what makes a road historic (Figure 5.7). 5.5.3.1 Lane Width Lane width is the width of the lanes in the travel way. It does not include shoulders, curbs, or parking lanes. Lane width is defined by values influenced by terrain, design speed, the volume and character of traffic, and the functional classification of the roadway. Widths generally range 9 feet for local roads to 12 feet for higher speed roads, like freeways, arterial highways in suburban areas, and rural arterial and collector roads. Exceptions do exist; for example, Georgia uses a 14 foot width for center turn lanes. The lane width value should also include consideration of the horizontal alignment, particularly along curves, and widths for left, right, and center two- way left turning lanes are often less than the lane widths for the through roadway. Reduced travel lane widths affect capacity (free flow of traffic), especially on high-speed roads. There are

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. For low-volume rural local roads, the basis for lane width is safety and risk assessment. In urban areas there is less direct evidence of a safety benefit associated with incrementally wider lanes than other cross sectional elements, and lane widths may vary from 10 feet to 12 feet for arterials. For full-depth reconstruction of rural two- lane highways, the Green Book notes that less than 12.feet lane widths may be retained "where alignment and safety record are satisfactory.‖ In other words, there is no mandate to widen an existing rural highway if its safety performance is acceptable. 5.5.3.2 Shoulder Width A shoulder is the paved or unpaved portion of the roadway contiguous to the travel way. It is considered part of the clear zone. The graded shoulder width is measured from the edge of the traveled way to the edge of shoulder slope. The paved or treated shoulder width is measured from the travel way to the edge of the paved portion of the shoulder. Shoulders perform a number of functions important to safety and traffic operations, including emergency storage for disabled vehicles, space for maintenance activities, area to maneuver to avoid crashes, accommodation of bicycles, and recovery area for drivers who have left the travel lane (Figure 5.8). Figure 5.7. Unimproved shoulders are generally a significant feature of rural roads and unimproved roads. One-lane rural roads in Ohio generally have passing shoulders (right) while in more congested New Jersey, gravel shoulders beyond the pavement are common (left). Maintaining the character of the historic treatment can be an important component of a balanced solution involving shoulder treatments. Photographs by M. McCahon

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. On streets and roads with curbs, shoulders store and carry water, keeping it off the travel lane. On narrow rural roads, shoulders serve as structural lateral support for the travel way pavement and additional width for meeting or passing drivers. Research has proven that the greatest safety benefit of shoulders is that they enable motorists to avoid crashes. Shoulders also have a measurable effect on traffic operations and highway capacity, particularly high-speed arterial streets and highways. Their effect is less on rural two- lane roads where the substantive safety effects of incremental shoulder widths are less. Green Book policy defines a range of values from 2 feet to 12 feet depending on the functional classification of the road with both paved and usable area counting toward that value. Appropriate Treatments for Lane Width and Shoulder Widths Lane and shoulder width treatments have been combined because normally they are evaluated in combination, particularly when there is limited cross-sectional width. The two criteria are also interrelated in terms of their effects on safety and operations, as is often the case with historic roads. From the preservation perspective, the two features are generally considered together and they can work in tandem to accommodate needed improvements and preserve historic Figure 5.8. There is a wide range of flexibility for shoulders depending on local conditions. One of several treatments could be applied to increase the safety and operations of this segment of state highway for its multi-modal uses, including incremental widening of the travel lanes and shoulders, which in this locale also serve as a buggyway. Note that even the buggies off track on the inside of the curve. Enhanced pavement markings could improve the ability of motorized vehicles to stay in the appropriate travel ways, especially on the inside of the curve as buggies labor up the hill. Photographs M. McCahon.

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. When the nature of the work is incremental safety improvements, advance it as a 3R project in order to increase opportunity for favorable outcome for preservation. Much of the work to existing roads is site-specific and may not require full-depth reconstruction to achieve the desired increase in safety performance. This approach will maintain more of the historical roadway geometry. Any widening of lanes and/or shoulders should always be done with sensitivity so as not to overpower the historic character and be as minimal as possible. Scale is often a critical consideration, especially in historic districts, with the amount of justified change predicated on the existing and historic scale. A good rule of thumb is that the larger the feature, the less intrusive making it bigger will be. For instance, it might be possible to widen a 150 feet-long pony truss bridge 6 feet to increase lane width without exaggerating the original proportions of the bridge, but doing the same thing to one 75 feet long will not work. The increase is just too great for the original proportions. If the historic road is sufficiently deficient that it requires reconstruction, consider bypassing the historic road or street with a new, full-capacity road on a new alignment. This approach has been successfully for urban bypasses and for arterial through routes. Upgrading routes by building on a new alignment and leaving the old road in place has been the keystone of transportation route improvements starting with the railroads in the middle of the nineteenth century. Location in a historic district or being a historic property in its own right are just two of the constraints encountered when upgrading or maintaining an existing road. When cross-sectional width is limited or constrained, make the best use of the width available instead of widening. To optimize safety and operations within existing cross section, analyze and then distribute cross sectional width based on optimal combination of traveled way and shoulder widths, site characteristics, performance history, highway type, traffic volumes and nature, geometry, crash history, and crash type. The intent is to reduce the incidence of the specific problem(s), like run-off-the-road crashes or truck off- tracking. When cross-section is limited, consider constructing a separate path and bridge, if needed, for pedestrians and bicyclists (Figure 5.5). It is noted that care needs to be used to blend the new pedestrian facility with the historic context into which it is being introduced (Figure 5.9). When compatible with the reason(s) the road is historic, improve the vehicle‘s ability to stay within the traveled way using pavement markings or delineation like reflective roadside delineators or wide pavement marking. Raised pavement markers are particularly useful to mark narrow lane and shoulder widths.

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) is precisely defined and well supported by crash history. Both the need for work and the effect of the improvements can be confirmed using the HSM or IHSDM. Safety and operational improvements at targeted locations result in preservation of historical features and character. When modifications to width are justified, match the existing edge conditions prior to construction, like relocating walls and fences, reestablishing drainage features including ditches, or reestablish vegetation in order to maintain historic character (Figure 5.10). Figure 5.9. The crossing is located in overlapping historic districts, each with its own significance. The bridge was originally built in 1907 as part of a lake developed for rowing regattas, and as such it was an incident in a manipulated landscape as well as a transportation facility. The highway itself was listed in the National Register as an evolved highway dating to the colonial era. It was taken into the state highway system in 1919 and subsequently upgraded using AASHTO design criteria. The National Register nomination was prompted by local desire to limit truck traffic or increase in roadway width. Consequently, the unimproved pavement edge treatment is very important to local stakeholders, who also wanted the new bridge and clear zone to accommodate pedestrians. In order to not adversely affect either historic district, the new bridge was finished with stone-veneered fasciae. It continues to serve as a frontispiece in the lake-centric historic district. The pedestrian bridge was integrated into the roadway bridge, and the desired sidewalk was finished as stabilized earth so as to maintain the historical unimproved pavement edge treatment. The standard safety-shape barriers were finished with tinted concrete. The bridge is in a suburban setting dominated by post-World War II houses in Princeton New Jersey. Photographs M. McCahon.

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. They contribute to the character of the road and setting. In order to maintain that aspect of the road‘s appearance, use a stabilized earth shoulder treatment that provides a recovery area and honors the character of the road. How the edge of pavement is addressed can be a significant issue to preservationists and the public. Figure 5.10. Sometimes historic features are safety hazards and the only prudent and feasible way to maintain them is to relocate them. Before the mortar was dry on the carefully restored parapets that flank a busy, 18‘-wide county road through the National Register-listed district, they were hit and damaged (A and C). When rebuilt and hit again, it was clear to estate managers, engineers, county maintenance forces, and historians that the parapets are simply too close to the road and that preservation would be better served if the culvert pipe was extended and the parapets reconstructed away from the travel lanes. Relocation would preserve the relationship of the feature with the historic landscape scheme. Those set back from the road do retain their original detailing, which reinforces the soundness of the relocation treatment (B). By 2010, the Duke Foundation had just given up trying to preserve the parapet and had finished it in "as found‖ condition (D). In this instance, the better choice would be to relocate the feature rather than leave it as an obvious safety hazard. Photographs M. McCahon.

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. When fixed objects with historic significance interfere with the desired shoulder width, treatments other than removal may reduce the severity of crashes. This could include redesigning a feature like light standards to make it break away or shielding with appropriately styled and finished barriers or naturalistic treatments, like slopes and berms. The barrier, however, may become a large obstruction. Lane Width and Shoulder Width Sources Guidelines for Geometric Design of Very Low-Volume Local Roads (≤ 400 ADT), AASHTO, 2001. Mitigation Strategies for Design Exceptions, FHWA, 2007. A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. Roadway Widths for Low-Traffic Volume Roads, NCHRP Report 362, TRB, 1994. Highway Capacity Manual, TRB, 2000. 5.5.3.3 Cross Slope Pavement cross slope is the transverse profile of the pavement, and it is important for both safety and operations. The purpose of the cross slope is to drain water from the roadway and to minimize ponding on the traveled way and shoulders, which in turn minimizes icing and promotes economy of maintenance. It is an important criterion for historic roads because addressing drainage is frequently the reason resurfacing projects are initiated. Cross slopes that are too steep can cause vehicles to drift or skid and become unstable when crossing the crown to change lanes. There is a range of minimum and maximum values for cross slope determined by factors like local climate conditions and the number of lanes. In general the maximum value of the cross slope break between pavement lanes is .032 ft/ft and, and where superelevated sections exist the break between the high side of the superelevated lane and the adjacent shoulder is .07 ft/ft. Appropriate Treatments for Cross Slope Since the primary concern for locations with insufficient cross slope is inadequate drainage, place "Slippery When Wet‖ signs to warn motorists of sections with insufficient cross slope. Groove pavement to improve ability to maintain control on slick pavement when the cross section is too flat or too steep and it is what makes the road historic. This would not be appropriate for streets with specialty paving, like brick streets.

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) in a manner that is compatible with the historic significance of the setting. Use historic treatment as basis for design if (1) drainage treatment is source of historic significance and (2) analysis demonstrates that it was adequate (Figure 5.11). Placement of standard curb and gutter treatments can be out of character with historic treatments. Reestablish the historic treatment, including width of tree lawns, width and pavement type of sidewalks, driveway cuts and other features that contribute to the historic character of historic settings if there are no safety or operational reasons supporting not using the historic treatment. The same consideration should be afforded the transitions from new to the existing cross section (Figure 5.12). Figure 5.12. Historic treatments of tree lawns or parkways, sidewalk pavement and curbing are often significant features in urban historic districts, and consideration should be given to maintaining the historic details. In the Jefferson (GA) Historic District, the concrete hexagonal pavers are noted in the nomination as a significant feature in the historic district (A). Likewise, the width of the tree lawn and the fact that stone edging is used for one side of the sidewalk is a contributing feature (B). Even when curb and gutter section is placed, attention to reusing historic treatments elsewhere whenever possible will preserve historic character in districts. Photographs M. McCahon Figure 5.11. A concrete apron set into the integral curb on the concrete roadway is a historic treatment that appears to continue to function well given the condition of the bypassed section of US 66 in Missouri. Modern curbs and drop inlets would change the historic significance of the geometric design of the 1943 dualized highway. Photographs M. McCahon.

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. Creating a false sense of history is not an accepted preservation practice, and it does not conform to The Secretary of the Interior’s Standards for Rehabilitation (Figure 5.13). Cross Slope Sources Mitigation Strategies for Design Exceptions, FHWA, 2007. A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. Highway Drainage Guidelines, AASHTO, 2000. 5.5.4 Clearances and Bridges 5.5.4.1 Vertical Clearance Vertical clearance is the height of an obstruction, like a bridge, over the roadway. Vertical clearances are to be maintained over the entire roadway width (travel way and shoulder). The criterion most directly affects overhead bridges and portal bracing on through truss and arch bridges. Insufficient vertical clearance affects safety and operations. The Green Book criteria provide vertical clearance values for the various highway functional classifications and whether the highway is rural or urban. The value is one-foot greater than the maximum legal vehicle height, and is at a minimum not less than 14 feet except for interstate highways where the minimum value is 16 feet. Typically, highway agencies will add additional vertical clearance in their initial design to permit future resurfacing projects. Where the vertical clearance is designed to a minimum, the depth of the proposed resurfacing material must be removed from the existing pavement before resurfacing can be done. Policy includes provisions for flexibility in urban areas where one route in a given direction must meet the requirements rather than every route. Figure 5.13. The stamped paisley pattern treatment and brick crosswalks added to a misaligned intersection with a vertical crest in the center of a small historic district is not only inappropriate from the historic perspective, it adds visual confusion to an already complicated intersection. Treatments that never existed in history should not be used in historic districts. It is more appropriate to use traditional pavement striping and signs. Photograph M. McCahon.

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. This could include a barrier with an audible warning (chimes) in advance of a bridge or tunnel. Advance warning signs can also be used to protect tree canopies or overhead bridges (Figure 5.14). On a road with more than two lanes, provide signing that moves tall vehicles to inner lanes (Figure 5.15). Raise obstructions like bridge superstructures. In many instances, raising can be done with no adverse effect unless the vertical profile of the facility carried is changed in a historic setting. It is a treatment that has been used historically. Lower the road under an obstruction if the vertical profile of the facility carried cannot be changed without an adverse effect. This has historically been a common treatment for streets and highways passing under railroads. When the bridge is not historic in its own right, replace a superstructure of an overhead bridge with one that is less deep in order to increase vertical clearance without changing the geometry of the road passing under the bridge. Vertical Clearance Sources Mitigation Strategies for Design Exceptions, FHWA, 2007. A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. Figure 5.15. There are a handful of approaches to improving vertical clearance, including advance and on-structure signing and pavement marking. Unrestricted height crossings of this rail line are nearby. The overhead bridge and the roadway are both contributing resources to a historic district. Oversized vehicles must move to the inside lane in order to avoid striking the bridge. Photograph M. McCahon. Figure 5.14.There advantages for posting vertical clearance signs in advance of the obstruction. They can prevent overhead bridges that contribute to historic districts from being physically damaged by oversized vehicles. It does little good to place warning signs after the overhead obstruction has been hit. Photograph M. McCahon

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. Adequate distance from these elements is provided in order to not affect a driver‘s speed or lane position and accommodate mirrors on trucks and buses and the opening and closing of vehicle doors. Lateral offset to obstruction is a common safety deficiency with historic roads and streets, particularly in urban areas and historic districts. In urban areas with curbed streets, the lateral offset is typically 1.5 feet from the face of curb. Lateral offsets on uncurbed rural designs vary depending on the functional classification of the roadway and volume of traffic. Lateral offset is an operational consideration. It is not the clear zone, which is a clear recovery area free from rigid obstructions and steep slopes that has a safety function. Clear zone guidance, which is not one of the controlling criteria, is addressed in AASHTO‘s Roadside Design Guide (see 5.7.1). Chapter 10 of the Guide provides guidance on roadside safety in urban and restricted environments and emphasizes the need to look at each location and its particular site characteristics individually, including site constraints associated with historic roads and roads in historic districts. Appropriate Treatments for Lateral Offset to Obstruction Assuming that an object cannot be removed or relocated, add reflective material around or appropriately attached to historically significant obstructions to make them highly visible to drivers. This applies to non-historic obstructions as well. Reflective marking should be installed to be completely reversible thus not marring historic feature. Depending on the historic significance of the obstruction, it may be appropriate to relocate rather than demolish obstructions in order to achieve balanced solutions. In many historic districts, the feature is an incident in a manipulated or evolved landscape with its historic significance and value as a contributor to the historic character of the district. If the feature can be reconstructed in its historic orientation, like the New Hampshire program to rebuild stone walls parallel to the roadway, historic character can be retained while safety and operational needs are met (Figure 5.10). Consider if the cross section can be reconfigured away from the obstructions with historic significance. This is particularly important in historic districts. Knowing the crash history can be useful in decisions on how to treat features with historic significance. For 3R projects in particular, unless there is a crash history related to the lateral offset or the roadside, any increase in existing width may be limited to that which may be reasonably attained. Lateral Offset Sources Mitigation Strategies for Design Exceptions, FHWA, 2007.

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. Highway Capacity Manual, TRB, 2000. 5.5.4.3 Bridges-Width and Structural Capacity Design criteria and treatments for bridges are addressed in detail in AASHTO‘s 2008 Guidelines for Historic Bridge Rehabilitation or Replacement and are not addressed in this guide. Bridge Width and Structural Capacity Sources Guidelines for Historic Bridge Rehabilitation or Replacement, AASHTO 2008. Guidelines for Geometric Design of Very Low-Volume Roads, AASHTO, 2001. A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. 5.6 Intersections Intersections are important components of highways, and their design affects efficiency, safety, speed, cost of operation and capacity of the facility. The objective of intersection design is to facilitate the convenience, ease and comfort of traversing it while enhancing efficient movement of vehicles and people. Although they account for a very small part of the highway system, nearly half of all vehicle crashes occur at intersections, with the left-hand turn representing the greatest risk. Intersections associated with historic roads and roads in historic districts can present a variety of challenges when it is necessary to bring them into conformance with current safety standards and meet operational needs. Providing adequate sight distance while preserving historic significance can also be challenging. Intersection design is complex, and it involves consideration of many factors, including existing site constraints like the presence of historic properties or the road itself being the historic property. AASHTO recognizes the inherent complexity of intersections in several ways. Firstly, it provides flexibility to consider and address the many factors by making its policy guidance rather than prescribed minimum dimensions. Secondly, it provides a great deal of guidance on the subject by devoting over one-third of the total content of the Green Book to all types of intersections. Thirdly, it uses FHWA‘s Manual on Uniform Traffic Control Devices (MUTCD) to govern the design and placement of control devices, including traffic signals, stop and other regulatory signs, and warning signs. In keeping with the policy of driver comfort and consistency to avoid surprises, many of the dimensions, treatments, etc. in MUTCD are mandated. For a more detailed explanation of the progression of treatments for making intersections safer and more efficient, refer to Appendix A: Factors Associated with Intersection Design and Operations. The information outlines the function of and thresholds for assigning right of way and issues to consider as design alternatives for geometric changes.

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). The basis for most of the intersection geometric features is accommodating turning paths of the design vehicle to prevent off-tracking. The larger the design vehicle, which generally minimizes encroachment of most vehicles into adjacent lanes and shoulders, the greater the size of the intersection. Consequently, selection of the design vehicle is among the most important intersection-related considerations when working with historic roads. The inherent flexibility in the Green Book allows for consideration of a design vehicle that arrives reasonably frequently at the intersection, rather than the largest vehicle that would ever use it. Once the design vehicle has been selected, the Green Book provides guidance, not policy, on radii and lane widths that are consistent with the design vehicle. Stopping Sight Distance Intersection sight distance criteria is consistent in principle with stopping sight distance, and is intended to provide sufficient clear sight distance for the driver in the intersection to avoid a potential conflict when moving through the intersection. Its purpose is to provide enough sight distance for motorists entering the intersection to either turn left or right from a crossroad and accelerate to a speed without oncoming traffic being forced to lower their speed substantially, or to cross the crossroad without oncoming traffic being forced to brake substantially to avoid a crash. It also considers the time a driver needs for perception and reaction, to make a decision, and to carry out the maneuvers associated with moving through the intersection. Figure 5.16. The design of intersections is significantly influenced by site-specific features or constraints as well as the potential users, from pedestrians to school and city buses. Shown here is a typical 90-degree street urban intersection within a historic district. Turning lanes and setbacks for traffic stopping lines have been installed, but the tracking of large vehicles still presents challenges. The basis for most of the geometric features is accommodating turning paths of the design vehicle to prevent off- tracking. Selection of the design vehicle is among the most important intersection-related considerations when working with historic roads. Photograph M. McCahon.

Chapter 5: Highway Design; Past, Present and Balanced 5-29 Appropriate Treatments for Intersections When desired turning lane arrangements cannot be developed, use different movement- control designs, including turn prohibitions, special signal phasing, or other measures. See Appendix A. Remove on-street parking and allocate roadway to most effectively accommodate movements. When it is a viable option, reroute large vehicles to roadways going the same directions that can accommodate them. Where intersection sight distance is limited according to the Green Book criteria, consider non-construction options depending on the nature of the restriction, from removing obstructions to positioning the vehicle so that sight lines are clearer (Figure 5.17.) Before additional lanes are considered at signal-controlled intersections, signal timings and phases should be reviewed to determine if the green time provided to the various phases is being used efficiently, or whether a redistribution of green time to the various Figure 5.17. In order to improve the preservation potential of a feature that contributes to a National Register-listed historic landscape and the safety performance and operations of high-traffic volume local roads through that landscape, the cross section was moved away from the obstruction (right). Note how natural stone curbing was to define the edge of pavement, pavement markings are used to position vehicles to provide adequate sight distance and room to execute turning maneuvers. The plain signal poles are painted to blend with the rustic setting. Even though the reconfiguring is a change to the old road pattern (left), which dated to an era when the estate was not surrounded by modern residential and commercial development, treatments like this should have no adverse effect on the historic property because the character and features that contribute to that character are preserved. This represents a balanced solution. Photographs M. McCahon.

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. Consider non-traditional intersection design to increase level of service without adding lanes. See Appendix A. 5.7 Safety Principles Refer to Section 3.2.5 and 3.2.6 for discussion of nominal and substantive safety, AASHTO‘s Highway Safety Manual and the Interactive Highway Safety Design Module. 5.7.1 Roadside Design (Clear Zone) Roadside design as a component of highway geometric design is a relatively new concept. Post- 1965 understandings of the importance of a forgiving roadside, one where a vehicle leaving the travel way is afforded the ability to recover or decelerate safely before striking a fixed object, resulted in development of an obstacle free, graded area known as the clear zone. Its affect on safety has been significant and irrefutable. Achieving an appropriate balance of maintaining historic significance and providing a forgiving roadside can be one of the greatest challenges when upgrading or improving historic roads and roads in historic districts. Design of the clear zone is addressed in AASHTO‘s Roadside Design Guide, that, as its name implies, is advisory and not policy. The intent of AASHTO is to be flexible with respect to roadside design treatments. The Green Book refers to the Roadside Design Guide as general guidance and also states that more than one solution may be evident or appropriate for a given set of conditions that include design speed, rural or urban location, and practicality. While clear zone widths are provided in the guide, they should not be viewed as absolute or precise. Rather, they represent national consensus and practice based in part on empirical research and testing of the consequences of road encounters and cost effectiveness. It is expected that roadside design criteria and the design of the roadside will be tailored to address site- or project-specific safety needs. The desired clear zone width is a function of the design speed, traffic volume, roadside slopes, and the horizontal alignment. Its design should be consistent with the expected speed of errant vehicles. Selection generally represents a compromise or balance based on engineering judgment between what can be practically built, the presence of constraints like historic properties, and the degree of protection afforded the motorist. Factors that can limit the clear zone width include the location, frequency and nature of roadside objects, valued historic resources, or the need to accommodate pedestrians. Because of the variables, treatments will differ significantly between urban settings, where speeds are lower and curbs can assist in redirecting errant vehicles, and rural settings where speeds are greater and the roadside is needed for recovery of errant vehicles.

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. To ensure that any risk assessment analysis is fair and complete, it should include using the body of performance history to identify specific features or locations that are not performing well and to assess the reason(s) for crashes. The Roadside Design Guide recommends a hierarchy of safety treatments for existing roadside objects: removal, relocation, modification, shielding, and delineation. Removal, relocation, and modification can have an adverse effect on features that contribute to the historic significance of a road, like walls, fences, trees, and monuments. In some instances, however, relocation of an object, particularly walls and fences parallel to the roadway, can be an effective treatment for historic roads when it is the feature as an incident in the landscape next to the road that is generally the basis for its historic significance. How any treatment will affect preservation of what makes the road historic needs to be thoughtfully and fully considered, as should ways to minimize or mitigate the impacts, like replanting trees beyond the clear zone or reconstructing walls and fences. Moving such features with historic significance can facilitate their preservation as well as improve safety and operations. In certain instances, shielding may be appropriate when obstacles and nonconforming features cannot be removed from the clear zone (Figure 5.18). The approach is discussed below. Barriers/Railings When crash history demonstrates that barriers are warranted and appropriate to shield fixed objects, roadside obstacles, or non-conforming cross sectional or drainage features as well as on bridges, it is important that they be designed to be compatible with sensitive settings. While placement of new barriers in historic settings can be a challenge, increased awareness of the value of maintaining historic character has resulted in an ever- increasing range of aesthetic barriers and railings that have been crash tested or judged crashworthy. The variety of treatments, from stone veneered safety- shape barriers to weathering or painted beam guide rail systems that blend well with many settings (Figure 5.19). The Figure 5.18. Barriers are an appropriate treatment to shield a historically significant object or fixture. Here barriers are placed between the roadway and the Art Deco-style pedestrian overlook at the Croton Reservoir Bridge on the Taconic State Parkway in New York.

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 steel- backed wood railings provides the opportunity to use a railing design that meets current safety criteria and is compatible with the setting (Figure 3.2). To provide the desired level of safety, barriers and their terminals need to be crashworthy for speeds at which they will likely be struck, regardless of the overall design speed, since operating speeds may vary along the highway. Crashworthiness is based on a barrier‘s capacity to effectively redirect an errant vehicle and to safely stop it in a controlled manner. These characteristics are determined by adequate tests and meet established guidelines based on test levels (TL) and speeds specified in NCHRP Report 350, Recommended Procedures for the Safety Performance Evaluation of Highway Features. The highest value TL-5 railings are used on federal-aid projects and meet full- scale, crash-tested criteria. The flexibility to use railings and barriers that are appropriate for historic roads and roads in historic districts is restricted in some states that have adopted a policy of requiring a test level that may exceed what can be considered appropriate for the speed, volume, and character of traffic using a facility For instance, Florida requires all barriers and bridge railings to meet at least TL-4. A good source for compatible railings and barriers that have been crash tested is a web site maintained by FHWA (http://www.fhwa.dot.gov/bridge/bridgerail/). Designs, including aesthetic ones, are grouped by type and include the TL rating. Many contemporary railings, like the open, tubular Wyoming Railing or the concrete Kansas corral railing, are often appropriate for use on historic roads and in historic districts because of the simple and unobtrusive design. Tubular metal railings should be painted to better blend with their settings (Figure 5.18). Figure 5.19. New York‘s Taconic State Parkway is an arterial freeway and a scenic byway that is eligible for listing in the National Register. Improvements are governed by Programmatic Agreement and a Scenic Byway Corridor Management Plan. The plan discusses appropriate treatments and guidelines for reconstruction, including lane widths, shoulders, lengthening of deceleration/acceleration ramps, drainage features, overpasses, and side slopes. A significant safety concern of the original design of the parkway was a demonstrated history of accidents caused by traffic crossing over the median into the opposing lanes. The solution is stone-faced barriers that reflect the original design. Photograph M. McCahon.

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. It is recognized that opportunities for addressing geometric deficiencies may be limited by site constraints, particularly in historic settings. In order to minimize changes to historic treatments, avoid establishing an arbitrary clear- zone width and use site-specific solutions to address deficiencies that are supported by performance and crash histories. The more historic fabric and features that can be preserved, the better the outcome for the historic road. When trees are a significant component of significance, limit removal to where it will substantially reduce the risk of crashes. Where trees are numerous, removal of isolated trees may not significantly reduce the overall crash risk, whereas removal or shielding of isolated trees noticeably closer to the roadway may in some instances be appropriate. Roadside barriers should be placed to shield trees only when the severity of striking the tree(s) is greater than striking the barrier. Knowing the crash history can be useful in decisions on how to treat roadside features with historic significance. For 3R projects in particular, unless there is a crash history related to the lateral offset or the roadside, any increase in existing width may be limited to that which may be reasonably attained. Use the age and health of trees as a consideration in decision making. The fate of some trees may be predetermined based on their condition. Place new landscape material so that in the future it will not become a fixed object requiring mitigation treatment. Steep side slopes can pose risks. Preservationists and designers should collaborate to develop solutions for flatter slopes that do not have an adverse effect on the overall significance of the resource. This can include acquiring additional right of way. Regrade to flatten embankment slopes in a manner that reestablishes the historic treatment when that treatment is significant. This is especially important for roadways in significant landscapes. It is important to maintain the integrated nature of manmade features in a designed landscape. Replace fixed object poles and supports with breakaway poles and supports that are detailed to be compatible with the historic significance. When the original design is not documented and known, consider using scale and color of compatible contemporary design rather than conjectural interpretations of period treatments per NPS guidance in The Secretary of the Interior’s Standards for Rehabilitation.

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. Consider an aesthetic treatment for barriers and railings. There are many crash-tested railings that meet TL-3 and TL-4 Report 350 requirements. When the desire is to use a custom barrier or railing, consider pursuing approval from FHWA. To minimize duplicate crash testing, FHWA may allow use of designs that are similar to crash tested designs based on an analytical comparison using their specified methodology. Roadside and Barrier/Railing Sources AASHTO. A Guide to Flexibility in Highway Design. 2004. AASHTO. Roadside Design Guide (Updated). 2009. www.fhwa.dot.gov/bridge/bridgerail/ FHWA. "Roadway Aesthetic Treatments 2001 Photo Album Workbook.‖ Note: This CD produced by the WFLHD Technology Development Team is inclusive and shows a wide variety of treatments, some of which do not represent sound preservation practices. It was also followed by a second CD dated 2002. FHWA. Summary Report on Aesthetic Bridge Rails and Guardrails. Report No. FHWA- A-SA-91-051. June, 1992. 5.8 Operations (Roadway Capacity) – TRB Highway Capacity Manual The Transportation Research Board‘s Highway Capacity Manual (HCM) is a guide on the relationship between traffic-carrying ability and roadway characteristics. It provides the user with the ability to determine the number of lanes required for the roadway to operate at a specified level-of-service (LOS). The HCM uses the LOS concept based on control delay per vehicle (seconds per vehicle) to define the congestion on a roadway. Levels of service are graded "A,‖ representing the least delay per vehicle, through "F‖ representing the most delay. As an example, the level-of-service for a freeway section is noted below. Table 5.1 Level of Services Criteria – Basic Freeway Segment Basic Freeway Segment LOS Control Delay per Vehicle (seconds/vehicles) A 11 B > 11 and 18 C > 18 and 26 D > 26 and 35 E > 35 and 45 F > 45

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. The desired LOS for many projects is C, which is defined as stable flow conditions by the 2000 manual. At LOS C, most drivers are comfortable, roads remain safely below but efficiently close to capacity, and posted speed is maintained. Typically, LOS C is the recommendation for design in rural areas and LOS D for urban and suburban areas. However, in many highly built-up urban areas, LOS E may be considered due to high costs or the lack of available additional right-of-way. When the LOS becomes intolerable and site conditions permit, a roadway is likely to be studied for improvement to raise its LOS. A similar LOS rating is used for intersections (see Appendix A). Appropriate Treatments for Highway Capacity If capacity of the road or level of service have been identified as the purpose and need for the project and is resulting in alternatives that adversely affect the road, develop an alternate route or reroute classifications of vehicles in order to preserve the historic road or a historic district.