A residential street.
|Refer to Chapter IV of the AASHTO Green Book|
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A residential street.
|Refer to Chapter IV of the AASHTO Green Book|
The cross section of a road includes some or all of the following elements:
Considered as a single unit, all these crosssection elements define the highway rightofway. The rightofway can be described generally as the publicly owned parcel of land that encompasses all the various crosssection elements (see Figures 6.1 and 6.2).
Some decisions about cross section are made during project development, such as the capacity and number of lanes for the facility. Other decisions, such as functional classification, are made earlier in the process. Within these parameters, the Green Book guidelines recommend a range of values for the dimensions to use for crosssectional elements. Deciding which of the elements to include and selecting the appropriate dimensions within these ranges is the role of the designer.
In selecting the appropriate crosssection elements and dimensions, designers need to consider a number of factors, including the following:
The most appropriate design for a highway improvement is the one that balances the mobility needs of the people using the facility (motorists, pedestrians, or bicyclists) with the physical constraints of the corridor within which the facility is located.
The number of lanes needed for a facility is usually determined during the concept stage of project development. It is usually the number of lanes necessary to accommodate the expected traffic volumes at a level of service determined to be appropriate for the facility (see Chapter 4 for a discussion of level of service). The number of lanes can only be added in integer units, i.e., a twolane highway can be widened to three or four lanes. Each additional lane represents an increase in the trafficcarrying capability of the facility.
Knowing future projected travel demands, the designer, using the analysis procedures in the Highway Capacity Manual, can provide input into the decisionmaking process during project development to determine the appropriate number of travel lanes for the level of service desired. Community input also plays a part in this decision. A community may decide through public involvement that a lower level of service is acceptable for the situation than the level of service normally provided for new construction projects.
In urban and suburban areas, signalized intersections are usually the predominant factor controlling the capacity of the highway or street. There may be more latitude in determining the number of lanes for these types of facilities. For example, a twolane facility approaching an intersection can be expanded to four lanes (one left turn lane, two through lanes, one rightturn lane) at the intersection itself and then returned to two lanes beyond the intersection. The need to distribute traffic safely will determine the need for any expansion of the approach roadway. The added lanes at the intersection can be in a variety of configurations to serve the travel desires of the traffic.
The width of travel lanes is limited by the physical dimensions of automobiles and trucks to a range between 2.7 and 3.6 m (9 and 12 ft). Generally, as the design speed of a highway increases, so must the lane width to allow for the lateral movement of vehicles within the lane. However, constricted rightofway and other design restrictions can have an impact on this decision. Chapter IV of the Green Book recognizes the need for flexibility in these cases:
Although lane widths of 3.6 m are desirable on both rural and urban facilities, there are circumstances that necessitate the use of lanes less than 3.6 m wide. In urban areas where rightofway and existing development become stringent controls, the use of 3.3 m lanes is acceptable. Lanes 3.0 m wide are acceptable on lowspeed facilities. Lanes 2.7 m wide are appropriate on lowvolume roads in rural and residential areas.
An important consideration in the design of any multilane highway is whether to provide a median and, if one is provided, what the dimensions should be. The primary functions of highway medians are to:
Depending on agency practice and specific location requirements, medians may be depressed, raised, or flush with the surface of the traveled way. Medians should have a dimension that is in balance with the other elements of the total highway cross section. The general range of median widths is from 1.2 m (4 ft), usually in urban areas, to 24 m (80 ft) or more, in rural areas. An offset of at least a 500 mm (1.5 ft) should be provided between any vertical element located within the median, such as a curb or barrier, and the edge of the adjacent traveled lane.
The design and width of medians again require tradeoffs for designers. In locations where the total available rightofway is restricted, a wide median may not be desirable if it requires narrowing the areas adjacent to the outside edge of the traveled way. A reasonable border width is required to serve as a buffer between private development along the road and the edge of the traveled way, and space may be needed for sidewalks, highway signs, utilities, parking, drainage channels and structures, proper slopes and clear zones, and any retained native plant material. On the other hand, wider medians provide more space for plant material, offer a refuge for pedestrians at intersections, and help soften the look of the roadway. Including and designing medians requires public input to find the design that meets the needs of the community.
The use of twoway leftturn lanes on urban streets in densely developed suburban commercial areas has increased as an alternative to raised medians with leftturn or Uturn bays. Although not as aesthetically pleasing as raised, planted medians, continuous leftturn lanes can improve capacity. Twoway leftturn lanes generally are not recommended in residential areas because they do not afford a safe refuge for pedestrians. Also, the number of driveways can create unsafe vehicle maneuvers.
Although the physical dimensions of automobiles and trucks limit the basic width of travel lanes, the treatment of that portion of the highway to the right of the actual traveled way, that is, the “roadway edge,” provides the designer with a greater degree of flexibility. This is true in both urban and rural areas, although different design elements are more appropriate in each location.
Shoulder widths typically vary from as little as 0.6 m (2 ft) on minor rural roads, where there is no surfacing, to about 3.6 m (12 ft) on major highways, where the entire shoulder may be stabilized or paved.
The treatment of shoulders is important from a number of perspectives, including safety, the capacity of the highway section, impact on the surrounding environment, and both the initial capital outlay and ongoing maintenance and operating costs. The shoulder design should balance these factors. For example, a designer must consider the impact of the shoulder width and other roadside elements on the surrounding environment and, at the same time, how these dimensions will affect capacity. Even with a maximum lane width of 3.6 m (12 ft), the absence of a shoulder or the presence of an obstruction at the edge of the travel lane can result in a reduction in capacity of as much as 30 percent, compared to an area where shoulder or clear zone exists that is a minimum 1.8 m (6 ft) wide. On the other hand, significant environmental, scenic, or historic resources may be adversely affected by a widened shoulder.
Another consideration is the accommodation of pedestrians and nonmotorized vehicles. In many parts of the country, highway shoulders provide a separate traveled way for pedestrians, bicyclists, and others (when no sidewalks are provided).
In addition to the dimensions of shoulders, designers have choices to make about the materials used. Shoulders may be surfaced for either their full or partial widths. Some of the commonly used materials include gravel, shell, crushed rock, mineral or chemical additives, bituminous surface treatments, and various forms of asphaltic or concrete pavements.
In a number of States, particularly in the southern part of the country where snow removal is not an issue, grass or turf surfaces have been provided on top of compacted earth embankments. The advantages of grass shoulders are that they provide both a natural storm water detention system and are aesthetically pleasing. The disadvantages can be that they are often less safe than paved shoulders and force pedestrians and bicyclists to share the road with motorists, if no offstreet facility is provided.
Shoulders represent an important element in roadway drainage systems by carrying surface runoff away from the travel lanes into either open or closed drainage systems. A variety of design treatments have been used to accommodate roadway drainage across shoulder areas. In rural and suburban areas, the most common technique allows surface runoff to cross over the shoulder and go directly into drainage ditches running parallel to the roadway edge.
In rural areas where significant physical and/or environmental constraints exist, more “urban” style solutions have been used. For example, along an older section of Maryland State Route 51, passing through the Green Ridge State Forest in Allegany County, steep, narrow cuts along the existing alignment severely limited the total roadway width. Asphalt curbing and a closed drainage system were constructed in conjunction with a recent pavement rehabilitation project. This allowed for a modest widening of the travelway and elimination of an area of steep and narrow ditches, without the need to engage in major earthwork.
An important consideration in defining the appropriate cross section for a particular highway facility is the width of the clear zone. As defined in Chapter IV of the AASHTO Green Book, the clear zone is “…the unobstructed, relatively flat area provided beyond the edge of the traveled way for the recovery of errant vehicles.”
The width of the clear zone is influenced by several factors, the most important of which are traffic volume, design speed of the highway, and slope of the embankments. The AASHTO Roadside Design Guide’ is a primary reference for determining clear zone widths for freeways, rural arterials, and highspeed rural collectors based on these factors. For lowspeed rural collectors and rural local roads, the AASHTO Green Book suggests providing a minimum clear zone width of 3.0 m (IO ft). For urban arterials, collectors, and local streets with curbs, the space available for clear zones is typically restricted. 1 Roadside Design Guide, AASHTO, Washington, DC, January 1996.
Used primarily in urban and suburban environments, curbs can serve some or all of the following functions:
There are basically two types of curbs: barrier and mountable. Flexibility in the use of either type is a handy tool for a highway designer when defining the cross section of an improvement project. Barriertype curbs are not, however, recommended for projects with design speeds above 65 km/h (40 mph).
Curbs can be constructed from a variety of materials, including concrete, asphalt, and cut stone. Figure 6.3 illustrates a variety of commonly used barrier and mountable curbs.
Sidewalks and Pedestrian Paths
The safe and efficient accommodation of pedestrians along the traveled way is equally important as the provisions for vehicles. Too often, pedestrians are a secondary consideration in the design of roadways, particularly in suburban areas. Although sidewalks are an integral part of city streets, they are much more rare in rural areas and provided only sporadically in suburban areas, despite data that suggest that providing sidewalks along highways in rural and suburban areas results in a reduction in pedestrian accidents.
When considering the placement of sidewalks, designers have several options. The sidewalk can either be placed flush with the roadside edge (if a curb is provided) or next to a buffer area, such as a planted strip (usually of grass or plant material), located between the sidewalk and roadside. The pros and cons of each option should be weighed and considered by the designer, using input from the community. For example, a planted strip has these advantages:
Buffers, or planted strips, may have the disadvantage of requiring additional rightofway that may negatively affect restricted rightofway corridors.
Another important consideration, and one in which the designer is given some flexibility, is in the width of the sidewalk and planted strip. Typically, sidewalks in residential or lowdensity commercial areas vary in width from 1.2 to 2.4 m (4 to 8 ft). The Americans with Disabilities Act Accessibility Guidelines of August 1992 set the minimum passing width on a sidewalk at 1.525 m (5 ft) at least every 61 m (200 ft). If a planted strip is provided between the sidewalk and the curb, it should be at least 0.6 m (2 ft) wide to allow for maintenance activities. This planted strip also provides space for street lights, fire hydrants, street hardware, and landscaping.
Sidewalks can also provide space for street furniture and necessary traffic poles and signals; however, additional width should be added to sidewalks to accommodate these fixtures. The wider the sidewalk, the greater the number of pedestrians that can be accommodated and the less difficult it is for them to maneuver around these fixed objects. When considering the placement of objects inside sidewalks, it is important not to overlook the need to maintain as unobstructed a pathway as possible. For instance, locating utility poles to the sides and not in the center, of sidewalks is important. This detail facilitates the movements of people with disabilities as well.
Adding sidewalks to a facility where none previously existed can be beneficial to a community. When the Lincoln Beach Parkway section of the Pacific Coast Highway (U.S. Route 101) was reconstructed in the early 1990’s, sidewalks were added along both sides of the facility. Not only did this result in a more aesthetically pleasing alternative to the shoulder section for the two travel lanes that previously existed, but the sidewalks made it safer for residents to walk between their homes and local commercial facilities. Residents can now interact with each other much more easily, which has fostered a higher level of community spirit.
Bicycles are recognized by many as a viable mode of transportation in the United States, both for commuting and recreation. Transportation designers should consider the needs of these users in the design of facilities. Basically, there are five types of bicycle facilities:
There are three primary factors to consider when designing facilities to accommodate bicycles and other nonmotorized vehicles:
Six factors are recognized by transportation planners and engineers as having the greatest effect on bicycle use:
Landscape Design and Selection of Plant Material
Landscape design is an important element in the design of all highway facilities and should be considered early in the process, so that it is in keeping with the character or theme of the highway and its environment. The AASHTO Green Book mentions three objectives of landscape design:
Landscape designs for urban highways and streets plays an additional role in mitigating the many nuisances associated with urban traffic and can help a roadway achieve a better “fit” with its surroundings.
An important aspect of roadside landscape design is the treatment of trees. Singlevehicle collisions with trees account for nearly 25 percent of all fixedobject fatal accidents annually and result in the deaths of approximately 3,000 people each year. This problem is most apparent on roads that have existing trees, where designers do not have direct control over placement. For landscape projects, where the type and location of trees and other vegetation can be carefully chosen, the potential risks can be minimized.
Integrating trees into the design of a facility has many advantages. Trees provide a visual “edge” to the roadway that helps guide motorists. Trees also add to the aesthetic quality of a highway. In urban and suburban areas, trees soften the edges of arterial and collector streets. If sight distance is a concern, taller trees with lower branches that are trimmed or lowgrowing (shorter than I m [3 ft]) herbaceous and woody plants can be another option along both the roadway edge and in raised medians.
It is important to select the appropriate species of tree for the highway environment. In particular, trees need to be chosen that can survive poor air quality, infertile and compacted soils, and extreme temperature fluctuations. Remember that maintenance, particularly during the first year after installation, is essential to the longterm health and viability of trees and other plants. Utilize the skills and knowledge of the city or town urban forester or arborist, the local agricultural extension service, or a landscape architect to identify the plant material that will be best suited for the location.
In addition to selecting a type of tree for its hardiness, the size and placement of trees is another important consideration. Generally, a tree with a trunk diameter greater than 100 mm (4 in) measured 100 mm (4 in) above the ground line is considered a “fixed object” along the roadway. Because most trees grow larger than this, their placement along the roadway needs to be carefully considered. Factors that affect this decision include the design speed, traffic volume, roadway cross section, and placement of guardrail. Trees should not be placed in the clear zone for any new construction or major reconstruction, nor should they be considered safe because they are placed just outside the clear zone. The safe placement of trees to prevent errant drivers from hitting them should be made in conjunction with a highway designer who is knowledgeable about safety. However, the decision to create a clear zone that requires the removal of existing trees is an issue that should be presented to the public and addressed by the multidisciplinary team early on.
Trees are an important aspect of community identity and carry a great deal of emotional ties with the residents. If communities consider existing trees a valuable resource, alternatives to complete eradication should be pursued. These include installation of traffic barriers, lowering of the design speed, or even complete redesign of the facility to incorporate the trees. It is not unusual for a community to value one specific tree and desire to preserve it. In general, transportation designers must balance safety with other community values when considering facility design and tree preservation.
One element of crosssection design that is often overlooked is the accommodation of public utilities. Overhead utilities typically include electric, telephone, and cable television. For new construction in urban areas, electric, telephone, and other telecommunication lines are now often placed underground.
Motor vehicle collisions with utility poles result in approximately 10 percent of all fixedobject fatal crashes in the United States annually. Utility poles also have a negative affect on the aesthetics of a roadway. It is important, therefore, whether designing in rural or urban locations, to consider accommodating utilities early in the design process.
The most desirable design solution, in terms of safety for overhead utilities, is to locate the utility poles where they are least likely to be struck by a vehicle. (The same is true for sign and luminaire supports.) The 1996 AASHTO Roadside Design Guidenotes the following options for the location and design of utilities:
Burying power and telephone lines, although the safest and most aesthetically pleasing option, is also the most expensive. For example, during the reconstruction of 1.66 km (1.03 miles) of Carson Street in the city of Torrance, CA, all the existing overhead utilities were placed underground at a cost of about $2.3 million, or approximately 37 percent of the total project cost. Because of these tradeoffs, the design and location of utilities requires public input and should be considered early in the design of each project.
The options available to designers for traffic barriers include deciding whether or not to include them in the design and, if they are included, deciding which type to choose. The purpose of the barrier, as stated in the AASHTO Green Book, is to “minimize the severity of potential accidents involving vehicles leaving the traveled way where the consequences of errant vehicles striking a barrier are less than leaving the roadway.” In addition to preventing collisions with fixed objects along the roadside, traffic barriers are themselves obstacles and have some degree of accident potential. The use of traffic barriers should consider these tradeoffs.
A wide variety of traffic barriers is available for installation along highways and streets, including both longitudinal barriers and crash cushions. Longitudinal barriers (such as guardrails and median barriers) are designed primarily to redirect errant vehicles and keep them from going beyond the edge of the roadway. Crash cushions primarily serve to decelerate errant vehicles to a complete stop (such as impact attenuators at freeway exit gore areas) .
The design of the traffic barrier is an important detail that contributes to the overall look or theme of roadway design; therefore, in addition to safety, the selection of an appropriate barrier design should include aesthetic considerations. In addition, all traffic barriers should meet crashtesting guidelines for the type of roadway being designed. Crashtesting guidelines have different levels, depending on the facility and the type of vehicles that will use the facility. For example, on parkways with restricted truck traffic, many aesthetic barriers have been designed and crash tested. The criteria used for these types of barriers are less stringent than the criteria for facilities with truck traffic. Because aesthetic considerations are usually a factor on parkways, many of these barriers are designed to add to the visual quality of the road. Even for roads that are not parkways, however, there are still many barrier designs that meet the criteria for facilities with truck traffic. Given these options, designers must balance their decisions based on safety, cost, and aesthetics.
A sample of available traffic barrier designs includes:
(e) An indepth discussion of the factors associated with the decision to install traffic barriers and guidance on the selection of a particular barrier design is presented in the AASHTO Roadside Design Guide.
A concern among some States when selecting a barrier design is cost. Aesthetic barriers might have a higher upfront cost than standard steel barriers and may be more expensive to maintain. One solution to this concern is to be consistent in the type of aesthetic barrier used throughout a State. For instance, a State might want to limit the type of barriers used to only two, an inexpensive barrier for highways where aesthetics are not a major concern and an aesthetic type for highways where visual quality is important. In this way, States can cut back on the cost to maintain multiple barrier designs.
Weathering steel guardrails are an example of an inexpensive barrier that may be considered acceptable in certain surroundings. For many States, weathering steel has been a good solution, because its rustic color helps the guardrail blend into the environment. Weathering steel has, however, had durability problems in a few areas.
Highways operate as truly multimodal transportation facilities, particularly in large urban areas. Accommodating public transit and other highoccupancy vehicles (HOVs) is an important consideration. On one end of the scale, this may involve including sidewalks to allow local residents to walk to and from bus stops. As higher levels of vehicle traffic and transit usage are expected, bus turnouts may need to be considered. At the higher end of the scale, such as on major urban freeways, dedicated bus lanes and/or HOV lanes may need to be incorporated into the design. The management of the local public transit operator should be consulted during the planning stage, if possible, so that these facilities can be incorporated into the design from the beginning.
Some of the challenging aspects of highway design have to do with crosssection elements. Decisions that designers need to make may include the number of lanes proposed for the improvement, the width of travel lanes and shoulder areas, the type of drainage proposed, or the desirability of including sidewalks or bicycle/pedestrian paths as part of the project.
Many roads currently exist that were not built to today’s standards. These roads may be located in restricted rightofway corridors that have scenic or historic resources adjacent to the roadway. It is necessary to try to avoid impacting these resources when considering highway improvements.
One option, as has been discussed previously, is to reconsider the functional classification and design speed of a particular section of highway, because these decisions go a long way toward defining the basic design parameters that can be used in connection with an improvement of the facility. Lowering the design speed or changing the functional classification results in a lowering of the minimum width dimensions for the crosssectional elements.
Another option is to maintain the road as is or as a 3R project. Design criteria established by States are generally lower for 3R projects than for reconstruction projects. A third option is to seek design exceptions. Whichever alternative is chosen, the designer should try to maintain consistency in the roadway cross section. If only a small stretch of highway is located within restricted rightofway, it would be unsafe to narrow that stretch while maintaining a much higher roadway width before and after it.
A successful resolution of the design of a highway cross section was found during the planning and design for the State Route 9A project along the Hudson River in Manhattan. The existing atgrade “interim” facility had two 3.6m (12ft) lanes in each direction, separated by a 4.6m (15ft) flush median with a Jersey barrier.
The preferred alternative, which is now under construction, replaces a rather unattractive urban street, with a six to eightlane divided urban boulevard that has a landscaped median. The new design incorporates extensive landscaping and separate bikeways and pedestrian walkways. The width of the travel lanes was reduced from 3.6 m (12 ft) on the existing surface street to 3.4 meters (11 ft) on the new urban boulevard. This cross section accommodates traffic demands and dramatically enhances the physical environment of the project area. More information about this project is in the case study section of this Guide.
The Design of CrossSection Details
Some highway facilities may be designed with the greatest concern to fit into their surrounding environments, but if the details are not carefully thought out, they can still leave the impression of an unappealing roadway.
The design of all elements of the highway cross section adds greatly to its appearance. Design details include the design and width of the median and traffic barriers and the selection of plant material. All these elements contribute to the theme of the roadway and should be considered as a unit. The best method for achieving a unified look is to work with a multidisciplinary design team from the beginning of the project development process through the last detail of the design.
Details are some of the first elements users of a facility will notice. For example, designers may go through a lot of trouble to preserve vegetation along the roadway because of its importance to the community and its scenic qualities, but if designers use concrete barriers as shields in front of this vegetation, that one element may catch the users’ attention.
Another option that aids designers in the details of crosssection elements is the use of computerimaging technology. The series of figures on the following page illustrates the application of various combinations of basic design elements to define a number of widening options for a portion of State Highway 23 in Rockville, MN. These options include the use of different median types and widths and incorporate different levels of rightofway acquisition.
The Minnesota DOT has found the use of such computerimaging techniques to be particularly useful in illustrating the impact of alternative design concepts on existing facilities for project area residents and businesses. Minnesota DOT has made this approach a standard element in all major project planning and preliminary engineering assignments.
With the increasing need to ensure meaningful and continuous public involvement on all such projects, the use of computer imaging to illustrate design alternatives to communities will help to alleviate potential conflicts and misunderstandings and lead to the best design decisions.
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