Choices & Guidelines
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Design vehicle is defined by ITE as the vehicle that must regularly be accommodated on a roadway without encroachment into other travel lanes. The design vehicle’s dimensions and movements can play a large role in the physical characteristics of a roadway, such as the appropriate lane width and the radii of curves at intersections and driveway corners.
AASHTO has developed several profiles for commonly-used design vehicles, the details of which are provided in Chapter 2 of AASHTO’s Policy on the Geometric Design of Highways and Streets. The profiled design vehicles range in size from passenger cars to interstate tractor-trailers. Larger design vehicles require larger roadway dimensions, particularly at intersections.
There are a number of tradeoffs inherent in design vehicle selection. The design vehicle selected for a given roadway should represent the largest vehicle that regularly or frequently, not occasionally, uses it. Selecting too large a design vehicle for a roadway or roadway segment will result in wider lanes and intersections, jeopardizing safety for other modes and leaving less space for pedestrian, bicycle and transit infrastructure.
Selecting too small a design vehicle can make turning maneuvers difficult or impossible for larger vehicles, potentially causing congestion and/or safety issues. Balancing these tradeoffs is an essential component of creating a great street which adequately serves regular users and is appropriate for the place type.
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Traffic signs, while often considered peripheral design elements, play a critical role in communicating a variety of information to users of the transportation system. Signs are used to guide, warn, regulate, and convey information to vehicular and pedestrian traffic along the roadway.
The Manual on Uniform Traffic Control Device (MUTCD) provides guidance on the design and placement of signs, including the size of the sign and text, placement along the roadway, and standard information to be conveyed. Most states, including Missouri, use the MUTCD as the primary guide for developing roadway signage. Some states, such as Illinois, provide supplemental guidance that builds upon the information in the MUTCD. Engineers and contractors also use standard drawings, similar to those provided by the Missouri DOT, to ensure that signs are designed and constructed appropriately.
In the various place types discussed in this guide, traffic signs are often one of many traffic control devices that users must process while navigating streets. Therefore, it is important to design and position signs in a manner which ensures that users have enough information to safely and efficiently navigate the arterial street network without becoming distracted or overwhelmed.
The MUTCD specifications help ensure a minimum level of nationwide traffic sign uniformity. Uniformity is a critical component of effective traffic sign communication, as users are better able to recognize and respond to familiar signs. Although some jurisdictions may wish to modify the standard sign types to be more attractive, the majority of traffic signs are required to conform to the MUTCD specifications. One exception is street name signs, which can be customized and used to help create a theme or identity for a place. Many agencies are developing artistic street signs, such as the Washington Avenue sign in downtown St. Louis shown at right, to add character to the streetscape.
The ITE Traffic Handbook indicates that traffic control devices, including traffic signs, should:
- Meet a need
- Command attention
- Convey a clear and simple meaning
- Command respect of the road users
- Give adequate time for proper response/reaction
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When rain falls in natural, undeveloped areas, storm water is able to permeate the soil and return to the water table below the surface. Developed areas have a greater concentration of impervious surfaces such as roads, sidewalks, buildings, and parking lots, which slow or prevent water from permeating the soil.
In developed urban and suburban areas, enclosed drainage systems are often used to collect and transport storm water during and after rain events. When rain falls onto an impervious surface such as a paved road, the water follows the slope of the pavement to a gutter. The gutter directs the water along the curb line to a series of intermittent inlets where it can enter the collection system below the pavement. A system of pipes below the pavement moves the collected stormwater offsite to creeks, streams, or other bodies of water in the surrounding hydrologic environment.
Low Impact Development (LID) is an alternative method of stormwater management which is used to decrease runoff in developed areas by increasing surface permeability and seeking opportunities to store or utilize stormwater on site, as shown in the images below (e.g. underground storage, rain gardens, downspouts that empty into landscaping rather than the street).
Drainage systems for arterial streets will in most cases require a combination of conventional and LID stormwater management methods. Combining these methods can be especially effective in retrofit situations where existing enclosed stormsewer systems are unable to handle the additional runoff created by new development. In this situation, using the LID method can be much more cost-effective than rebuilding the storm sewer system.
Effective stormwater management is imperative, regardless of which method or combination of methods is utilized. Flooding of roadways, bike paths, sidewalks, and adjacent properties can create safety and access issues and significant property damage.
Considerations for stormwater management strategy selection:
- Enclosed drainage systems are typically costly to construct and/or rebuild.
- Enclosed drainage systems tend to have significant maintenance requirements. Without routine maintenance, debris can build up in inlets, curb and gutter systems, and pipes, limiting the system’s effectiveness.
- Enclosed drainage pipes increase the velocity at which stormwater travels, which can exacerbate the erosion of stream banks and channels. Installing erosion control devices and/or energy dissipators creates an additional cost.
- The design process for an enclosed drainage system is generally more straight-forward than utilizing LID strategies. While there is an established set of guidelines and software to direct the design and sizing of enclosed system pipes and inlets, additional research on LID controls and the development of models to validate proposed solutions is needed.
- Reviewers and regulators may be hesitant to approve non-standard drainage system approaches due to the lack of existing design standards and procedures. Further development of regional guidance to assist reviewers and regulators and pointing to local models of successful implementation could help reduce uncertainty.
- LID strategies can result in some puddling which, although innocuous, some may find aesthetically objectionable or bothersome. However, many LID strategies (e.g. landscaping, green roofs, and rain gardens) offer significant aesthetic benefits.
- Shared use of parking (e.g. daytime use for offices, nighttime use for a movie theater or restaurants) is a strategy which can be used to reduce the amount of space needed for parking, decrease impervious surface area, and presumably reduce stormwater management requirements.
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Credit: East-West GatewayPervious pavement can also be used in parking lots to further reduce the impervious surface area. Pervious pavement is more permeable than regular pavement and allows stormwater to more easily drain through to the soil. See the EPA Porous Pavement Fact Sheet for a detailed breakdown of the pros and cons of pervious pavement, or visit perviouspavement.com for more information.
- Detention ponds are most often used to manage stormwater in residential areas and employment districts; however, they can enhance streets in many types of places. Pocket parks, trails, and greenways can be situated adjacent to a detention pond, providing opportunities for recreation and creating a more visually appealing corridor.
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Credit: CH2M HILLOpportunities to use LID strategies to supplement enclosed drainage systems are often overlooked or underutilized. For example, landscaping is often placed higher than the surrounding pavement (e.g. a raised median on a sidewalk or in a parking lot), thus limiting its ability to absorb runoff, as in the image at right.
The images at right illustrate two simple LID strategies. In the first image, a downspout from the roof above is channeling rainwater directly into the landscaping instead of directing it through an enclosed stormwater management system.
This strategy reduces stormwater capacity requirements while helping to sustain the aesthetically and ecologically beneficial landscaping. However, the curb and gutter system used for the landscaping island prohibits stormwater flowing across the surrounding impervious surfaces from being absorbed by the landscaping.
Placing intermittent curb cuts around the island, as shown at right, allows some rainwater to flow from the parking lot into the landscaped area for absorption. Eliminating the curb and setting the landscaped area lower than the adjacent pavement would allow even more stormwater to permeate the natural surface, further minimizing stormwater capacity requirements.
These simple, effective LID strategies can have significant environmental and economical benefits if applied consistently and appropriately in developed areas.
LID State of the Practice Example: 12th Avenue Green Street Project, Portland
The 12th Avenue Green Street Project in Portland, Oregon is a fantastic example of the successful use of LID stormwater management techniques. The American Society of Landscape Architects recently awarded the project a General Design Award of Honor (2006), and stated that the project was “…the best example of this type of work we’ve ever seen.”
The initiative was part of a street retrofit project started in 2005. The project uses stormwater planters to collect, treat, and distribute stormwater from the street, bypassing the existing enclosed drainage system. The planters aesthetically enhance the streetscape, while providing excellent stormwater management, as shown at right
For more information about the project, visit the 2006 ASLA Professional Awards Page.
Bicycle-Friendly Design Considerations:
Because bicycles are typically expected to use the rightmost edge of the travel lane (a bike lane, paved shoulder, or wide outside lane), they may encounter various obstacles, such as manholes and grates, which are commonly used along the curbside as part of enclosed drainage systems.
It is important to consider the needs and safety of bicyclists when planning the location and design of these items. Bicycle-friendly grates and manhole covers should be used along established bicycle lanes and routes, but should also be considered in areas where the potential for bicycle traffic exists.
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Lighting impacts the street environment in two major ways: the performance and quality of the lighting affects safety and navigability, and the street light equipment (usually a series of poles and luminaires) can help shape the character and aesthetics of the streetscape and neighborhood.
Street Lighting Basics: The appropriate design and placement of street lighting is determined according to several interrelated criteria including street use, street type, the degree of pedestrian use, the presence of street trees, the intended illumination levels for drivers and pedestrians, the need for lighting uniformity, energy efficiency considerations, dark sky ordinances, and the design character of the street.
These criteria will affect decisions about the height of the light source above grade, the horizontal spacing of pole/luminaire combinations, luminaire type, lamp type, and the architecture of the pole/luminaire. For most roadways, an electrical engineer or lighting designer recommends particular treatments and design elements, and an urban designer, landscape architect or architect may also be involved for streets with a more specific or intricate intended design character. The Illuminating Engineering Society of North America (IESNA) establishes national standards for the engineering and performance of roadway lighting.
Pole and luminaire configurations: Street lighting poles and luminaires can be divided into two main categories based on size and purpose: roadway height (16’ to 40’) and pedestrian height (10’ to 16’ feet). Luminaire mounting configurations are referred to as post-top if the fixture is mounted atop a pole, or pendant mount if the fixture is mounted on an arm extending from the pole, either suspended from above or from the side.
Lighting control and distribution: To the degree possible, lighting designers generally strive to create a uniform level of light throughout a street and/or sidewalk environment. These designers will typically begin by setting the height, spacing, and orientation of lamps whose light is modified and controlled by the design of the luminaires.
Brightness: In the past, lighting for most streets was designed to a 0.1 footcandle “maintained” minimum standard; however, many retail areas and pedestrian-oriented streets are now designed to a 1.0 footcandle maintained level, with some cities such as Phoenix, AZ requiring 5.0 footcandles maintained in downtown. As urban areas develop, glare and light pollution are becoming increasingly problematic.
Glare, an uncomfortably strong light source, is both a nuisance and a safety hazard for drivers and pedestrians, while light pollution of the night sky is typically considered an aesthetic and efficiency issue. Cut-off lighting, a treatment designed to block any light emitted above the horizontal plane of the lamp (shown at right), can be used to minimize light pollution and shield upper floor windows of buildings, and will soon be required in most states.
Reflectors, fixtures that control light using mirrored surfaces within a fixture, and refractors, which use lenses (typically Fresnel lenses, like those of a lighthouse) to control and direct light, are also used to control and minimize light pollution.
The globe of a light fixture (the outer transparent or translucent shell around a lamp) is referred to as refractive, when it contains Fresnel lens elements. Diffuser, globes are translucent and distribute the lamp’s brightness over a larger surface area without directional control, thus reducing glare but not necessarily light pollution. Using a combination of the lighting control methods described above within a fixture can allow for greater spacing between luminaires and minimize installation and operating costs.
Lamp types, energy efficiency, and lighting color: No current lamp technology is perfect in terms of energy efficiency, longevity, and color quality. The most energy-efficient lamp technologies commercially available for street lighting at present include high-intensity discharge (HID) lamps, mercury vapor, and fluorescent lamps. The four types of high-intensity discharge lamps - high-pressure sodium, low-pressure sodium, metal halide, and induction - are all efficient in “lumens per watt” and relatively long-lived.
Light-emitting diodes, or LED lighting, is considered moderately efficient, but is not yet a cost-effective option, nor is it widely distributed for commercial use; however LED use and affordability are expected to increase over the next few years. Mercury vapor and fluorescent lamps are less commonly used for street lighting today due to slightly lower efficiency, a shorter lamp life, power/size limitations, and other technical issues. Both standard and halogen incandescent lamps are considered non-efficient lighting sources due to their high energy consumption and short lamp life. Incandescent lamps are rarely used for area illumination, but can be useful for accent lighting.
Lamps also have two important color characteristics: the color of the emitted light (measured in degrees Kelvin/color temperature) and the accuracy of colors as seen under the light (measured as a Color Rendering Index – CRI).
High-pressure sodium measures at about 2200K (peach-colored orange, as shown in the image below), a household incandescent lamp rates 2700K (considered the most desirable color range), and a bright white metal halide lamp used in stadium lighting might be 4000K.
The most commonly used lamp type for street lighting in North America is the high-pressure sodium lamp, due to its high energy efficiency in terms of “lumens per watt” and its relatively long lamp life (20,000+ hours).
The peach-orange light emitted by high-pressure sodium lamps tends to distort colors with a poor CRI. This type of lighting is adequate for roadway illumination, but it is not ideal for pedestrian-oriented environments. Efficient white lighting sources in the warm white range (2900K-3200K) such as metal halide, induction lighting, and compact fluorescent are best for pedestrian-oriented environments or wherever color is critical (e.g. automobile dealers and high-end shopping malls use warm white lamps to light their outdoor environments). Color accuracy is also important for nighttime safety and policing (see image at right).
However, in discussing the benefits of warm-white halide lighting, it is also important to note that these sources generally have slightly higher energy consumption, and in some cases, a shorter lamp life than high-pressure sodium. The very long life of induction lamps may help offset their slight energy disadvantage when compared to high-pressure sodium lighting.
Utilitarian pole/luminaire types: The majority of streets in the United States are illuminated with a basic, utilitarian pole/luminaire combination. Street lighting technology has evolved to provide energy-efficient lighting with the lowest possible installation and operating cost in the form of “cobrahead” streetlights and poles equipped with high-pressure sodium lamps (as shown in the image at right).
The shoebox light is another common utilitarian pole/luminaire treatment, often used in parking lots. While shoebox and cobrahead lights are quite efficient, these pole/luminaire combinations generally do not enhance the physical character of a street or help create an attractive, pedestrian-scale environment.
Urban Design considerations: Streetlights can significantly contribute to the design and character of the overall streetscape environment; however, as with a number of other streetscape furnishings such as crosswalks and trash cans, treating streetlights as strictly utilitarian roadway elements is a missed opportunity.
In many cases, streetlights are one of very few public streetscape investments. Due to their vertical orientation, streetlights are highly visible and can noticeably change the look and feel of a streetscape.
As such, there are several points of urban design to consider:
- The size and character of streetlight poles/luminaires should correspond to the importance of the street within the district.
- Streetlight spacing should be dictated by engineering, safety, and placemaking considerations.
- Streetlights may be architecturally aligned with each other or with focal points along the street, to add to placemaking.
When both pedestrian and vehicle-appropriate lighting are required in an area, pedestrian-height luminaire heads may be added to roadway height poles (as shown in the image above).
Pedestrian-height luminaires/poles may also be added at more frequent intervals between the roadway height poles.
The architectural style or character of a streetlight should draw from and complement the architectural and historic character of the district, as in the image at right.
Individual streetlight poles should include elements designed to help create a more human-scaled streetscape and corridor (e.g. a decorative base treatment).
Supplementary Lighting: Additional lighting of building facades (sconce lighting, wall-wash lighting), pathways (bollards), and landscaping (uplighting and holiday lighting) can be useful tools in special placemaking efforts (as shown at right).
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The term "setback" refers to a distance, prescribed by zoning ordinances, that buildings must be literally ‘set-back' from the property line or public right-of-way. While conventional zoning ordinances may still include setback requirements, many communities are replacing them with "build-to-lines," particularly in commercial areas, to encourage spatial definition and the development a street wall.
Establish Build-To Lines. Great streets create a sense of space and enclosure that makes it attractive and convenient for pedestrians. Build-To lines help reduce distances pedestrians must travel from the public sidewalk to building entrances and a sense of enclosure and by setbacks suggest a minimum distance buildings must be "set back" from the public right-of-way.
Zoning regulations often require commercial shopping centers to be set back 100-200 feet from the public right-of-way, thus creating a "strip" shopping center fronted by parking. Because sidewalks are located next the roadway, in the public right-of-way, the pedestrian zone is then sandwiched between a parking lot and traffic. Reducing setback requirements and locating parking on the street, behind, or at a minimum, beside buildings would help create a more pedestrian-friendly environment and encourage shoppers to walk to and between shops.