Flexible Design in Practice

View case studies and examples of flexible design elements in practice around the U.S. and internationally. This section features hundreds of case studies and images of road-design elements like barriers, bicycle facilities, crosswalks, curb extensions, medians, parking, shoulders, sidewalks, transit facilities, and more.

Excerpt IconExcerpt Decision Sight Distance (DSD): Mitigating Insufficient Decision Sight Distance
Designers should strive to provide three-dimensional alignments that produce DSD as part of location planning and studies for new alignments, and in considering proposals to add new intersections and interchanges to existing highways. more...
from  A Guide for Achieving Flexibility in Highway Design
Excerpt IconExcerpt Decision Sight Distance (DSD): Flexibility in the AASHTO Guidelines
AASHTO refers to DSD as "desirable" and "appropriate." Providing DSD is clearly recommended as an element of a high-quality alignment. more...
from  A Guide for Achieving Flexibility in Highway Design
Barriers and Clear Zones
In CSS projects, close scrutiny is given to the design of pedestrian and traffic barriers, sound and visual barriers, and the dimensions and design elements within a "clear zone."
Bicycle Facilities
Making streets more bikeable is often a goal of CSS projects. The integration of bicycles onto streets and roads can be done in a number of ways, depending on the context, space available, and demand for bicycling.
Bridges are important functional and historic structures in both urban and rural settings. In CSS projects, designers are addressing the aesthetic design of a bridge, its linkages to the surrounding context at each end of the bridge, and the functions that bridges can serve for pedestrians, bicyclists.
Crossing Islands / Channelized Intersections
Research shows that crossing islands help reduce vehicle speeds in intersections and provide refuge space for pedestrians to cross halfway, wait for a break in the traffic, and then continue across.
Crosswalks are an essential element for a safe, continuous and comfortable pedestrian environment. Design decisions in CSS projects specifically deal with the crosswalk location (including at unsignalized or mid-block urban locations), materials to enhance their character and visibility, width, and relationship to crossing islands, curb extensions, and other pedestrian enhancements.
Curb Extensions / Neckdowns
Neckdowns and bulbouts reduce the crossing width of streets for pedestrians, make pedestrians more visible in a crosswalk, and add space to sidewalks that can be used for pedestrian amenities and activities - which is why they are now common in downtown projects. By forcing drivers to slow down when they turn the corner, makes it safer and more comfortable for pedestrians. Research shows they have a minor impact on traffic speed.
Highway interchanges are often gateways to communities and regions.
Intersections present challenges in any transportation project. With CSS, innovative techniques and tools are being developed to address this important aspect of road design. These techniques help make streets and roads safer and easier to cross, and reduce accidents and conflicts. In cities and towns, intersections can become lively street corner environments.
Landscaping and Amenities
Broadly termed, these features include trees, lighting, information signs, benches and other features which make a street more comfortable for its users and more attractive.
Lane Widths
In CSS projects, designers are using the flexibility inherent in design criteria and standards to select a lane width that is appropriate to a site's context, the desired vehicular speed, and types of vehicles served, recognizing that roadway space is often constrained and excess space for vehicles can be given over to other purposes, such as wider sidewalks for pedestrians or a bicycle lane.
Medians can be major amenities for adjacent communities. They have many useful functions including reducing lane widths, providing a safe space for pedestrians as they make their way across the street, splitting-up a lengthy curb-to-curb distance, and becoming a location for trees, flowers, art, and other amenities. They can also present challenges in terms of their design and landscape treatments.
Modern Roundabouts
While not suitable for all situations, roundabouts are being used in CSS projects because they can offer several advantages over conventional intersections: they eliminate conflict points, while increasing vehicle capacity; they reduce delay and costs of site acquisition and signal maintenance; they can be very attractive and become a community focal point; and they have a terrific safety record.
In CSS projects, the layout of parking on a street is being used to achieve broader goals as well, such as reducing vehicle speeds and buffering pedestrians from moving traffic.
Public Utilities / Valley Gutters
One element of cross-section design that is often overlooked is the accommodation of public utilities. Overhead utilities typically include electric, telephone, and cable television.
In CSS projects, designers are tailoring the width and materials of shoulder to better fit in with the adjacent context, and eliminating shoulders in urban settings where they are not appropriate.
Sidewalks are not just pedestrian thoroughfares: they are social places in communities as well which serve adjacent land uses. Design of sidewalks responsive to context deals with the full variety of functions the sidewalk will serve.
Signals and Timing
  Traffic signalization is another tool of CSS designers seeking to adjust vehicle speeds and balance vehicle operating goals with the goals of adjacent communities.
Stormwater Management/Water Quality
  Transportation projects provide the opportunity to implement sustainable stormwater management strategies that mitigate the negative impacts of construction activities that disturb the natural hydrologic cycle.
Transit Facilities
The design of transit facilities on streets - shelters, stops, and special lanes - is being viewed as very important to increasing ridership by making transit service more comfortable and efficient for passengers.
Excerpt IconExcerpt Horizontal Alignment: Mitigating Tight Curvature
Where acceptance of nominally sharper-than-normal curvature appears to be an appropriate solution, the designer has many tools available to mitigate the potential adverse safety impacts. Widening the road and/or shoulder through the curve and improving the roadside may address crash severity. Relocating or closing intersections or driveways within the curve removes additional risk factors. Spot resurfacing or wedging of the pavement through the curve can increase available friction for cornering. Mitigation measures for horizontal curvature below typical ranges may include one or more of the following: more...
from  A Guide for Achieving Flexibility in Highway Design
Excerpt IconExcerpt Horizontal Alignment: Flexibility in the AASHTO Guidelines
The AASHTO Green Book (2) presents horizontal curve design values for a range of maximum superelevation rates ranging from 0.04 to 0.10. Agencies are free to adopt one or more ranges consistent with their terrain, climate, and other factors. Within a given design range for maximum superelevation, the full range of curvature and recommended superelevation is presented for the range of design speeds.  more...
from  A Guide for Achieving Flexibility in Highway Design
Excerpt IconExcerpt Vertical Alignment (Grades): Mitigating Steep Grades
Where it is necessary to accept a steeper than normal grade, designers should evaluate the operational effects of the grade on heavy vehicles. The AASHTO Green Book (2) includes a set of design curves that enable estimation of vehicle speeds on upgrades.  more...
from  A Guide for Achieving Flexibility in Highway Design
Excerpt IconExcerpt Vertical Alignment (Grades): Flexibility in AASHTO Guidelines
(..)[T]he information on grades reflects design practices related to cost and operational efficiency. The AASHTO Green Book (2) refers to "reasonable guide values for maximum designs," and further indicates that such guidelines are based primarily on traffic operational considerations as opposed to direct safety impacts.  more...
from  A Guide for Achieving Flexibility in Highway Design
Excerpt IconExcerpt Stopping Sight Distance (SSD): Mitigating Limited Stopping Sight Distance
Risk assessment of SSD offers insights into mitigating a location with limited SSD. Increasing sight distance through the reconstruction (lengthening) of a vertical curve is typically considered first. This measure, however, is often expensive and may result in adverse impacts to access or adjacent rights-of-way. more...
from  p. 60
Excerpt IconExcerpt Stopping Sight Distance (SSD): Flexibility in AASHTO Guidelines
Determination of SSD values is highly dependent on the selected design speed. Flexibility within the Green Book (2) is thus primarily provided by the ranges of design speeds afforded designers. more...
from  p. 60
Excerpt IconExcerpt Intersection Sight Distance (ISD): Mitigating Limited Intersection Sight Distance
Where limited ISD exists according to the AASHTO Green Book (2), the designer has a number of options depending on the nature of the sight restriction. Every attempt should be made to eliminate sight restrictions such as trees, vegetation, signs, and movable obstacles. Where such obstacles cannot be moved, field investigations can determine whether the driver can safely position the vehicle such that actual sight lines (versus those specified in the design model) are clear. more...
from  p. 61
Excerpt IconExcerpt Intersection Sight Distance (ISD): Flexibility in the AASHTO Guidelines
Provision for sufficient ISD is recognized in the AASHTO Green Book (2) as important for overall intersection operations. For unstopped approaches, the Green Book (2) notes that the provision of SSD will generally provide sufficient distance for drivers to perceive conflicts. Intersection sight distances that exceed stopping sight distances for unstopped vehicles "enhance traffic operations." more...
from  A Guide for Achieving Flexibility in Highway Design
Excerpt IconExcerpt Passing Sight Distance (PSD): Mitigating Limited Passing Sight Distance
Insufficient PSD can degrade operations and increase risk-taking by drivers. The effects of insufficient PSD may not be evident or significant except where traffic volumes approach the capacity of the two-lane highway, or where the volume of heavy- or slow-moving vehicles is unusually great. more...
from  A Guide for Achieving Flexibility in Highway Design
Excerpt IconExcerpt Passing Sight Distance (PSD): Flexibility in the AASHTO Guidelines
There is no requirement that PSD be provided for any two-lane road. Where it is possible to provide, PSD enhances the operation of the road, but it is not normally considered a "safety essential" design feature. Furthermore, PSD applies only to two-lane highways, and is not considered in the design of multilane facilities.  more...
from  p. 62
Excerpt IconExcerpt The Roadside: Flexibility in the AASHTO Guidelines
The AASHTO Green Book (2) is intended to be flexible with respect to roadside design treatments. It refers to the Roadside Design Guide (3) for use in general guidance; as noted previously, there may well be more than one solution that is acceptable for a given location. The Roadside Design Guide (3) also states this in its preface. more...
from  A Guide for Achieving Flexibility in Highway Design

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