A Common Sense Approach to Street Design
Using Performance-Based Design to Optimize Roadway Development
What is Performance-Based Design?
Performance-based design is a structural engineering methodology whereby each project is evaluated based on the needs of its surroundings and users rather than only following design manual requirements. The application of this methodology to street design is emerging as a way to maximize budgets while still achieving desired outcomes. By comparing alternatives through specific goals and measures, you can determine the optimal design for the specific environment.
Projects have different performance benchmarks and can intentionally be designed for aspects such as target speed guidelines, heavy pedestrian usage, and operation and safety (crash history and delay). The movement toward performance-based street design has become even more important in the current market as it affords agencies the luxury of maintaining flexibility in their design standards and project budgets.
What is Motivating the Move to Flexible and Performance-Based Design?
With rising costs and fewer resources available, agencies are taking a common sense approach to distributing dollars across projects. By focusing on flexible design that achieves short-term performance metrics despite expansion phasing, available resources can be used to further advance multiple projects.
In addition to cost considerations, context is important for communities that are urbanizing and discovering that traditional highway design standards are insufficient for retrofit or high-density challenges. The Vision Zero initiative, which is a national focus on zero deaths from transportation, is another strong motivator towards adapting to flexible and performance-based design methods. This initiative places greater emphasis on appropriate design speed and design for vulnerable street users as opposed to only for vehicles.
Flexible street design may allow quickly growing regions to deliver projects at an accelerated rate while still meeting budgets. Across the United States, agencies and municipalities are adopting this approach in design manuals for pavement design and context-sensitive cross sections. Performance-based design methods will likely be seen in the 2022 edition of AASHTO’s Policy on Geometric Design of Highways and Streets (Green Book) 8th Edition, AASHTO’s Guide for the Development of Bicycle Facilities, and multiple Department of Transportation’s (DOT) Roadway Design Manual (RDM) updates.
What Design Standard Changes Do We Anticipate?
Since the upcoming 8th Edition of the AASHTO Green Book emphasizes context areas and performance-based metrics, we anticipate agencies following suit in the coming years and updating design standards, procedures, and policy. We primarily see this resulting in agencies establishing performance-based models for design or clarifying AASHTO’s models in the Green Book and adapting design standards to accommodate considerations for context-sensitive design. In addition, future considerations may include designing for technology advancements, such as wider pavement marking striping for emerging vehicle technologies and other changes in the next version of the Manual on Uniform Traffic Control Devices (MUTCD).
What Are Some Practical Applications of Performance-Based Design?
When considering how these larger national initiatives apply to your local agency, several different applications may be helpful to consider:
- Retrofits can be difficult from a budgetary standpoint since design standards are usually focused on new development. With new flexible design standards, practical design for retrofits is possible within an updated project-specific design framework.
- Asset Management for pavement conditions, utility systems, or traffic signals can benefit from performance-based design by prioritizing investments where they are needed most and assisting in making data-informed decisions.
- Performance Assessments can be completed through a performance-based lens by tracking pre-design and post-design performance and safety metrics to paint a picture that is understandable for non-technical audiences, decision makers, and the public.
- Policy, design standards, and specifications can be evaluated holistically by agency staff and decision makers to ensure performance-based design is feasible.
Putting it into Practice: Alternative Typical Section
In the diagram shown above, the alternative typical section relocates the bike lanes from the pavement to the two, 10-foot wide shared-use-paths to accommodate both pedestrian and bicycle traffic. Separating bicycles from motorized traffic where the speed differential and traffic volumes are higher will greatly increase the safety and comfort of riders. In addition, a 15-foot-wide roadway border will easily accommodate utilities, ADA, and provide flexibility to reduce right-of-way in constrained areas. Additionally, this calculated construction cost savings of roughly $2M benefits agencies by reducing roadway pavement and accommodating bikes via shared-use-paths.
What are the Benefits of Performance-Based Design for Cities or Agencies?
With performance benchmarks tailored for each project, performance-based roadway design gives agencies and cities the luxury of flexibility in their design standards and project budgets. This design approach allows the optimal design to be determined by comparing alternatives through specific goals and measures. Cities and agencies that want to achieve desired outcomes while making the most of their budgets are using flexible, performance-based design.
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