CHAPTER 2. RESEARCH APPROACH

RESEARCH QUESTIONS

The activities in this project focused on answering several key research questions:

• Determine the effects of roadway, roadside, and non-roadway elements on operating speeds on roadways with a target speed between 30 and 40 mph. This research question was later refined to determine the relationship of operating speed to road elements for roads with a posted speed limit between 25 and 40 mph.
• Identify current practices regarding target speed.
• Determine how to relate the results from the first two research questions to findings that can be incorporated into the design process.

RESEARCH METHODOLOGY FOR NCHRP PROJECT 15-76

The research was conducted within eight tasks. Each task listed is followed by the objectives of that task:

• Task 1. Identify Current Knowledge and Practices. The objective of this task was to review the existing body of knowledge related to target speed, design speed, and operating speed. In addition, the review identified and summarized known effects of roadway, roadside, and non-roadway elements on speed and how those effects are used in practice.
• Task 2. Develop Proposed Analytical Approach and Identify Sources for Data. The objectives of this task were to (a) provide a detailed description of the proposed analytical approach to be used to develop methods for target speed analysis; and (b) identify and compare potential sources for the needed data on speed; relevant elements; and supplemental information to build the dataset needed for the proposed analytical approach. This task also recommended the data sources with the most potential for successful use in the project.
• Task 3. Develop Research Study Approach. The objective of this task was to develop the data collection plan that will be recommended for use in Phase II.
• Task 4. Develop Interim Report and Phase II Work Plan. The objectives of this task were to: (a) develop the Work Plan to be carried out in Phase II of the research project; (b) develop the Interim Report that will summarize the results of Tasks 1 through 3; (c) submit the Phase I Interim Report and the Phase II Work Plan; and (d) participate in a panel meeting.
• Task 5. Develop List of Study Sites and Analysis Elements. The objective of this task was to develop a comprehensive list of potential study locations and categorization of the elements.
• Task 6. Conduct Phase II Studies. The objective of this task was to conduct the approved work plan.
• Task 7. Develop Recommended Process for Target Speed. The objective of this task was to recommend a process by which appropriate elements should be considered to achieve a target speed.
• Task 8. Prepare Final Deliverables. The objective of this task was to prepare the final deliverables, which included the following: (a) Practitioner’s Guide that will

• assist an engineer or planner with guidance on the steps needed to design for target speed. This guide is anticipated to be published as part of the NCHRP Report series, to facilitate its distribution to a wider audience of practitioners and promote implementation. (b) Final Report that documents the entire project (this document). (c) Prioritized recommendations for future research to guide subsequent research efforts based on findings from this project. (d) Suggested modifications to standard references that will describe, in detail, where the findings from this project may be incorporated into documents such as the AASHTO Green Book (AASHTO 2018), the TRB Highway Capacity Manual (TRB 2016), and other relevant references. These suggested modifications are included in Appendix H. (e) Presentation (Power Point format) with speaker notes that described the project background, objectives, research approach, findings, and conclusions suitable for use in a webinar. (f) Technical memorandum titled “Implementation of Research Findings and Products” that was provided as a standalone document. (g) Webinar that described the activities and findings of the project, as well as recommendations for implementation, to a diverse set of stakeholders. (h) Draft article suitable for publication in TR News.

PHASE I EFFORTS

During the kickoff meeting for NCHRP Project 15-76, the panel identified three topics for consideration in the project and in the Phase I review efforts:

• Current practices and existing guidance on defining, setting, and designing for target speed.
• A broad project development process, to include planning, design, and operations (and the corresponding practitioners within those areas of the profession as the intended audience).
• The location (i.e., roadway, roadside, or non-roadway) of different factors that have an effect on speed.

In accordance with the direction set by the panel, the research team focused the Phase I review efforts on these three topics. Overall, Phase I efforts were grouped into the following broad areas:

• Conduct project management.
• Review literature with an emphasis on speed concepts and definitions of commonly used speed terms, the use of speed in the design process, and identifying factors that influence speed.
• Identify current practices for incorporating target speed into the design process.
• Develop a conceptual data framework.
• Identify potential datasets for operating speed, site characteristics, and other relevant data.
• Develop a proposed analytical approach for Phase II.

The research team met with the panel at the end of Phase I. In addition to approving the research team’s proposed Phase II activities, the panel set the following directions during the Phase I panel meeting:

• In addition to the states with sites where the team had already collected data in Phase I (California, Oregon, Texas, Utah, Virginia, and Washington), the panel members’ consensus was to identify a state in the Southeast and the Northeast. Florida and Massachusetts, respectively, were chosen to meet those regional requests.
• On the topic of factors to use as site selection criteria, panel members agreed with the research team to keep the number of through lanes to 2 or 4, to use only two-way roads, and to emphasize horizontal alignment with tangents or minimal horizontal curves. Consensus was to keep the 25-mph sites in the databases that the team already had on hand, while selecting only 30-40 mph in the identification of additional sites. The panel instructed the team to keep both curbs and shoulder as edge treatments, and to remove median type as a control.
• In a similar discussion on the study matrix, the panel agreed with the research team’s initial suggestions of bicycle lane presence, building setback, and the census-based factors of gross residential density and total road network density. Based on the panel’s comments during the meeting, researchers added the presence of center line markings, the presence of edge line markings, and lane width in the matrix. Median type was also included in the study matrix, as a factor for which researchers wanted sites with a range of values. A variety of other factors were measured (e.g., presence of rumble strips, presence of features near the road such as trees and street furniture, etc.) as part of the site characteristics data collection but were not part of the selection of study sites.
• The research team used 2019 speed data from INRIX XD, specifically excluding more recent data that may have an unclear “pandemic effect”.
• The team performed quality control checks on the data. Researchers also looked at more than one speed measure, beginning with average speed, to find the speed measure that has the strongest relationship with the factors. The activities and findings related to these checks and reviews were summarized in a technical memorandum.
• The team defined the speed measures and terms that are used, including a review of the working definition of target speed to refine it as appropriate based on the results of the analysis.

PHASE II EFFORTS

Following the Phase I panel meeting, the following research efforts were conducted in Phase II:

• Develop a list of study sites for data collection.
• Develop a list of analysis elements (i.e., roadway, roadside, and non-roadway factors) for data collection to be used in the analysis.
• Perform the requested checks on available sources of speed data to confirm that the INRIX XD speed dataset is appropriate for this research.
• Identify and collect site characteristics data in multiple rounds of data collection for the chosen study sites.
• Analyze the data to identify relationships between speed and various road elements.

• Develop a process for incorporate target speed in the design process, and develop a practitioner’s guide for implementing that process.
• Conduct selected technology transfer activities.
• Document the efforts within the project.