Synthesis of Variable Speed Limit Signs
CHAPTER 2. RESEARCH METHODOLOGY
The research team used two methods to collect information for analysis on existing policies, procedures, and practices by highway transportation agencies on variable speed limit (VSL) systems. A literature review compiled information from resources such as published research, dissertations, presentations, guidelines, and other relevant publications. To supplement the literature review, agencies operating VSL were interviewed to gather descriptions of their systems and background information on lessons learned. This report reviews all VSL systems but uses those that have been investigated through both a literature review and agency interviews as examples when discussing different VSL system components.
LITERATURE REVIEW
The research team conducted a comprehensive literature review using a variety of resources. This literature review included published research related to VSL planning, policy, and operations; resources obtained from operating agencies, policy manuals, operating documentation from operating and law enforcement agencies, and documentation directly related to VSL-instigated legislative action on speed limits; and additional public information campaign materials, such as web sites, that are used to interact with the public. The team reviewed VSL sites in the United States and other countries. A list of most relevant references is shown in Appendix A.
AGENCY OUTREACH
In addition to the literature review, the research team selected representative VSL systems that are currently active and collected VSL data from corresponding agencies directly. The team documented how agencies are operating their respective VSL systems and their experiences using this particular traffic management strategy by interviewing agency representatives and reviewing various documents provided by highway transportation agencies. During the process of contacting agencies, the team identified some States that are considering VSL systems as well as those that have deactivated their VSL systems. Information collected from these agencies was documented as well.
The data collection methods included phone interviews with agency staff and a review of various VSL documents and materials (e.g., video clips, reports, presentations, operations documents, etc.) provided by the agencies. The research team used the following resources to identify agencies using VSL systems:
- Results of the literature review.
- Historical information from Guidelines for the Use of Variable Speed Limit Systems in Wet Weather.
- Members of the Transportation Management Center Pooled Fund Study (TMC PFS).
- Personal knowledge of the research team.
The research team interviewed 13 agencies about their VSL systems, 9 of which currently have active VSL systems. In addition to the phone interviews, the team also obtained VSL documents from nine agencies. These documents include VSL activation procedures, operations manuals, equipment/software specifications, signing protocols, algorithms, checklists, and internal/external educational materials. Table 1 summarizes the data sources.
Table 1. Summary of variable speed limit data sources.
| Agency |
Conducted Phone Interview |
Provided Documents Used in Synthesis |
| Arizona Department of Transportation (DOT) |
Yes |
N/A |
| Florida DOT |
Yes |
Yes |
| Georgia DOT |
Yes |
Yes |
| Minnesota DOT |
Yes |
Yes |
| Missouri DOT |
Yes |
N/A |
| Nevada DOT |
Yes |
Yes |
| New Jersey Turnpike Authority |
Yes |
Yes |
| Oregon DOT |
Yes |
Yes |
| Pennsylvania Turnpike |
Yes |
N/A |
| Tennessee DOT |
Yes |
Yes |
| Virginia DOT |
Yes |
Yes |
| Washington State DOT |
Yes |
Yes |
| Wisconsin DOT |
Yes |
N/A |
N/A = not applicable (no variable speed limit systems).
The research team requested information in the following categories to gain a comprehensive description of each agency's VSL system:
- Planning and policies.
- Design, deployment, and standards.
- System operations and control.
- Maintenance and life-cycle costs.
- Costs and benefits.
- Liability issues.
- Enforcement issues.
To perform the interviews, the research team developed a list of questions and classified each as key or auxiliary. In consideration of agency staff time, interviews were scheduled to last 30 minutes. During that time, the key questions were discussed first with the intention of collecting information on the auxiliary questions from agency-provided documents. Time permitting, some auxiliary questions were discussed during the calls as well. On some occasions, agencies provided feedback to the full set of questions and/or emailed supporting documents prior to a phone interview. These instances provided the team with an opportunity to become familiar with the agency's VSL system to better tailor the phone discussion. The interview questions were not generally provided to an agency, but the interviews were completed by project team members using a form and gathering as much information possible during each interview. A summary of each interview is included in Appendix B. Agency Interview Summary.
SYNTHESIS AND ANALYSIS
Both during and following each interview, all State representative responses were compiled and recorded in an Excel spreadsheet, which served as the research team's main database. This database allowed the team to organize all responses into categories (e.g., general VSL information, setting speed limits, equipment and costs, enforcement, VSL signs, etc.). After all responses were appropriately categorized, the information was synthesized and incorporated in the appropriate sections of the final report. Additional documents provided by various agencies were also utilized to better understand and describe VSL systems located throughout the United States.
