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FHWA Home / Safety / Speed Management / Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

Chapter 4. Guidelines for the Design, Installation, Operation, and Maintenance of Wet Weather Variable Speed Limit Systems

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This chapter provides guidelines for developing a comprehensive concept of operations for a VSL system, determining where weather sensors are appropriate, ensuring that all necessary equipment is incorporated in the requirements, and developing an operations and maintenance plan.

4.1 Guideline 13: Develop a comprehensive concept of operations for the variable speed limit system.

Once the decision has been made that a VSL is appropriate for a location, the first step in the design process should be to develop a concept of operations. A concept of operations is a document that helps ensure that the system contains all required features. This document will describe the characteristics of the proposed system and may involve the full system engineering process for developing it. FHWA's Systems Engineering Guidebook for ITS can be used as a guide in this process (15). In the operational concept, it is important to keep issues related to system operations and maintenance in mind and not to lose sight of the primary purpose of a VSL system, which is to reduce speed variance and avoid accidents caused by drivers traveling at an unsafe speed.

4.2 Guideline 14: Install appropriate weather sensors or use accurate weather data at problem locations.

It is important to determine the data needs of a VSL system, which data sources should be used to meet these needs, and when they should be used to ensure that adequate data is collected. The data that should be collected for wet weather VSLs are traffic, weather, and pavement conditions. Either real-time traffic cameras or sensors can be used to collect traffic data, but data on weather conditions can be collected in a variety of ways, including video, pavement sensors, weather radar, and sensors for rain and fog. Additionally, it is important for personnel on the ground, such as law enforcement, first responders, and other incident management personnel, to be able to request speed reductions based on their observations in cases where there are weather conditions that the system operator is not yet aware.

4.3 Guideline 15: Ensure that equipment required in the design process is incorporated into the installation requirements.

In order to avoid potential issues, there are a variety of details that need to be considered when designing any VSL system. These details include sensors, communications, power, operator confirmation and override, and a police notification process. When designing a VSL system, it is important to have adequate speed and visibility sensor coverage in the vicinity of the VSL sign. The most cost-effective time to add these sensors is when power and communication are being added at the beginning of the design phase. Although sensors may be costly to begin with, they are even more expensive to add after the system is designed. It is important to budget for and include a reasonable number of sensors in advance.

The communications and power for a VSL system should be designed in conjunction with other nearby devices that use similar technology, such as Closed Circuit Television (CCTV). If running landline communications and power is cost prohibitive, wireless communication and solar power may be more cost-effective options. However, it is important to consider overall power requirements and whether solar energy will provide enough power to the entire system. Additionally, cost and maintenance of batteries should be considered since battery life may be reduced in harsh environments. Additional technology that should be included in the design of a VSL system are speed detectors, rain sensors, Bluetooth detectors, and a method to provide police notifications. It is crucial for police officers to be aware of the speed limit that a VSL is displaying at any given time. Because police officers typically take their communications gear with them when shifts change, it is important that the notifications are broadcast and not sent directly to individual officers.

Another important detail to ensure is that the system outputs information into a log of all actions that the system makes. Regardless of whether changes in the system are made automatically or by the operator, it is extremely important to maintain a log. Any changes made to the speed display should be logged, as should the source of the change. This log should be designed in a way that ensures the data will be archived and backed up.

4.4 Guideline 16: Develop an Operations and Maintenance Plan.

A plan for operating and maintaining the VSL system should also be included in the process. It is easy for maintenance to be overlooked when budgets are being created, but it is a necessary element. To maintain both driver safety and driver trust in the system's accuracy, it is important that a VSL system operate properly and reliably. Agencies should develop plans in advance for regular cleaning, checking, and calibration, not just of the signs, but of all the equipment, including road weather and traffic sensors, cameras, and communications. This should be at least a 5-year plan and include information on funding requirements. Additional studies should be completed after the VSL has been implemented in order to measure the effectiveness of the system. If these studies show that the VSL system is not operating effectively, changes may need to be made regarding either the system itself or related enforcement efforts. Guidance for developing operations and maintenance manuals can be found in FHWA's Systems Engineering Guidebook for ITS (16).

Important elements that are relevant to VSLs and should be clearly addressed in the plan include:

Page last modified on October 15, 2014.
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