Movable bridges in today’s transportation networks: Balancing mobility, navigation and long term reliability

Situated at the intersection of regulation, design complexity and community expectations, movable bridges continue to play a critical role where surface transportation and marine navigation must coexist

By Christian Brown and Greg Harrell | HNTB

For transportation agencies responsible for keeping people, freight and commerce moving across navigable waterways, the choice between a fixed or movable span is rarely simple. Each option carries implications for regulatory approval, operational reliability, community impact and long term stewardship.

Movable bridges continue to serve as critical transportation assets in many regions, supporting surface mobility while preserving the navigation that waterways require. In corridors where vertical clearance, right of way and multimodal demands constrain traditional fixed spans, movable bridges offer a flexible approach that balances competing needs.

When thoughtfully planned and modernized, today’s movable spans can deliver reliable multimodal mobility while maintaining essential marine access on which regional economies depend.

Benefits of a modern movable bridge solution
Movable bridges offer a range of benefits that help transportation agencies balance surface travel with uninterrupted waterway navigation.

• Clearance in constrained environments
  In many corridors, constrained environments limit the ability to adjust roadway and track profiles, raise approach grades or shift alignments to accommodate a fixed span. Surrounding development, adjacent rail corridors, utilities, existing right‑of‑way and construction access often restrict elevation changes. When these constraints make it difficult to meet required navigation clearances through a fixed structure — or when maintaining multimodal connectivity is essential — a movable bridge can provide a viable alternative. By reducing the potential need for extensive approach structures, steep grades, increased right-of-way and environmental impacts, movable spans can help minimize cost, construction complexity and overall impact while still meeting navigation and mobility requirements.

• Maintaining essential navigation
  The U.S. Coast Guard Bridge Program supports marine safety, security and stewardship and helps preserve freedom of navigation and the nation’s Marine Transportation System. This is accomplished by approving the location and plans of new bridges, modifications to existing bridges and causeways over navigable U.S. waterways, as well as international bridges across waterways. Movable bridges make that possible by providing the vertical clearance needed for a wide range of vessels and uses, serving as operational safeguards that ensure waterways remain dependable and resilient under all conditions.

• Improved operational reliability
  Advances in technology, including digital controls and system automation, allow movable bridges to operate more precisely, improving reliability and safety. Integrated diagnostics and real‑time monitoring provide a clear understanding of system behavior, allowing operators to preserve bridge health by detecting emerging issues before they result in disruptive shutdowns. Modern mechanical, electrical and control systems operate more smoothly and efficiently, reducing stress on critical components, minimizing wear and tear on parts and reducing long-term maintenance costs.

• Support for safe, multimodal operations
  Technology also supplements human control of today’s movable bridge systems. Added reliability and safety balance the needs of all users: roadway, rail, pedestrians, cyclists and marine vessels. A programmable interface can provide visibility into every step of the functional sequence, allowing the control system to verify that roadway or rail traffic is stopped and the bridge is clear of other users before the span begins to open for waterway traffic. That same logic ensures the bridge stays open until the waterway is clear and confirms that the bridge is fully seated, aligned and locked after closing before traffic resumes on the bridge.

• Remote and technology-enabled operations
  Controlling the movement of movable bridges from off-site locations through remote operation is enhanced through improvements in closed-circuit television cameras, thermal imaging devices and other detection systems that interface directly with modern bridge controls. With software‑based control systems, operators can monitor conditions, verify safety steps and manage span movements from centralized dispatch centers without compromising safety. These systems also have the potential to reduce on-site staffing needs while improving situational awareness. The Coast Guard is receptive to remote operation, approving this approach on a case-by-case basis that considers the relevant benefits of remote operation, redundant safeguards that protect against an operational failure and timely response plans to address navigation disruptions.
• Manageable long-term ownership
  A movable bridge is much more than a structure; it is a large machine that plays a critical role in the success of a multimodal transportation network. Long-term performance depends on diligent inspection, preventive maintenance and timely repairs. Effective maintenance protects public safety, maximizes reliability and helps ensure the bridge remains operational for generations.

Considerations when exploring a movable bridge solution
A movable bridge may be the preferred solution when rail and vehicle crossings must coexist with marine navigation that cannot be restricted by a fixed span, or when waterway trends indicate that navigation needs may change over time.

Certain additional considerations will help transportation, transit and rail organizations evaluate whether a movable span is the right choice.

• Site constraints and feasibility
  Built environments often impose limits on roadway profiles or require tight coordination with adjacent rail or transit facilities, leaving little room to adjust grades or shift alignments. Bridge replacement projects may inherit fixed-pier locations, narrow rights‑of‑way and other constraints that cannot easily be altered.

Considering these factors early in the planning process helps determine whether a movable bridge is feasible on a given site and guides designers toward cost-effective solutions that respect existing constraints while still meeting navigation and mobility needs.

• Regulatory and stakeholder requirements
  The length and height of bridges crossing navigable waterways are dictated by the needs of navigation within the waterway being crossed. Movable bridges can offer solutions that minimize the overall project footprint and reduce environmental and other impacts relative to a fixed bridge that may need longer approach structures or embankments to provide enough vertical clearance. Early coordination with regulatory agencies and other stakeholders is critical for ensuring the project efficiently and successfully completes environmental review under the National Environmental Policy Act (NEPA) and finalizing permits needed for construction.

The environmental planning process provides insight into the project’s potential impacts and how those impacts can be mitigated. It starts with early consultation with the Coast Guard to establish the necessary clearance requirements and other reasonable needs of navigation, shaping decision-making about structure type, to conduct a comparative analysis between movable and fixed bridge solutions. Public engagement within the local community, early and often throughout early planning stages of a project, also helps clarify expectations and operational impacts.

By engaging various groups at the beginning of the planning process, project owners and designers can make informed decisions about movable bridges and set the project on the right trajectory from the start, reducing the risk of costly redesigns and years of delay.

• Cost and lifecycle expectations
  Modern movable bridges are designed to achieve a service life of at least 75 years, and historically, many movable bridges have served for much longer than that. Before a bridge is even built, lifecycle planning through early design decisions can positively influence how these bridges age over time.

For example, decisions related to structure type, materials, operating machinery and control system components are driven by how frequently the bridge will operate and how easily and cost-effectively it can be maintained. By integrating lifecycle considerations into design, transportation agencies and rail companies are better positioned to protect their investments and prolong the service life of their movable bridges.

• Operational and maintenance considerations
  A key consideration in movable span type selection is what the owner’s staff already knows — what systems and parts they are familiar with and accustomed to maintaining. Aligning the structure type with that institutional knowledge, when possible, increases the likelihood that a bridge and its systems will be maintained properly and reduces the apprehension that often comes with adopting unfamiliar parts and technology.

Even when a replacement bridge uses the same general span type as the legacy structure did, newer components and systems will require staff training. Together, these operational and maintenance considerations ensure that the owner’s team is prepared to manage the bridge safely and effectively.

Effective maintenance requires efficient access. Design criteria for movable bridge projects should prioritize safe, easy access to components and systems that require regular attention and inspection without special tools or equipment. Thoughtful access planning, integrated with the overall bridge design, simplifies maintenance and inspection activities to directly influence a bridge’s durability, reliability and long‑term health.

Coordinated closures are an important consideration in operational planning. While the Coast Guard’s baseline expectation is that movable bridges open on demand, early coordination can establish tailored operating restrictions to minimize congestion by limiting bridge openings when roadway or rail demand is highest. These negotiated closure periods protect mobility for all users and help owners plan for consistent operating patterns.

As transportation networks become larger, more complex and more resilient, movable bridges remain an important tool for addressing challenging crossings. Thoughtful planning, modern technology and long-term maintenance strategies can help owners balance navigation, multimodal mobility, environmental constraints and responsible stewardship over the life of the structure.

ABOUT THE AUTHORS

Christian Brown
Senior Vice President and Senior Project Director
HNTB 

Christian Brown is a senior vice president and senior project director at HNTB and currently serves as the firm’s national movable bridge program lead. For the last three decades with HNTB, Brown has been responsible for delivering movable bridge projects through their entire lifecycle, including environmental planning and permitting; preliminary and final design; and engineering services during construction.
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Greg Harrell
Associate Vice President and Senior Project Manager
HNTB 

Greg Harrell is an associate vice president and senior project manager at HNTB, specializing in planning, design, and construction of complex and movable bridges for more than 25 years.
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