When a bridge closes, a city may keep essential trips possible by routing traffic over existing roads, building a temporary road or detour bridge, or—in some circumstances—using a ferry. The right choice depends on the traffic that must cross, the safety and capacity of alternatives, how long the closure may last, and what can be built or operated at the site. A backup crossing can preserve access without restoring normal travel times, lanes, or routes.
What happens when a bridge is closed?
Officials assess the risk and decide whether the bridge needs a load restriction, a lane closure, or a full closure. They then determine how people and essential services can reach destinations on the other side. Depending on local conditions, that may mean signing an existing-route detour, creating a temporary roadway connection or bridge, arranging ferry service, or combining a detour with construction that shortens the closure.
The Federal Highway Administration (FHWA) describes essential traffic service as a reasonable, safe connection—not necessarily a return to normal service. A route may have fewer lanes, lower speeds, extra travel time, or a different travel mode. In a 2022 memorandum, FHWA states that restoring essential traffic does not mean restoring normal service or fully rebuilding the site: FHWA’s 2022 Emergency Relief memorandum.
Which backup crossing options can cities use?
Detours over existing roads and bridges
A detour uses roads and crossings already in service. Agencies identify and sign an alternate route, then consider whether it can safely accommodate the additional traffic and the vehicles that need to use it. The route’s distance and travel time matter, but so do pavement and bridge condition, emergency access, and connections to essential destinations.
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FHWA’s plan-of-action guidance specifically calls for identifying alternative routes and considering scour vulnerability of bridges on them. A detour is not automatically safe or adequate simply because it appears on a map: FHWA plan-of-action guidance.
Temporary road connections and detour bridges
A temporary road connection or bridge can carry traffic around the closed structure while repairs or replacement work proceeds. FHWA lists both among examples of emergency repairs. Its guidance says temporary connections should match the anticipated volume and type of essential traffic, reasonable speeds, safety, term of use, and cost: FHWA Emergency Relief Manual.
These are public infrastructure projects, not off-the-shelf fixes. They require a site-specific design and agency decisions about whether a temporary structure can be built, connected safely, and kept in service for the needed period.
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Temporary ferry service
Where an alternate route or temporary structure is not reasonable or practical, a ferry may provide a substitute highway connection. A workable service needs a suitable crossing and landing facilities, arrangements to operate and maintain the ferry, and a capacity appropriate to the traffic that needs to cross. Ferry service also differs from a road crossing in how it handles vehicles, schedules, and access.
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FHWA’s Emergency Relief Manual identifies ferryboats, ferry operation and maintenance, and docking and loading facilities as possible eligible costs in its emergency-relief funding context. That is not a promise that a particular project will qualify; eligibility depends on program conditions and agency review.
How do cities choose among the options?
There is no universal ranking that makes a detour, temporary bridge, or ferry the right choice everywhere. A practical comparison considers:
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- Safe capacity: Can the option handle the volume and types of vehicles that need to cross?
- Access: Does it connect people, emergency services, and essential destinations?
- Extra travel: How much added distance and time will it impose?
- Route condition: Are alternate roads and bridges suitable for the added traffic, including their vulnerability to hazards such as scour?
- Timing: How soon can the option be put in place, and how long must it operate?
- Cost and impacts: What are the construction and operating costs, and how will the option affect neighborhoods, businesses, evacuation routes, navigation, or rail operations?
These are planning considerations drawn from FHWA guidance, not a formal FHWA scoring formula. Local engineers and agencies must weigh them against site conditions and the particular trips that need to continue.
Can construction choices shorten the disruption?
Accelerated Bridge Construction
Accelerated Bridge Construction (ABC) uses planning, design, materials, and construction methods intended to reduce onsite work time and traffic impacts. Prefabricated components can allow parts of a bridge to be assembled away from the site and installed during a shorter, planned closure. FHWA’s framework says planning should consider factors such as an evacuation route’s importance, traffic impacts and detours, navigation or railroad constraints, and whether closures can be concentrated in off-peak periods: FHWA ABC framework.
When a viable detour exists, closing the bridge for the full construction period may make work more efficient and safer than maintaining traffic through the work zone. If that is not practical, agencies may instead schedule shorter night or weekend closures. The choice depends on the route and project.
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Slide-in bridge construction
With slide-in bridge construction, a replacement is built on temporary supports, often beside the existing bridge. Once it is ready, crews move it into position and connect it to the approaches. FHWA also describes a variation in which the existing bridge is moved to a parallel alignment for temporary use as a detour while the replacement is built: FHWA slide-in bridge construction overview.
This method depends on engineering feasibility and site constraints; it is not suitable for every bridge or location.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How are closure decisions and temporary crossings kept safe?
A closure plan should establish the conditions that trigger restrictions or closure, who has authority to close and reopen the bridge, how decisions will be communicated, and what detour instructions the public will receive. FHWA’s scour-focused example identifies indicators such as observed scour, riprap movement, water level, discharge, rainfall, flood forecasts, and debris buildup. Which indicators apply depends on the bridge and hazard being managed.
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A temporary bridge carrying public traffic while a permanent bridge is closed is subject to inspection under the National Bridge Inspection Standards (NBIS). FHWA addresses this requirement in its NBIS questions and answers; a temporary structure cannot simply be opened to public traffic without applicable review and inspection.
Does federal emergency relief pay for a backup crossing?
FHWA’s Emergency Relief Manual lists examples that can include temporary detour bridges, detours, and temporary ferry service, subject to program conditions and FHWA review. Whether federal assistance applies to a specific city project depends on the event, facility, documentation, and agency determinations. The manual is guidance about possible eligibility, not a guarantee of reimbursement.
The federal materials discussed here describe U.S. guidance. They do not determine what a particular city should build, its route capacity, schedule, costs, local authority, ferry availability, or project funding eligibility; those questions require location- and project-specific engineering and agency information.
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