---
title: "Interstate 90 Floating Bridges | Washington State History"
description: "Interstate 90 Floating Bridges are pontoons-supported highway bridges across Lake Washington that show how Washington solved a major transportation challenge."
canonical: "https://fiveable.me/hs-washington-state-history/key-terms/interstate-90-floating-bridges"
type: "key-term"
subject: "Washington State History"
unit: "Unit 7"
---

# Interstate 90 Floating Bridges | Washington State History

## Definition

Interstate 90 Floating Bridges are the pontoon-supported bridges that carry Interstate 90 across Lake Washington between Seattle and Bellevue. In Washington State History, they show how bridge engineering shaped the highway system.

## What It Is

Interstate 90 Floating Bridges are the two floating highway bridges that carry Interstate 90 across Lake Washington in Washington State, mainly the Lacey V. Murrow Memorial Bridge and the Homer M. Hadley Memorial Bridge. They connect Seattle and Bellevue over a body of water that would have been much harder to cross with a standard fixed bridge.

In Washington State History, these bridges are a concrete example of how the state adapted transportation to geography. Lake Washington is deep and wide, so engineers used large hollow concrete pontoons that float on the water and support the roadway above. Instead of building a tall bridge with many high piers, the road surface sits on a platform that rises and falls slightly with the lake.

That design matters because it shows a different answer to the same problem highway planners faced all over the state: how do you move cars efficiently when terrain, water, or distance gets in the way? The floating bridges were first introduced in Washington in the 1940s, and they quickly became one of the most recognizable pieces of the state’s road network. They were not just a local curiosity, they were part of a bigger push to modernize transportation in a growing metropolitan area.

The bridges are also a good reminder that highways are not only about pavement. They depend on engineering, funding, maintenance, and long-term planning. Because the pontoons are exposed to weather, waves, and heavy traffic, they need regular inspection and repair. That ongoing upkeep is part of the story too, because a major highway structure is only useful if the state can keep it safe and operating.

Students often remember these bridges because they are unusual, but their historical value is bigger than the novelty. They represent Washington’s willingness to use innovative infrastructure when geography made standard road building difficult. In a unit on the highway system, they fit right alongside road expansion, automobile growth, and the broader effort to connect population centers across the state.

## Why It Matters

This term matters because it shows how Washington’s highway system grew through engineering choices, not just new road miles. The floating bridges are a real-world example of the state solving a transportation problem with local conditions in mind, since Lake Washington separated major urban areas that needed faster connection.

In a Washington State History class, this term helps you explain how highways changed daily life, commuting, and regional development. The bridges made Seattle and Bellevue more connected, which supported suburban growth, business travel, and the movement of goods and people across the Puget Sound region.

It also gives you a way to talk about the relationship between infrastructure and public investment. Building and maintaining a floating bridge takes planning, money, and long-term state responsibility. That connects the bridges to broader themes like modernization, urban growth, and the way transportation projects shape where people live and work.

If a question asks you how Washington adapted its road system to geography, this is one of the clearest examples you can use.

## Connections

### Pontoons

The floating bridges literally depend on pontoons, which are the hollow flotation structures that keep the roadway above the water. In a Washington history context, this is the engineering piece that makes the whole design possible. If you see a question about how the bridge stays afloat or why it can span Lake Washington, pontoons are the key feature to name.

### Bridge Engineering

Interstate 90 Floating Bridges are a strong example of bridge engineering because they solve a geographic problem with a specialized design. Instead of copying a standard land bridge, engineers matched the structure to the lake environment. That makes this term useful when you are describing how infrastructure responds to local terrain and water conditions.

### Highway System

These bridges are part of Washington’s larger highway system, which expanded as the state needed better connections between cities and regions. They are not just isolated structures, they are links in a transportation network. When you write about the highway system, the floating bridges can serve as a specific example of how that network crossed natural barriers.

### [construction of the Seattle freeway system](/hs-washington-state-history/key-terms/construction-of-the-seattle-freeway-system)

The floating bridges fit into the broader buildout of highways and freeways around Seattle. They helped carry growing traffic between Seattle and the Eastside, which made the freeway network more functional. If your class is discussing urban transportation growth, these bridges show how regional road construction changed commuting patterns.

## On the AP Exam

On a quiz or short answer question, you might be asked to identify why the Interstate 90 Floating Bridges were unusual or how they fit into Washington’s highway system. A strong answer should name the pontoon design, place them across Lake Washington, and explain that they connected Seattle and Bellevue. If you get a map, photo, or infrastructure prompt, use them as evidence of how Washington adapted transportation to geography.

In an essay or discussion, you can use the bridges to support a bigger argument about modernization, urban growth, or state investment in transportation. They work especially well in a response about how highways changed the Puget Sound region. If the prompt asks about engineering solutions, mention that the bridges float because of large concrete pontoons and that this made a major water crossing possible without a traditional fixed span.

## Interstate 90 Floating Bridges vs Columbia River Bridges

Both are major Washington crossings, but they solve different geography problems. Interstate 90 Floating Bridges cross Lake Washington with pontoons, while Columbia River Bridges cross a much larger river system and usually use more traditional bridge designs. If you mix them up, focus on the body of water and the structure type.

## Key Takeaways

- Interstate 90 Floating Bridges are pontoons-supported highway bridges that cross Lake Washington between Seattle and Bellevue.
- They show how Washington used bridge engineering to solve a transportation problem shaped by local geography.
- The bridges are part of the state highway system, not just a stand-alone landmark, because they connect major population centers and support daily travel.
- Their design and upkeep show that transportation history is also a story about planning, maintenance, and public investment.
- When you see them in class, think of modernization, regional connection, and the way infrastructure changes how people move around Washington.

## FAQs

### What is Interstate 90 Floating Bridges in Washington State History?

They are the floating bridges that carry Interstate 90 across Lake Washington, mainly the Lacey V. Murrow Memorial Bridge and the Homer M. Hadley Memorial Bridge. In Washington State History, they are used as an example of how the state solved a major transportation challenge with bridge engineering. They also show how highway development supported growth in the Seattle area.

### Why are the Interstate 90 Floating Bridges important?

They are important because they connect Seattle and Bellevue across a wide lake that would have been difficult to cross with a standard bridge. They also show how Washington’s highway system changed to meet the needs of a growing urban region. Their design makes them one of the most distinctive infrastructure projects in the state.

### How do floating bridges work in Washington?

Floating bridges rest on large hollow pontoons that float on the water and support the roadway. That lets the bridge span deep water without relying on many tall supports. The structure still needs regular inspection because weather, waves, and traffic affect the bridge over time.

### Are the Interstate 90 Floating Bridges the same as Columbia River Bridges?

No. They are both major Washington bridges, but they cross different bodies of water and use different designs. The Interstate 90 Floating Bridges cross Lake Washington with pontoons, while Columbia River Bridges cross a river corridor and usually use more conventional bridge structures. The difference matters when you are identifying infrastructure on a map or in a history question.

## Related Study Guides

- [7.3 Highway system](/hs-washington-state-history/unit-7/highway-system/study-guide/BpAECXMeWY2GB2cH)

## About This Document

Canonical Fiveable pages are available as Markdown at the same path plus `.md`.

- [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns
- [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing
- [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`)
- [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`)

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