What are the advantages and disadvantages of fixed vs movable bridges for urban traffic?

1 Fixed bridges

Fixed bridges are bridges that do not change their position or shape to allow passage of vessels or other obstacles underneath. They are usually built high enough to accommodate the expected clearance of the waterway or the terrain. Fixed bridges are generally cheaper to construct and maintain than movable bridges, as they do not require complex mechanical or electrical systems to operate. They also offer more stability and durability, as they can withstand higher loads and stresses without compromising their structural integrity. However, fixed bridges also have some drawbacks, such as limiting the size and type of vessels that can use the waterway, creating potential bottlenecks or conflicts with navigation. Fixed bridges may also have a negative visual impact on the landscape, especially if they are too tall or bulky for the surrounding environment.

2 Movable bridges

Movable bridges are bridges that can change their position or shape to allow passage of vessels or other obstacles underneath. They are usually built lower than fixed bridges, and have some mechanism to lift, swing, rotate, or retract their span or segments. Movable bridges are generally more expensive to construct and maintain than fixed bridges, as they require complex mechanical or electrical systems to operate, as well as staff and control systems to coordinate their movements. They also pose more challenges for reliability and safety, as they are more prone to malfunctions, delays, accidents, or vandalism. However, movable bridges also have some benefits, such as allowing more flexibility and diversity of vessels to use the waterway, enhancing the connectivity and accessibility of the urban area. Movable bridges may also have a positive visual impact on the landscape, especially if they are designed with aesthetic or historical considerations.

3 Factors to consider

Deciding between fixed and movable bridges for urban traffic is not easy, as it depends on a variety of factors. These include the characteristics of the waterway or terrain, such as its width, depth, flow, and tidal range; the characteristics of the bridge, such as its length, width, height, load, and design; the characteristics of the traffic, such as its volume, speed, direction, and mode; and the characteristics of the stakeholders, such as their needs, preferences, expectations, and budgets. Each factor has its own implications for the cost, reliability, safety, and environmental impact of the bridge. As a result, it may be necessary to make trade-offs or compromises.

4 Examples of fixed and movable bridges

To illustrate the advantages and disadvantages of fixed and movable bridges for urban traffic, here are some examples from different cities around the world. The Golden Gate Bridge in San Francisco is a fixed suspension bridge that spans 2.7 km over the Golden Gate Strait. It is renowned for its iconic design and carries about 112,000 vehicles daily, making it a popular tourist destination. However, it also limits the size of ships that can enter the San Francisco Bay and has a high maintenance cost due to its exposure to salt water and wind. London's Tower Bridge is a movable bascule bridge that spans 244 m over the River Thames. It is a famous landmark in the city and carries about 40,000 vehicles and pedestrians daily. It also features a museum and a glass walkway, but causes frequent disruptions to traffic and river traffic as it opens approximately 800 times per year to allow passage of ships.

5 Tips for using fixed and movable bridges

When you're out and about, using fixed and movable bridges for urban traffic may require some preparation. Before you go, check the schedule and status of the bridge online, on mobile apps, or on radio stations. Follow the rules and signs of the bridge, such as speed limits and lane markings, and respect the authority and instructions of bridge operators or police. Be ready for delays or detours in case of unexpected events that affect bridge operations. And don't forget to appreciate the view and experience of crossing the bridge - it can offer a unique perspective on the urban landscape.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?