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Grade separation is a process used to improve traffic flow at intersections and junctions. With grade separation design, each road or rail surface is placed at a different grade, or elevation. This difference in elevation is accomplished using tunnels, ramps, bridges and interchanges at every point where the two roads or rails cross one another. For example, an elevated train running above a roadway is considered an example of rail grade separation. An urban light rail system, where the train cars move with traffic at street level, is not grade separated.
There are many different types of grade separation projects, which are categorized by the type of surfaces each is separating, as well as how the difference in elevation is accomplished. These projects may involve railroads, freight trains, subways or metros, monorails, or even pedestrian walkways. Even the process of separating two separate automotive roadways is considered a type of grade separation.
In a fully-separated or free-flowing grade separation design, traffic in all directions can continue to flow without stopping or slowing where the roads, rails or paths cross. An example of this would be a pedestrian bridge or a "cloverleaf" intersection, where raised ramps allow vehicles to travel from one highway to another without stopping. Grades that are partially separated include intersections where motorists or pedestrians might have to slow or yield, such as a roundabout. A traditional intersection that uses traffic signals involves no grade separation.
This type of project design offers a number of advantages over roads and rails that are all built at the same elevation. All types of traffic are able to flow more freely, with little to no interruptions, and speed limits are usually higher. The greatest benefit is the separation of different types of traffic, including cars, trains and pedestrians, which lowers the risk of accidents for all parties.
At the same time, many residents who live near grade-separated intersections often oppose them. They are generally built fairly high, which can obstruct views. The appearance of grade-separated roadways and bridges in particular is generally unpleasant, and consists of massive concrete or steel structures that extend high into the air. It is much more costly to build tunnels or bridges than to build at ground level, and these projects take up a great deal of space, both during and after construction is complete. Finally, the complexity of these projects often means that they take a long time to complete, which can disrupt traffic flow for years.
I'm a big fan of grade separation, especially for subways. I used to live in a city with an above ground light rail, and it made me so nervous. You would just be driving along, and a fast moving train would be right next to you. I think grade separation is the way to go.
I especially think it's a good idea if it allows pedestrians to walk on a pedestrian walkway instead of into traffic. So many people don't pay attention to the traffic lights and accidents happen. If pedestrians aren't trying to cross a road cars drive on, this problem is alleviated!