I don't get the point of building a stopway. If you have the space to build a stopway, you could just extend the length of the runway.
Why are stopways necessary?
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Overrun areas are almost always filled with some form of loose material... sand, for example... that will slow a plane down with extra drag on the landing gear. The idea is that if a plane has reached the end of the runway, it's already in trouble and is probably having problems braking.
Regardless of how long a runway is built, a stopway is needed for other purposes. Stopways (also known as blast pads or overrun areas) is often not as strong as runways, so that airplane's tires have more friction and can slow down faster than runways. Also, the length of a runway does not include the length of stopway.
Why runways are not built longer? It is because real estate is real estate. It is expensive and is always on short supply. Also, if you find area around a runway which is not used, there could be rules for that region for open space.