This article is from the Star Trek Tech FAQ, by Joshua Bell inexorabletash@hotmail.com with numerous contributions by others.
In "Force of Nature" [TNG] it is discovered that within the Hekaras
Corridor, a region of space where warp travel is hindered except for a
narrow path, the intense use of warp drives in an already sensitive
area can, over time, cause subspace rifts to form, where subspace
manifests itself in real space on a macroscopic scale. To alleviate
the problem the Federation imposed a speed limit of Warp 5 on all
starships which remained in place until at least the following year.
In "The Pegasus" [TNG] Admiral Pressman gives Picard permission to
travel faster than Warp 5 for the duration of the mission. In "Eye of
the Beholder" [TNG] Picard is given permission to exceed the speed
limit to delivery needed medical supplies.
There is a general assumption is that engine modifications later
solved the problem; there has been no further mention of the speed
limit. This is fueled by rumors that Voyager's back story includes an
updated warp core that is "environmentally friendly."
 
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