Certain authorities impose speed limits in certain locations, limiting the warp factor at which starships can transit the area.
The Hekaran scientist Serova opened a subspace rift in the Hekaran sector in 2370, demonstrating that conventional warp engines caused damage to the fabric of spacetime. The Federation Council imposed a speed restriction of warp factor 5 on all Federation vessels in all but extreme emergencies. (TNG: "Force of Nature")
In 2378, The Doctor, disguised as Kathryn Janeway, informed the crew that hostile aliens had imposed a complete restriction on the type of warp drive used on USS Voyager and ordered the warp core ejected, in an attempt to exchange it for Janeway's freedom. (VOY: "Renaissance Man")
Known regions of space where warp speeds were physically limited by local subspace effects include the Hekaras Corridor and the Lantaru sector. (VOY: "The Omega Directive")