Speed limit

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")

By 2371, certain Intrepid-class vessels operated with newer warp drive designs, and were not subject to the Federation speed limit. (Star Trek: Voyager)

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 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 are physically limited by local subspace effects include the Hekaras Corridor and the Lantaru sector. (VOY: "The Omega Directive")

Background

In DS9: "By Inferno's Light", Dax was incredulous that a ship would go to warp in a star system, suggesting a physical limitation of, or hazard from, warp drive, but given that runabouts were traveling at warp in the Bajor system in DS9: "Past Prologue", it is possible that her reaction stems from a speed limit imposed by the Bajoran Provisional Government sometime after 2369.

The countless times Federation starships have warped directly out of planetary orbit in routine circumstances even after the Hekaran incident would indicate there is no physical hazard to inhabited space. Additionally, the slingshot effect specifically requires warp flight very close to a star or other high-gravity body.

Voyager's lack of speed limitations is often attributed to its variable geometry pylons.