Speed limit

Speed limits are imposed by certain authorities in the Milky Way Galaxy, limiting the warp factor at which starships are allowed to travel in their territories. The intention of the speed limit is to minimize the cumulative destructive effect of subspace emissions to the interstellar environment.

History
In the late-2360s, Hekaran scientists presented preliminary research of the danger to the Federation Science Council for evaluation. The evidence didn't hold up at that point. 

In 2369, unusual energy readings of elevated thoron emissions in the plasma field were detected in the Denorios belt. They were speculated to be the environmental byproduct of the increased traffic and presence of many warp and impulse engines in the area. The effect appeared to be the formation of a subspace rupture. It was however discovered that the rupture was a manifestation of what Jadzia Dax imagined it to be, caused by aliens from the Gamma Quadrant. The elevated emissions turned out to be harmless. 

In 2370, the Hekaran scientist Serova opened a subspace rift by overloading her ships engines to a warp core breach in the Hekaran sector, demonstrating that conventional warp engines did in fact cause severe damage to the fabric of spacetime. In order to slow the rate of the damage, the Federation Council shared the findings with all known warp-capable species and imposed a speed restriction of warp factor 5 on all Federation vessels in all but extreme emergencies. The areas of space most damaged by that point were restricted to essential travel only. The Klingons were expected to follow the restrictions, but Deanna Troi and Worf thought it unlikely that the Romulans, the Ferengi and the Cardassians would. 

Later that year, the USS Enterprise (NCC-1701-D) was authorized by Admiral Margaret Blackwell to exceed warp speed limitations during the assignment to salvage the USS Pegasus. . They were also given permission to exceed warp speed limitations when delivering urgently needed medical supplies to Barson II. .

In 2378, The Doctor, disguised as Kathryn Janeway, informed the crew that hostile ecological extremist aliens had completely outlawed conventional warp drive inside their territory. They supposedly also believed all conventional warp drives cause damage to subspace, and had ordered the warp core of the USS Voyager to be ejected. The Doctor also referred to a fictional type of warp drive, supposedly decades ahead of Federation technology, called transphasic warp drive that didn't cause the damage. 

Known regions of space where warp speeds were physically limited by local subspace effects include the Hekaras Corridor and the Lantaru sector. 

Background information

 * In Captains' Logs: The Unauthorized Complete Trek Voyages, page 294, Brannon Braga noted that in his opinion "when you limit warp drive, the rug is being pulled out from under Star Trek." And, according to Star Trek Encyclopedia 2nd ed., page 187, since starships used speeds above warp 5 after 2370, a solution was presumably found to the problem, even though it was not mentioned on-screen.


 * According to the unpublished VOY Season 1 edition of the Star Trek: Voyager Technical Guide, by Rick Sternbach and Michael Okuda, it is suggested that because of the variable geometry pylons used on Voyager, the generated warp fields might no longer have a negative impact on habitable worlds as established in "Force of Nature". However considering the plot of "Renaissance Man", Voyager at least was definitely not fitted with an environmental friendly warp drive.