I've heard it said that a science-fiction author is allowed one major change in the laws of physics. I'm not certain how true that is for authors, but it's a good rule of thumb for game designers. Each tunnel is relatively short, but the drive cycles at a rate of millions of warps per second and thus gives the illusion of considerable speed. The apparent speed of a starship is affected by the sheer power of its drive, the warp frequency of the drive, the mass of the ship, and the presence of a gravity well. The sheer power of the drive, when compared to the mass of the ship, determines the average length of the warp tunnel.
Gravity severely truncates the length of the warp tunnel.
Warp frequency indicates how many times the ship will tunnel per second. All of these combine to produce an apparent speed. The activity of the drive itself at high cyclic rates produces a gyroscopic effect that is referred to as pseudo-momentum. It is not true momentum in the Newtonian sense, but limits the magnitude of immediate changes in direction and velocity. Aside from the star drives themselves, the science of starships is a relatively conservative linear projection of current technology.