Originally Posted by
sleepy
Dumb question I can google but don't want to: when trying to maintain fitness level in a time crunch, which is more beneficial; riding 14 miles of level surface on a road bike, or Fixed?
If you pedal the road bike in the same gear and don't coast for the full interval, then there's no difference.
However, that's not a usual scenario, so fixed adds an extra margin of training. How much depends on how much you "cheat" with the road bike.
IME a fixed gear probably makes for the biggest improvement when riding in gently rolling terrain. It imposes the need to increase your effective band for both cadence and force. Folks who rode fixed for a while tend to be very good climbers of "bite sized" hills. Fixed gear riding in hills will also improve your top end cadence, and many of the people who can spin small gears at super high cadence can trace the skill to descending with a fixed gear.
So, while it may not apply for your hypothetical, fixed gear riding is a great way to improve a number of core skills as you become stronger and faster.