The Prologue is a dedicated Time Trial helmet that is CPSC certified and is appropriate for use in triathlons.  As you can easily see, it is far more aerodynamic than a normal road or mountain bike helmet, with its sleek and smooth exterior culminating into a tail-like apparatus and its flush windshield.  Once you have a proper fitting bike, aerodynamics (besides strength and fitness) is the most important criteria for a top bike split. 

The Prologue helmet is very comfortable and easy to adjust via the Spiderlock retention system and Keylock strap locks; it fits as nice on your head as any top-notch helmet.  I also like the way the included clear windshield flips up and down and that it's removable.  With the windshield down, it effectively keeps the wind out of your eyes so that you don’t need sunglasses.  But if you prefer sunglasses for a particular race, simply remove the windshield.  If you get a little hot during a race, just flip it up for a few minutes.  If you prefer, you can get both a smoke and/or yellow windshield, which are sold separately for $24.95 each.

The Prologue is also reasonably light at 350 grams and includes 3 vents for airflow.  Although the Prologue doesn’t provide as much airflow as normal road helmets, it’s not too terribly hot.  The bottom line, of course, is that numerous and large air vents do not equal aero.

There is no doubt the Prologue and its big brother, the Rocket TT, are more aerodynamic than standard helmets.  Importantly, though, I have no idea how much time the helmet alone will shave off of your bike split.   The longer and faster the bike leg, the more likely it will make a difference.  One thing I am sure about, though, is that you will need plenty of self-confidence to wear any manufacturers' aero helmet as they aren’t particularly flattering.

KDG