  The distances to individual wind-driven bubbles such as Planetary Nebulae
(PNe) can be determined using expansion parallaxes: the angular expansion
velocity in the sky is compared to the radial velocity of gas measured
spectroscopically. Since the one is a pattern velocity, and the other a matter
velocity, these are not necessarily the same. Using the jump conditions for
both shocks and ionization fronts, I show that for typical PNe the pattern
velocity is 20 to 30% larger than the material velocity, and the derived
distances are therefore typically 20 to 30% too low. I present some corrected
distances and suggest approaches to be used when deriving distances using
expansion parallaxes
