Bill, taking into consider the more mass a car must haul down from speed compared to a motorcycle, but also taking into this equation the larger and more (four vs two) contact patches a car has, in most cases and(here comes the operative wording) on a DRY and reasonably non-slippery road surface(i.e. no oil or sand or leaves, etc), the stopping distances will be somewhat the same.
Take a look at this video and which I think demonstrates this principle...
http://vimeo.com/44130890 (...but once again, take note of how conducive the road surface is to this "test")
