Compared with Northern Europe, bicycle use in the US is rare and appears to be aimed more at recreation than transportation. Recreation is fine and bike riding is great for fitness, but it is still a rare activity. This is unfortunate a for short trips it is a much more efficient mode of transit than a car with a potential for great savings. But getting to the point where Holland or Denmark are is a very non-trivial task and requires serious attention to infrastructure. Safety and the perception of safety are critical elements and tend to be more important for women than for men. In areas where bike riding is perceived to be safe the percentage of female riders increases and in some countries more women ride than men - women are an indicator species for bicycling as transportation.
Here is a video on biking in Holland - how they got where they are today.
Cycling For Everyone from Dutch Cycling Embassy on Vimeo.
Another piece of the puzzle is the right bicycle. Practical commuter bikes are beginning to become available in the US, but few are designed with women in mind. You hear about "wsd" - women's specific designs - but they are frequently little more than downsized men's frames in different color schemes - "shrink and pink"...
There are exeptions. Given enough money there is the bespoke route and a few custom builders specialize in women's designs, but $4,000+ bicycles are hardly practical for the average commuter. Terry Bicycles is a line focused on women. Run by Georgena Terry, they tend to concentrate on the enthusiast. The bikes are very well reguarded and, like enthusiast bikes, tend to be spendy. This is changing a bit. The blog bikesfortherestofus noted they will be releasing the Terry Burlington by the end of the year. A geometry appropriate for most women, fenders, a rear rack, kickstand, a seat that actually fits women, and even a resonable price at about $750. There will be a range of frame sizes including one that is 41cm - probably suitable for someone who is as small as 4'11".
It would be nice to see something that is lower maintenance - an internally geared hub and perhaps drum brakes and even a carbon belt, but they would add cost and this is a very positive sign. Just the thing for those short trips where a car is a very inefficient choice.
The Trek Cocoa looks like a women's European design and may be a good candidate at a good price. If the bicycle as transportation becomes more important this will be one of the main growth areas.