The massive influx of women cyclists-making up at least a third of the total market- was perhaps the most striking and profound social consequence of the mid-1890s cycling boom. Although the new, improved bicycle had appealed immediately to a few privileged women, its impact would have been modest had it not attracted a greater cross section of the female population. It soon became apparent that many of these pioneer women bicyclists had not taken up the sport as an idle pastime. Rather, they saw cycling as a noble cause to be promoted among all women as a means to improve the general female condition. Not only would cycling encourage healthy outdoor exercise, they reasoned, it would also hasten long-overdue dress reform. To feminists, the bicycle affirmed nothing less than the dignity and equality of women.
There is a sentence
... its impact would have been modest had it not ...
I can't understand what grammar of "would have been adj. had" is?