According to the Air force and the forensic science digest: 40% - 60%
As for what motivates false accusers:
There is the attention issue. Feminists have insisted that "rape accusers are to be taken seriously", for many women this is an end in itself. The idea they will be "taken seriously" is too tempting for them.
The most common reasons the women gave for falsely accusing rape were "spite or revenge," and to compensate for feelings of guilt or shame (Forensic Science Digest, vol. 11. no. 4, December 1985).
Here are sources that put the number at around 40% - 60% of rape accusations being false.
According to a nine-year study conducted by former Purdue sociologist Eugene J. Kanin, in over 40 percent of the cases reviewed, the complainants eventually admitted that no rape had occurred (Archives of Sexual Behavior, Vol. 23, No. 1, 1994). Kanin also studied rape allegations in two large Midwestern universities and found that 50 percent of the allegations were recanted by the accuser.
In 1985 the Air Force conducted a study of 556 rape accusations. Over one quarter of the accusers admitted, either just before they took a lie detector test of after they had failed it, that no rape occurred. A further investigation by independent reviewers found that 60 percent of the original rape allegations were false.
The most common reasons the women gave for falsely accusing rape were "spite or revenge," and to compensate for feelings of guilt or shame (Forensic Science Digest, vol. 11. no. 4, December 1985).
Dr. Charles P. McDowell, Supervisory Special Agent, U.S. Air Force, Office of Special Investigations found that:
"To my considerable chagrin, we found that at least 60 percent of all the rape allegations were false."
Dierdre O is blatantly lying to justify her ideology. It is point blank not true that physical evidence of assault is required for a rape case to go forward. For example, look at the Duke case. Was there evidence of an assault? There was evidence of "intercourse", yes.. but no physical evidence of attack was present. On the contrary feminists are currently pushing for "evidence of intercourse" to be considered de facto evidence of rape. Not that that compounds the problem of false accusations severely or anything.
You might also want to look at the site:
http://www.falserape.net/
For more information on false rape allegations.