In answer to your question AOG, no woman can be 100% safe when using cabs, but there are sensible provisions she can make, the biggest of this is that the cab should be ordered from a taxi company.
No-one should ever simply jump into a car touting for business. Practically everyone has a mobile phone nowadays, and they should keep the telephone number of their preferred cab office in their contacts.
Another safeguard is that when waiting for the cab, if a car pulls up, the person waiting should always ask for the name of the passenger on the booking. That way they will know they are getting into the cab that they ordered.