While that may be a true statement, it isn't true that the North was a "safe place" to which slaves could flee. Because of the "Dred Scott" decision and the fact that it was federal law to mandate state and local government assistance to accredited "slave hunters" who were hunting escaped slaves in the North, there was no "safe place" in the North. Anyone assisting in the escape or hiding of escaped slaves was guilty of a federal offense, which was not likely to change without a sea-change in the attitudes of Southern politicians in Washington and elsewhere.
As deplorable as that was, was it not US Constitutional law that caused this?