We have determined the dependence of the transition voltage (minimum in a ln(I/V2) vs. I/V plot) on the vacuum gap width in ultra-high vacuum scanning tunneling microscopy junctions. We have performed dual bias room temperature experiments with a W tip and Au(111) as well as polycrystalline Pt surfaces. For both type of surfaces the transition voltage decreases linearly with increasing inverse gap width. This is in marked contrast with the standard models for quantum mechanical tunneling, which predict a linear increase of the transition voltage with increasing inverse gap width. This remarkable discrepancy can only be partly explained by the incorporation of an image charge effect and therefore there is a clear need for a revision of the standard models for quantum mechanical tunneling in vacuum scanning tunneling microscopy junctions.