  VI-curves of resistively shunted single Josephson junctions with different
capacitances and tunneling resistances are found to display a crossover between
two types of VI-curves: one without and another with a resistance bump
(negative second derivative) at zero-bias. The crossover corresponds to the
dissipative phase transition (superconductor-insulator transition) at which
macroscopic quantum tunneling delocalizes the Josephson phase and destroys
superconductivity. Our measured phase diagram does not agree with the diagram
predicted by the original theory, but does coincide with a theory that takes
into account the accuracy of voltage measurements and thermal fluctuations.
