The energy band description of resistively shunted single Josephson junctions is studied experimentally. At zero bias and low temperature, the junctions are found to undergo a dissipative phase transition, a crossover between two different types of IV-curves, at which macroscopic quantum tunneling delocalizes the Josephson phase and destroys superconductivity. The transition is washed away by increasing temperature, bias current and magnetic field. At low currents and temperatures, the current dependence of the junction resistance is found to scale with the temperature dependence, pointing out to the existence of a zero temperature quantum transition. At higher currents, Zener tunneling between energy bands is observed and compared with existing theories.