The results of potential mapping surveys of bridge decks of the Swiss highways, using a new eight-wheel electrode measurement system, are reported here. A comparison of the state of corrosion of the rebars (obtained by visual inspection after removing the concrete cover), including the potential fields in addition to theoretical considerations, clearly demonstrates that an absolute potential value (that is, -350 mV copper sulfate electrode (CSE) as proposed in ASTM Test for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete (C 876-80) for the identification of active corrosion of steel in concrete does not exist. Combining the local potential gradient on the surface with information on the electrical resistivity of the concrete (measured with the four-point method or a-c impedance) allows determination of a rough estimate of the corrosion rate in a single macrocell. Impedance spectroscopy, well suited for studying corrosion mechanisms in the laboratory, was found to be too time consuming and difficult to interpret in field measurements. Instead, a very rapid new technique—using galvanostatic pulse measurements—was tested successfully on site. It gives clear, unambiguous results on the corrosion state of the rebars, when half-cell potential measurements are uncertain.