Tests carried out by the Transport and Road Research Laboratory (TRRL) showed that the corrosion of steel under sheltered conditions could differ considerably from corrosion under open exposure (McKenzie 1978). In non-marine environments, sheltered corrosion rates on bridges were initially lower than open exposure rates. However, open exposure corrosion rates tended to reduce considerably with time but, in some cases, sheltered rates did not. In marine environments sheltered corrosion rates were higher than in open exposure and pitting developed. This suggested caution about the use of weathering steel for motorway overbridges where salt spray from the use of deicing salt might lead to similar problems.