The re-use of bottom ash in road construction necessitates a tool to predict the impact of trace metals on the surroundings over the lifetime of the road. The aim of this work was to quantify the effect of environmental factors that are supposed to influence leaching, so as to suggest guidelines in developing a leaching procedure for the testing of incineration residues re-used in road constructions. The effects of pH, L / S (liquid-to-solid ratio), leaching time, and leaching atmosphere on the leachate concentrations of Cd, Cr, Cu, Ni, Pb, and Zn were studied using a two-level full factorial design. The most significant factor for all responses was the pH, followed by L / S, though the importance of pH and L / S is often ignored in leaching tests. Multiple linear regression models describing the variation in leaching data had R2 values ranging from 61-97%. A two-step pH-stat leaching procedure that considers pH as well as L / S and leaching time was suggested.