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Estimates of the construction time and cost are of key importance in the early phases of the project – they serve as a basis for the decision whether to commence with planning or not, and are used as input for budgets and programmes. The project’s success depends on reliability of these estimates. It is thus crucial to answer the question: what do the project time and cost depend on? or easier to answer: correlated with? The answer can be based only on experience – personal as well as recorded in databases or mathematical models. Tools facilitating construction project planning on the basis of past experience are the object of research for many years. The paper discusses the time-cost relationship proposed by Bromilow in nineteen-sixties and adopted in many later multifactor models of construction duration. On the basis of a sample of 100 Polish public road projects, the statistical validity of the Bromilow’s model was confirmed. The model was compared with multifactor models based on statistical regression and regression trees. Applicability of the models was discussed with respect to their errors and confidence intervals.