Swiss highway project cost estimate performance: Deviations from norms and expected trends

• Swiss highway construction project median cost overrun is 4.7%. • Projects overrun costs during planning phases, and underrun during construction. • Each additional year of planning increases cost overrun by 4%. • Cost estimating norms do not reflect observed cost deviations. Cost overruns in construction projects are a common issue internationally, and project cost performance in individual countries varies considerably. Countries that lack systematic project data may rely on foreign data for informing cost estimating practices and norms, which may lead to errors. This study considered the case of Switzerland, a country with limited research on construction project cost performance. Swiss highway construction projects were analysed to assess the accuracy of cost estimating methods and norms. Different factors influencing cost overruns were considered: project size, type, planning duration, start year, and historical policy changes. The results revealed that the general patterns of cost overruns observed internationally do not apply to Switzerland: projects tend to overrun estimated costs during the planning phase but underrun during the construction phase, leading to accurate overall cost estimates (4.7% median overrun). Projects exhibited large and asymmetrical cost accuracy ranges throughout the planning and construction phases. These results deviate from expectations set in norms and comparable international project data. Based on these findings, the study challenges the effectiveness of current Swiss cost estimating norms, suggesting a need for more country-specific data analysis and policy development.