Influence of Project Planning Practices on Cost Overruns in The Construction of Trunk Roads in Kenya

In ideal situation, road infrastructure projects should be completed within the planned budget, schedule, and scope to ensure efficient utilization of public resources and timely delivery of transport services that support economic growth and social development. Currently, cost overruns are a persistent challenge in Kenya’s road construction sector, particularly in national trunk road projects, where final expenditures frequently exceed planned budgets. These overruns undermine timely project delivery, strain public finances, and reduce value for money, hindering the achievement of Vision 2030. This study examined the influence of project planning practices that is scope definition, design planning, and funding allocation on cost overruns in national trunk road construction. The study was guided by the Systems Theory, Design Structure Theory, and Contingency Theory, the study adopted a retrospective design using secondary data from 46 KENHA projects implemented between 2005 and 2023. Data were extracted from KENHA and Kenya Roads Board. Document reviews were done on parliamentary reports, audit reports, and financiers including the World Bank and African Development Bank. Descriptive and inferential statistics were applied, with multiple linear regression determining relationships between planning practices and cost overruns. Results showed that scope definition, design planning changes, and funding allocation were significant drivers of cost overrun. The study concludes that strengthening feasibility studies, integrating modern design technologies, enhancing stakeholder engagement, and ensuring predictable funding flows are critical to minimizing budget deviations. The findings would offer actionable insights for policymakers, project managers, development partners, and scholars to improve planning frameworks, emphasizing efficiency, accountability, and sustainable infrastructure development. This underscores the need for well-structured project planning practices that minimize cost overruns and enhance the long-term performance of Kenya’s road infrastructure sector.