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An intermittency model that is formulated in local variables is proposed for representing bypass transition in Reynolds-Averaged Navier-Stokes (RANS) computations. No external data correlation is used to fix transition. Transition is initiated by diffusion, and a source term carries it to completion. A sink term is created to predict the laminar region before transition and vanishes in the turbulent region. Both the source and sink are functions of a wall-distance Reynolds number and turbulence scales. A modification is introduced to predict transition in separated boundary layers. The transition model is incorporated with the k − ω RANS model. The model is validated with several test cases. Decent agreement with the available data is observed in a range of flows. An extended model for roughness-induced transition is proposed based on this intermittency model. To predict roughness effects in the fully turbulent boundary layer, published boundary conditions for k and ω are used. They depend on the equivalent sand grain roughness height, and account for the effective displacement of wall distance origin. Similarly in our approach, wall distance in the transition model for smooth surfaces is modified by an effective origin, which depends on equivalent sand grain roughness. Flat plate test cases are computed to show that the proposed model is able to predict transition onset in agreement with a data correlation of transition location versus roughness height, Reynolds number, and inlet turbulence intensity. Experimental data for turbine cascades are compared to the predicted results to validate the proposed model.