The paper attempts to explain sources of surplus storm water runoff in urban areas, particularly in relation with functioning of inlets. Inlet capacity (quantity of captured water) and inlet efficiency (portion of the approaching flow rate) have been defined and their relationship with relevant parameters (approaching flow, longitudinal and lateral street slopes) was established through laboratory measurements. Effects of clogging of inlets on inlet capacity were also investigated in laboratory conditions. As a consequence of decreased inlet efficiency, there is a portion of approaching flow that is not captured by the inlet (pass-over flow). If the pass-over flow is considered along a street having numerous inlets, it is easy to estimate the quantity of flow that would accumulate on the pavement. Inlet inefficiency can be significant when overestimation of inlet capacity results in increased distance between consecutive inlets, and when clogging of grates or inadequate placing of inlets causes significant decrease in inlet capacity.