Use case modeling (UCM) is commonly applied to document requirements. Use case specifications (UCSs) are usually structured, unrestricted textual documents complying with a certain template. However, because they remain essentially textual, ambiguities are inevitable. In this paper, we propose a new UCM approach, which is composed of a set of well-defined restriction rules and a new template. The goal is to reduce ambiguity and facilitate automated analysis, though the later point is not addressed in this paper. We also report on a controlled experiment which evaluates our approach in terms of its ease of application and the quality of the analysis models derived by trained individuals. Results show that the restriction rules are overall easy to apply and that our approach results in significant improvements over UCM using a standard template and no restrictions in UCSs, in terms of the correctness of derived class diagrams and the understandability of UCSs.