Nowadays, several approaches for developing web applications have been proposed in the literature. Most of them extend existing object-oriented conceptual modeling methods, incorporating new constructors in order to model the navigational structure and the content of web applications. Such new constructors are commonly represented in a Navigational Model. While navigational models constitute the backbone of web application design, their quality has a great impact on the quality of the final product which is actually implemented and delivered. In this paper, we discuss a set of metrics for navigational models that has been proposed for analyzing the quality of web applications in terms of size and structural complexity. These metrics were defined and validated using a formal framework (DISTANCE) for software measure construction that satisfies the measurement needs of empirical software engineering research. Some experimental studies have shown that complexity affects the ability to understand and maintain conceptual models. In order to prove this, we also made a controlled experiment to observe how the proposed metrics can be used as early maintainability indicators.