This paper presents a formal specification and analysis method motivated by issues faced during early stages of requirements development for automotive features. At this early stage of development, only overall goals of features are understood, and there is a need to discover all possible scenarios of operation. We have developed a formalism — Structured Transition Systems (STS) — that facilitates the rapid evolution of specifications. STS supports multiple idioms of specification : transitions, state-diagrams, scenarios etc. It also supports constructs for hierarchical organization of a specification. We have further defined analyses that are useful for review and inspection of STS specifications. A distinctive feature of our method is the ability to use analysis results to refine and reinforce parts of the specification by importing analysis results into STS specifications. In practice, this leads to a feedback loop where requirements can be rapidly refined using analysis engines to drive the development of requirements. We have experimented using our technique on a number of automotive case-studies, and we present some of our experiences with these case-studies.