Vehicle Safety Communications (VSC) is advancing rapidly towards product development and field testing. While a number of possible solutions have been proposed, the question remains open as how such a system will address the issue of scalability in its actual deployment. This paper presents a design methodology for congestion control in VSC as well as the description and evaluation of a resulting rate adaption oriented protocol named PULSAR. We start with a list of design principles reflecting the state of the art that define why and how vehicles should behave while responding to channel congestion in order to ensure fairness and support the needs of safety applications. From these principles, we derive protocol building blocks required to fulfill the defined objectives. Then, the actual protocol is described and assessed in detail, including a discussion on the intricate features of channel load assessment, rate adaptation and information sharing. A comparison with other state-of-the-art protocols shows that "details matter" with respect to the temporal and spatial dimensions of the protocol outcome.