The deployment of autonomous vehicles is contingent on trust in their ability to operate safely. However, the assurance that they can accommodate failures and changing weather conditions to maintain limited functionality requires the development of rigorous design and analysis tools. This paper presents a formal approach for the design of multisensor data fusion systems that support adaptive graceful degradation through the smart use of sensor modalities. A coloured probabilistic time Petri net is used to model known algorithms in a multi-sensor fusion scheme. The specification of safety requirements in terms of confidence levels conditions the outcome of the reachability analysis. The characteristics of a credible solution are then provided to the embedded safety module as support for online reconfiguration and decision making tasks. The validity of the approach is illustrated through an example outlining the capabilities of currently available perception systems, for the purpose of deploying autonomous vehicles on public roads.