Wet roads lower the tire traction and, hence, increase the risk of accidents. The exact friction coefficient between tire and road depends on a variety of influences, one of them being the road wetness or water height. To support drivers and autonomous systems in safe guidance of the vehicle, information about road wetness is helpful. In this paper, we discuss different system layouts of how to estimate the current road wetness, focusing on acceleration sensors in the front wheel arches or on the side skirts that measure tire spray. In addition, we analyze a new method that uses sensors at the underbody of the vehicle. We provide information on tire spray behavior, sensor types, and positioning. Our results show that underbody sensors are suited to detect especially high levels of road wetness that are associated with the risk of hydroplaning. In the conclusions, we combine the knowledge of new and known findings to provide a recommendation for the optimal system layout for road wetness quantification via tire spray.