Splash and spray conditions created by tractor-trailer combinations operating on the Federal highway system have been studied and tested for many years with mixed results. Past events are reviewed briefly in this paper. In additional testing during 1983, using new state-of- the-art splash/spray suppressant devices, some encouragement was provided that these devices could work.The 1984 Motor Vehicle Manufacturers Association (MVMA) test program was designed to develop practicable and reliable test procedures to measure effectiveness of splash and spray reduction methods applied to tractor-trailer combination vehicles.Over 40 different combinations of splash/spray suppression devices on five different tractors and three van trailer types were tested. The spray-cloud densities for some 400 test runs were measured by laser transmissometers and also recorded by still photography, motion pictures, and videotape. On-site observers made subjective ratings of spray density. In general, this research demonstrated that improved visibility can be achieved; however, performance varied by vehicle configuration and could not be quantitatively predicted.This paper explores the complex relationship that exists between the amount of spray generated and various vehicle aerodynamic characteristics. Ambient conditions such as wind direction and velocity and their effect on repeatability are discussed in detail.