Splash and spray production from heavy trucks under wet weather conditions has been a concern for many years.. The history of this area of research and development is described, in the context of Federal mandates in recent years. The laser transmissometer technology pioneered by Systems Technology Inc. has been further developed by the Texas Transportation Institute over the past four years. This methodology is described, and data reduction/analysis approaches are discussed, especially accounting for the influence of wind direction and velocity. General findings and recommendations for future R and 0 are provided for the two basic approaches to diminishing splash and spray from heavy trucks: redirection of water and aerodynamic treatment of the vehicle. BACKGROUND Splash and spray clouds raised by heavy trucks have long been a source of irritation to motorists. Since the late 1960's, groups such as the Western Highway Institute (WHI), the Motor Vehicle Manufacturers Association (MVMA), and others have tested proprietary designs and generic ideas to reduce splash and spray. The National Highway Traffic Safety Administration (NHTSA), early on, proposed rulemaking to reduce the annoyance and possible dangers of splash and spray produced by heavy commercial vehicles. However, until the relatively recent introduction of textured spray suppressing material for flaps and of flexible wheel 01 48· 7191/87/1 1 16·2279$02.50 Rodger J. Koppa and Olga J. Pendleton Texas Transportation Institute Texas A & M University System skirting, there seemed to be no practicable yet effective solution to the problem. Congress declared in the Surface Transportation Assistance Act of 1982 • ••• that visibility on wet roadways on the Interstate System should be improved by reducing, by a practicable and reliable means, splash and spray from truck tractors, semitrailers, and trailers,• requiring the Secretary of Transportation to •establish minimum standards with respect to the performance and installation of splash and spray suppression devices for use on truck tractors, semitrailers, or trailers" within one year, and ~~~ use of the devices on new vehicles within two years of the enactment of the law.(l)•.Oevices would also have to be retrofitted on vehicles in use within five years. Prior to rulemaking, NHTSA ran a limited number of full-scale sprayreduction tests at the Transportation Research Center of Ohio. In these tests, subjects visually evaluated or rated the effectiveness of the devices tested, rather than the researchers using the objective laser measurement scheme developed earlier by the Western Highway Institute and used by David Weir, then with System Technology, Inc.(2) The preliminary rating results· indicated that device combinations heretofore believed to be effective did not consistently make a substantial difference to the naked eye, using subjective ratings as an indicant. Subsequent HVMA tests *Numoers in parentheses designate references at end of paper. Copyright 1987 Society of Automotive Engineers, Inc.