The Federal Railroad Administration (FRA) has undertaken a rulemaking process to address the use of locomotive horns at public highway-railroad grade crossings [1]. This rule includes a provision to regulate the sound level output of railroad horns. This letter report supports the rulemaking by describing the process used to determine a railroad horn output sound level required for motorist detection. This sound level is defined as the sound level at which there is a 95% likelihood that a person with normal hearing will hear (detect) an average train horn at the instant in time at which detection must occur to avoid a collision. Generally, detection is based on the relative strength of the signal in the motorist’s ambient noise environment. The sound level is tied to an estimate of an average maximum motor vehicle speed and an average maximum locomotive speed. Locomotive horn sound level data measured by the Volpe Center Acoustics Facility at the Transportation Test Center (TTC) in April 2001[2], along with automotive insertion loss and interior noise data measured and documented by the Volpe Center in earlier research [3] provide the basis for the determination of the likelihood of motorist detection.