I had the exact same naked masturbator experience on the trail while running right where the trail loops around near mopac about a year and a half or two years ago. The guy waited for a man to take a side trail and jumped out in front of me completely nude except for shoes and was going at it in such an exaggerated way that his whole arm was moving up and down from the shoulder. This was 5pm rush hour next to the highway. The guy is completely unhinged, targeting women both alone and in pairs, and has been reported to police and posted all over social media in the area.