Discription of Events: Man driving was dangerous. he was in fast lane on US Route 30 heading west, driving about 35 mph in a 55, then 45, then 40, then 45 mph zone. I was terrified to pass him. He rode in the middle of the fast & slow lanes. He was weaving. Occasionally he would increase speed significantly, then slam on brakes - FREQUENTLY - for no reason. Waiving & flailing his arms like there was a bee in his vehicle. I witnessed this from behind his vehicle in both the left and right lane. I stayed behind him for about 30 miles (Greensburg PA to N. Versailles PA on US rte 30 W). I was literally waiting for him to cause an accident.