Discription of Events: incident happened on 44 west coming from Plymouth, at the point where 44 merges to one lane from 2 lanes. Signs say "lane ends, merge left". There was another driver directly ahead of me and I was in the left lane, ahead of the the driver of the red pick up to my right. He became irate when he could not get around me, tried to pass me in the breakdown lane and proceeded to throw what sounded like a handful of gravel at the passenger side of my car. Since getting home Ive discovered that it was likely an iced drink.