That's one reason it pisses me off when semi, 1/2 semi, or ppl towing trailers get in the fast lane of any road. If your not passing ppl on the right, get over. You create traffic, create blind spots, and .... damn I should just stop now before I say something I'll regret later.
Another reason I don't closely follow two giant vehicles that you can't see past. You don't know what's ahead of em.
Too bad the driver of the trailer didn't see the 2x4; or if he did, too bad he/she had terrible judgment and ran it over. Just glad it didn't hit the driver or a passenger. I think my head shot back about 3 inches when it hit the windshield.
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On another note, this exact thing happend to me this past Friday on my way down to LA for RideLikeAPro. I was in the fast lane, it was about to merge and close up with the lane next to me. Two lanes over on the right was two semi's with a van in between em.
The 1st semi was carring a ton of 2x4s. The semi behind the van, to my right, and a bit ahead was carring a ton of cars.
I didn't see the board drop off the 1st semi, but I did see it on the ground ahead of the van. All I could think was oh crap. I quickly accelerated and as I came up next to the semi my lane merged with the lane next to the semi. At the same time, the semi carring cars ran over and smashed the 2x4 into a million projectiles. They all flew just behind me and pelted all the cars behind. I could only imagine what would have happened had I not see the possible danger and accelerated out of the path of the shrapnel.
This also reminds me of a time when a truck in front of me (a dually), also in the fast lane, ran over a full sheet of dry wall and kicked it up in front of me. We were doing about 75. I was able to slow down to about 55/60 when I ran into said dry wall which was fully vertical when I hit. Felt like I drove into a house. All the cars behind me slammed on their brakes thinking my car just exploded -- lots of white smoke from the obliterated dry wall.
That was fun as there was no damage to the car -- thankfully.
I probably got a million other close calls. Commuting 120 miles a day in the bay area, you start to rack em up.