The shocking incident happened on the eastbound carriageway of the M62 at Tingley on Friday night.
The filled sandbag was thrown over the Thorpe Lane bridge.
READ: F1 legend Nigel Mansell backs Highways England tailgating campaign that gives nod to classic arcade game Space InvadersThe driver suffered minor injuries from the impact but managed to move his vehicle to the hard shoulder.
Police have warned that the attack could have resulted in a fatality.
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