The latest blitz on speeding and un-roadworthy trucks  on the Hume Highway and other parts of NSW has seen more than 400 vehicles  stopped, one doctored so it can travel at 140 kilometres an hour.

Police Operation EXPLORER is the fourth blitz by the Highway Patrol  since a truck crushed a car between Mittagong and Campbelltown, killing three memebrs of the one  family two years ago.

PETER WELLS from the ROADS and MARITIME SERVICE says the government  knows its NOT only truckies to blame.

Mr WELLS   says truck drivers are relaying accounts of being pressured by suppliers to  speed, or lose out, and that's becoming a new focus of investigation.