Trucks may not be the direct cause of congestion, but they are certainly indirectly causing it. Some people sh*t themselves when there's a truck nearby and don't speed up and go around it, also once traffic is ruined and slowed down by some "merge point" by a pack of morons who have no idea how to merge, then the trucks take forever to get going again, especially if the trucks have to go even slightly uphill. Granted, this would be fine if the trucks stuck to the left-most lanes, but they don't, it's not unusual to see trucks four-wide doing 40km/h on a 100km/h freeway.