Lots of if's. When coming up to an on ramp, always have a look for possible conflicts for space & if possible move left right away. If moving left is not possible, watch the driver/s on the on ramp. Are they paying attention or doing something like yapping on the phone? I'll flash lights & let those who can speed up & get in front of me do so. The dawdlers who expect me to slow for them take their chances. The law of tonnage comes into play. If the car is beside me & running out of acceleration lane it had better find some brakes & fall in behind me. 55' & 20 something thousand pounds is no Mini Cooper in agility.
On the reverse side, truckers usually move over if they can & see you on the ramp. Cars are another story. It is like they don't know what is about to happen. Here again the law of tonnage rules. Make room I am coming to join you. Move over, speed up, slow dow, make a hole.