There is no way to stop oncoming traffic without sending an officer to the other lane to redirect or stop traffic.
I have removed the big trucks from the spawns in the new version. I also included a "delete traffic" button just to be sure.
To remove the vehicles, go into the editor. Load up "freeplay.e4m". Once you are in the map, press the F9 key.
Choose the spawn point that gets stuck and delete the trucks using the "X" button on the right side.
http://www.emergency-planet.com/uploads/monthly_2018_11/5bf6093b4c3a8_Em4Deluxe2018-11-2209-34-58-28.thumb.png.707db6c6c8f9b61d037e4143b132dfd2.png