Terraforming/Tunnel under Runway

I managed to create a tunnel under the runway and add a plane mesh with road material to cover it. Now all the airport ground traffic are driving in the tunnel. I did set the falloff distance for the rectangles to 1.0 but is not working well. Is still affects a big part of the runway/taxiway. I did expand the mesh to cover the affected areas but as you can see in the video the affected areas are snapping where they supposed to be only at a short distance.
Is there a way to set the distance higher in SDK? or somehow to stay in place regardless the distance?
I am concerned about when that runway is active it might not snap in time for the lower/mid computers and it might result in crashing the airplane.