In a long game, the number of vehicle tire marks on the terrain gradually increases until it causes performance and graphical problems.
Evidenced by the count of the array returned by (allMissionObjects "#track") increasing gradually over time.
Eventually the terrain starts flickering, which I presume is related to the problem with the tracks.
For some reason, there seems to be no scripting command to delete tire marks, so it seems there is no remedy for this other than to restart the mission.
The missions I encounter this on have vehicles spawning periodically, but the number of vehicles existing in the world at any one time is fairly stable. And that number is in the vicinity of about 20 vehicles.