I have done small tests, and yes, it doesn't depend from buildings/vehicles.
With settings: 240p, prefetch levels 17, max terrain level generated 21, tiles per frame 6, tree coef 0.14 it runs quite smoothly.
1440x900, off vert sync, FXAA on, shadows 1024x1024 with 4 samples, shadowmap kernel size 2, shadow distance 600.
But for example on default (480p, prefetch 12, max 22) the problem returns.
I'm not sure yet, but while driving the problem seems to be bigger, but while I'm in ufo mode flying (especially farther from ground) this problem is smaller.