NOTE: The foundations are plain, see below for possible explanation.
Confirmed. Driving over 80 km/h is very risky and very unstable. Sometimes the vehicles steering changes up to 15°. It depends on the current performance, if save freeze happens, how many other buildings are around and so on.
You can see this well by looking at the wheels especially from the Explorer and Tractor. It jumps sometimes quite a lot for a split-second. After that it can happen the steering changes a little.
The foundations are flat, the problem is the transition from one to another. The collision box used for the gravity calculation seem to be not 100% perfect probably to avoid false positives on placements and/or rounding issues of floating point. So it seem the collision check sometimes "peek" through on the ground and think there is no foundation for a split second.
A solution might be to calculate more collision points per wheel and use some kind of median/avg logic or discarding of extreme values to avoid those micro-gaps having influence to the steering.
EDIT: Might be an explanation why the Trucks are more stable on that because it has at least 6 collisions points to calculate, but Trucks have other issues. They are too sensitive on the sides and/or with their weight. You just need to touch a Wall or Walkway and with enough speed your Truck jumps into the air and lands on it's back.