How can I get rid of strange bounce?

Sometimes when an object slides along a curved road it gets pushed very hard in a random direction. Here’s a video that demonstrates the issue: Unity physics issue - YouTube .
My question is - How can I make sure this never happens? The car consists of two colliders - no friction on the bottom, half friction and half bouncy on the top. Collision detection is set to continuous dynamic. Fixed timestep is 1/256.
I noticed that most often this push happens around small triangles in the road mesh.

perhaps a few of the triangles are tilted in such a way as to push the vehicle like you described.
since the road is curved the change in angle may provide the above problem with the triangle pushing the car?
hope this helps!