It depends on how much weight they could be capable of handling as they were inevitably buried, I would imagine.

[He 'hrrm's thoughtfully as he drives, though Roller apparently has no shame. He prrums at the petting, only to beep in distress at the bumps, immediately moving to her side to help her balance against them.]

...If he gets in your way, feel free to push him aside.

[He sounds...almost embarrassed.]