Walking pace, I suppose. IIRC, imperceptible rises in the road would require passengers dismount.
On my most recent iteration, I pedaled up an incline, did a u-turn, and coasted down. The speed was alarming and a curve was approaching, so I braked fairly aggressively.
Sadly there is no photo evidence, but I'm most certainly the only person to execute a front wheelie on a sofa bike.