Driving a pedicab with a large passenger

When we were downtown the other night, we saw a large number of pedicabs driving around. As we were sitting at dinner looking out at the street and watching the people, my wife asked what if a large person wanted a pedicab ride (or say three adult males) and the pedicab driver couldn’t physically pedal? Can the driver discriminate based on weight? Would the driver just have to attempt to pedal and go nowhere? I’m sure this has come up before; there doesn’t seem like a good solution besides maybe having the large person do the pedaling.