I was doing a little reading on local bike laws in Massachusetts, and I came across this list of restrictions:

"You must ride astride a regular, permanent seat that is attached to your bicycle. You must keep one hand on your handlebars at all times.
...
You may not carry a passenger anywhere on your bike except on a regular seat permanently attached to the bike, or to a trailer towed by the bike. You may not carry any child between the ages of 1 to 4, or weighing 40 pounds or less, anywhere on a single-passenger bike except in a baby seat attached to the bike. The child must be able to sit upright in the seat and must be held in the seat by a harness or seatbelt. Their hands and feet must be out of reach of the wheel spokes. You may not carry any child under the age of 1 on your bike, even in a baby seat. You may not use a siren or whistle on your bike to warn pedestrians. You may not park your bike on a street, road, bikeway or sidewalk where it will be in other people's way. You may not carry anything on your bike unless it is in a basket, rack, bag, or trailer designed for the purpose. You may not modify your bike so that your hands are higher than your shoulders when gripping the handlebars. You may not alter the fork of your bike to extend it."

I haven't yet been able to find the actual text of the law, but this came from the Mass Bike Coalition.

I was amused by the actual legal requirement of a seat, but surprised by there being laws addressing handlebar height and fork length.