I'll say it again. The only federal laws in the US pertain to safety standards imposed on ready-made e-bikes sold to the public. Prior to 2001, the National Highway Transportation Safety Administration asserted authority to impose moped/motorcycle safety standards on electric bikes. In 2001, the US Congress (in response to lobbying) passed legislation that defined two categories of ready-made e-bikes for purposes of safety regulations, "low-powered" electric bicycles and other electric bicycles that don't fit this category (Public Law 107-319). The term "low-powered" electric bicycles is defined as a bicycle with an electric motor of no more than 750W and speeds no more than 20 MPH. Those ready-made e-bikes that meet this definition only have to meet consumer bicycle safety standards set forth in regulations issued by the Consumer Product Safety Administration. Other, more powerful ready-made e-bikes with motors over 750W or speeds over 20 MPH must meet safety standards for mopeds/motorcycles set forth in regulations issued by the National Highway Transportation Safety Administration. The more powerful e-bikes must have beefier frames and better brakes as well as signal lights and headlights and tail lights. At present, conversion kits aren't covered, but I expect that to change in the near future as more powerful hub motors continue to become available. The Congress has left it to the 50 states to decide whether to allow electric bicycles to be operated on public roads and whether to require licensing and insurance. One state, New York, has a complete ban on riding e-bikes on public roads. (There is legislation pending in the NY Legislature that would overturn this ban, but it's stalled in the Senate.) Many states treat e-bikes that can travel over 20 MPH as moped and require them to be licensed and insured.