Bicycles are vehicles, not toys. They are legally required to obey all relevant parts of the Uniform Vehicle Code (in the US), as modified by local ordinances. They have no business on sidewalks, unless it is really a marked multi-use path. Bicycles should be on separate bike trails or lanes. If there aren't any, then they belong on the paved roadway, staying as far to the right as is reasonably practical. That being said - and that being the law most places - cyclists need to be aware that cars are a lot harder than they are and watch out for idiots.