Cars and bicycles share the same travel surface. In order to interact safely, they need to follow the same rules. Using your example, semis still need to follow nearly all the same rules as cars. There is a base ruleset for everyone who uses a roadway (including, one must come to a complete stop at a traffic control device that directs them to do so), and only specific modifications to certain rules for additional safety for vehicles in certain classes.

Here in Saskatchewan, bicycles fall under the Traffic Safety Act if they are on public roadways. That means they can be ticketed for exceeding speed limits or disobeying traffic control devices.

If different modes interact on the same travelway, they must share the same set of rules. If they don't, you get conflicts, which means collision between vehicles, pedestrians, bicycles, and other wheeled modes of travel.