DETROIT (AP) — Amazon’s self-driving robotaxi unit is being investigated by the U.S. government’s highway safety agency after two of its vehicles braked suddenly and were rear-ended by motorcyclists.The National Highway Traffic Safety Administration said in documents posted on its website Monday it will evaluate the automated driving system developed by Zoox.Both crashes involved Toyota Highlander SUVs with autonomous driving technology. They happened during daytime hours, and the agency confirm...