A series of letters sent by autonomous-vehicle (AV) developers to Democratic US senator Ed Markey of Massachusetts sheds the most light yet on the human side of robot vehicle operations. In the documents, submitted to Markey as part of an investigation into self-driving-vehicle technology and released on Tuesday, seven companies, including Tesla , Amazon-owned Zoox , and Uber- and Nvidia-funded Nuro , released new details about their “remote assistance” programs. All the companies that responded to the senator's office say they use remote assistants —humans charged with responding to autonomous vehicles when they get confused, stuck, or in emergencies. The programs, experts say, are an important part of any autonomous vehicle company’s safety considerations, a backstop for a technology that’s becoming safer by the year but will continue to run into new situations on the road indefinitely. In a report also released Tuesday , Senator Markey said the new details were not enough.…