One frequently given definition of an Orthodox Jew is one who abides by the 15th Century code of Jewish law — the Shulchan Aruch.
In his third lecture on Rabbi Abraham Samuel Sofer (Ketav Sofer) for Torah in Motion, Orthodox rabbi and history professor Marc B. Shapiro says: “Every single Orthodox synagogue in the United States does something not in accord with the Shulchan Aruch and I’m sure many haredi minyanim as well. According to the Shulchan Aruch (261), Sabbath begins and ends with the Rabbenu Tam‘s opinion (which holds that the Sabbath starts 72 minutes after sunset, which was the dominant Jewish position in Eastern Europe a hundred years ago).”
Marc Shapiro sent this inquiry to various Orthodox Jewish sites online that invite halachic (Jewish law) questions: “If I’m driving in my car just before the Sabbath and I’m stuck in traffic, can I hold by the Rabbenu Tam opinion?”
Two of the halachic sites said you could not hold by the Rabbenu Tam in this emergency but another site said you could.