I said hello in quite a few yards before the message registered that this wasn't normally done. (Para. 6)
Meaning: When I said hello to them, in the many yards around the homes where I collected garbage, people's nonverbal responses lead me to understand that I shouldn't have spoken to them.
"The message" here refers to "that this wasn't normally done". The response the greeted people made to his greetings helped him to understand that he was doing something other garbagemen never did and were not expected to do. This can be seen from what follows in the next sentences and paragraphs.
register: vt. 1) record, show, make mental note of
The earthquake registered 5.3 points on the Richter scale. 這次地震是里氏五點(diǎn)三級(jí)。
That he left the city registered his negative attitude toward it. 他離開這個(gè)城市表明他不喜歡這里。
2) write in a list or record 登記
Visitors are asked to register their names on the book. 請(qǐng)來客在簿子上簽名。
All students have to register before the beginning of the new school year. 在新學(xué)年開始之前，所有學(xué)生都要登記注冊(cè)。