A letter is always good.
About two weeks ago, I had a long truck pass me, then come over way to soon. I had to go into the gutter to avoid becoming a ghost bike under the wheels.
Caught up at the light ... got the company name and tag number ... fired off an e-mail (mine was formal in nature but gave specifics of what happened).
Got a nice response back-- two in fact, one from the company owner and one from a safety officer.
They apologized, said they had spoken with the driver and would hold a companywide driver meeting to discuss with the others.
I was impressed.