It doesn't appear you have a lot of experience in the US, legal, insurance, or law enforcement investigation.That's the whole point isn't it.
DOT is the engineering specifications behind public road use. . You hit a non-road worthy vehicle in the fog, dark or rain on the interstate or making a left turn in an intersection & law guarantees it's going to be his fault and liability not yours.
Federal DOT and State laws overlap but differ in some cases.
* Post military I did ODOT vehicle inspections and alignment with my Federal FAA license.
The ruling document in Oregon is what is called Oregon Revised Statutes and State Police, Oregon department of transportation, county sheriff's and city police follow those, in addition each county and city can & do have additional rules called " ordinance".
Here's a example, City of Canby Oregon has a rule that all lighting equipment must be turned on and visible from Dusk till dawn and if you are driving a car with marker lights only, not headlights, you can be cited. That's not state or county, or federal.
An officer can initiate a traffic stop for federal DOT but it has to align with state, county and city. I could go on & on, but you are pedantically over thinking this.
I have been in accident investigations and nobody tries to blame the victim for not having Federal DOT turn signals.
Now... One exception. If you are intoxicated, even if sitting at a stop light, and some idiot crashes into you, you are at fault as well. Nothing to do with turn signals, but recently a friend processed a crime scene where both drivers were drunk.
If you like, look up ORS rules on vehicle lighting. Only reason you would get stopped by the police on lighting, is if they are looking for a pretext to stop you such as a patch club.
* How do you know you are a British bike owner? If police pull you over for dim lighting and you express surprise the lights were working at all! " A gentleman does not motor about after dark" Joseph A Lucas.
** In Oregon you don't HAVE to add turnsignals if the bike didn't originally come with them ( pre 71) however you should not ride in the dark because hand signals are not visible. This is in contrast to seatbelts and shoulder belts in cars. You HAVE to install seat/shoulder belt's.
But nobody enforces it. Been riding since the 70s.