i've seen Truss Arms similar to this on Interstate 20 going along the outskirts of terrel,Texas...but the straight masts are rather boring to me.

No.....those straight ones aren't boring.....not in Washington DC...for some reason....those masts are different from most...they aren't really straight but kinda tapered actually....they are different compared to a lot of different types I have seen....The only ones I know similar arms here in this world is Toronto, Canada! I donno too many places with something like those LOL..yet it's simple but specular! LOL Those mast are VERY WASHINGTON DC!!! Once you see em, you know you are in Washington DC! They have had those for a long time, some of them are really old like those

They are actually installing brand new ones that look pretty much exactly the same...and even using GE M-400A2 FCO!!!

Why are the M-400 A2 FCO's so common and used so much in Washington DC? What's the utility name there?

When I was last in Washington DC was right when I started getting into streetlights, I recognised the M-400 A2 FCO's. And saw LOTS Of them there. Everywhere. And there was ONE OV-25 next to the Hotel I stayed at... Most likely converted to HPS. Back then I didn't know it was an OV-25 but I knew it was different. And now I Know it was an OV-25.

@Jace i still think the straight mast's are boring thank you...no matter who uses them...or even if they are slightly "Tapered" that is my opinion.

The utility is PEPCO...but PEPCO used to work on ONLY the wooden poles....but not since 1995 The city of DC bought pepco lights and do ALL the lights themselves, and DC actually does a great job with streetlights, they do the highways, roads, all of them themselves....but those are POWERED by PEPCO