It may depend on where you're at, but these marked routes do not necessarily get a lot of bike traffic. I've ridden that 280 route to White Rock Lake several times, and to Love Field once. I've seen plenty of riders at White Rock Lake, and a few at that lake over by Love Field, but have only seen a couple of other riders out on the marked route itself. They may get more commuting traffic during the week or something.

Drivers vary a lot. Some of them give you a whole lane even if you're at the far right. Some don't. Some are TOO polite, and refuse to pass you, and chug along right behind you, which is not very helpful, either.