Arc de Triomphe lines?

Yes, tickets can be purchased right there (12€ adults)
http://www.paris-arc-de-triomphe.fr/en/

There wasn't much of a line at all when we were there.

I was there on an evening in August and there was no line at all. I also walked by during the afternoon on a couple of other days and there was never any line. Now, that's not to say that sometimes there may be a short line but I doubt it's ever very long, it's not as popular as the ET for example. It's a great place to go after sunset to see the lights come on along all of the avenues radiating out from there, including the Champs Elysees and also to see the ET sparkle on the top of the hour, and there is an elevator if you can't handle stairs.

Great visit with a loooong spiral staircase to the top. The view, especially at dusk, is worth it. Lines are generally short, often there is a memorial ceremony in the late afternoon. Admission is included with the museum pass.

If you're planning to visit in the near future, you won't be able to access the Arc de Triomphe, because the underground passageway leading to the stairs will be blocked due to possible flooding.

You can still get a magnificent view of the Arc de Triomphe - front, back and both sides - from the Champs-Elysees.

Thank you so much for the helpful hints and suggestions. We will be in Paris at the end of the month so are keeping fingers crossed that the rain and flooding have subsided by then!

We were there last June on a busy afternoon. Everything was very crowded, but lines to go up were not a factor. Going up the spiral staircase wasn't so bad, but I personally didn't like descending it. I'm getting old enough that I'm very conscious of not wanting to fall and I had much younger people practically breathing down my neck on the way down. That said, it was well worth it, both going to the top for the views and, just as impressive, seeing it from directly below on ground level. It's quite massive.

We experienced virtually no line around 5:00 p.m., on a weeknight, a few weeks ago. (That said, we didn't take the stairs. A guard spotted my cane and escorted us to a speedy lift that shot us to the top. I hadn't requested special attention. He may have been concerned that I would slow the flow of others on the stairs.) If you make the effort to get to the Arc, you should certainly consider the trek to the top. The views are terrific, and the traffic action around the Arc is particularly entertaining at rush hour.