When we were driving to Yuma, we had to stop at an intersection that a ton of bike riders were passing through, and we had to wait until the cops let us sneak through between riders. We thought it must've been some sort of race, but nobody really looked like they were in much of a hurry.
I found an article in the Republic about it. Turns out it was a fund-raising ride for MS. They rode to Gila Bend & back- 160 miles! Wow!