In addition to keeping your pants away from your drivetrain while commuting, Randonneurs USA and many other events that involve nighttime cycling require reflective ankle bands. And many jurisdictions require either reflective ankle bands or pedal reflectors. Plus, they're just a good idea for nighttime visibility.
These are basic reflective ankle bands made from 3M's excellent Coast Guard-approved SOLAS reflective fabric. The material is soft and comfortable and reflects just as well when wet as it does when dry, the reflective surface is sturdy and doesn't easily wear off, and it is very bright! The second photo was taken with the flash, in otherwise identical lighting conditions to the first one.
The generous 2" width holds your pants leg securely. But since the hook-and-loop fastener is only 1" wide, there is still ample visible reflective area all the way around even when part of the hook-and-loop is exposed. The softer loop side faces out and the rough hook side faces in; but the hook side is shorter so it won't snag your socks if the entire length doesn't overlap.
Each leg band is 16 1/2" long, and can fit ankles, legs, arms, etc, from under 9" in circumference to over 14". Sold in pairs.