OK, I recant my bit about "most" impacts, as measured per hit. But I stand by my argument regarding plasma, since I'm pretty sure that's far more prevalent in higher velocity impacts, and so for real velocity you want to look at plasma-creating dust, not so much debris.
@Bill Gray "Low-earth orbital speed is 8 km/s, relative to the earth's center. Most satellites, and therefore I assume most dust, is in lower/medium-inclination orbits,"
Urm, to me (out of the field for a few decades, I admit) Dust=natural, debris=paint flakes, dropped zips, ASAT tests, and of course impact ejecta.
Dust, by the time it's dropped down the gravity well can be prograde or retrograde, at any inclination, and (back when I was studying this stuff) is/was considered a significant source.
Hmm... looked it up. Dust impacts on the space face of NASA's 69 month LDEF experiment consistent with the natural flux at an average impact velocity of 15km/s. So, 15km/s average, with a random inclination, and you're going 7.8km/s or there-abouts, in LEO circular orbit, faster for elliptical orbits, of course.
I'll leave the calculation about if we can or can't just average the max & min (15 +/- 7.8km/s) because of the "running into the rain effect" as an exercise to the student. I know it's spawned papers....