What really matters is their velocity relative to each other, which of course will vary, especially for objects on different orbits that happen to intersect.
The closing velocities involved can still be very high, and the "bullet" metaphor still applies in most cases, but it's misleading to label them as travelling at "17,500 mph (28,160 kph)".
The other thing that always bothers me is the graphical depictions that show a "point cloud" of debris in orbit around Earth. These always underplay the size of the earth and overplay the size of the orbital objects.
When discussing space debris, it's always worth remembering that there's approximately 10,000 planes in the air at a given time, and yet if you look up, how many can you see in the sky above you?
Since orbits are much higher, they're consequently much larger, providing even more available space than there is to commercial aviation. Not only that, but there's a far greater altitude range available for satellites than for planes, making the available space out there even more massive.
I don't doubt that space debris is an (emerging) problem, but the current severity of it often seems overplayed by the mainstream media.