Adding a little context for those not familiar with orbital mechanics: the speed of a circular orbit is directly related to its distance from the barycenter (i.e. the Sun). The asteroid belt is between the orbits of Mars (orbiting at 24km/s) and Jupiter (13km/s). Whilst an asteroid that strays far enough to hit Earth (orbiting at 30km/s) is by definition not in an exactly circular orbit, nor one always between Mars and Jupiter, the difference in speeds isn't that great. That accounts for ddahlen's point about the limited range of velocities.
A very rough calculation of mine involving a hypothetical asteroid in a elliptical orbit extending as far as Jupiter and right down to Earth, assuming no difference in orbital inclination to Earth and no significant gravitational perturbations, would result in a relative speed of 5km/s. The actual impact speed would be greater due to Earth's own gravity, adding an extra 11km/s.
Not all asteroids are from the asteroid belt, but I am under the impression that visitors from the outer solar system (which could be as fast as the upper bound that ddahlen mentions) are much more infrequent than stray asteroid belt objects, so the median impact speed would still be relatively slow.
Earth's orbital diameter is: ~30 x 10^7 km
number of seconds in a year is: ~ π x 10^7 s
so, Earth's orbital velocity is: ~30 km/s
speed of light, c, is: ~30 x 10^4 km/s
so, Earth's orbital diameter is: ~ 1,000 light seconds
and, Earth's orbital velocity is: ~ 0.0001 c
A very rough calculation of mine involving a hypothetical asteroid in a elliptical orbit extending as far as Jupiter and right down to Earth, assuming no difference in orbital inclination to Earth and no significant gravitational perturbations, would result in a relative speed of 5km/s. The actual impact speed would be greater due to Earth's own gravity, adding an extra 11km/s.
Not all asteroids are from the asteroid belt, but I am under the impression that visitors from the outer solar system (which could be as fast as the upper bound that ddahlen mentions) are much more infrequent than stray asteroid belt objects, so the median impact speed would still be relatively slow.