Well. This could be a game changer. New Warp Drive theory

[Bear]

Right... But where a pebble the size of a wad of bubblegum hits with the force of a nuke, it's not really the distance between kilometer-size rocks that we're worried about.

The colony ship spots the kilometer-plus rocks as it goes by and maybe in a few years somebody shows up to explore them in person or make a new habitat on them.

Or if the colony ship is near maximum population and a tenth or so of them are interested in starting a colony now instead of waiting centuries more for a proper planet near a star, then that group and its equipment get themselves ejected as reaction mass leaving them with a small relative velocity to the rock and the ship with a slightly larger relative velocity away from it. Or if the colony ship has already hived off a dozen colonies this way then they probably need to stop somewhere to resupply with - well, with practically everything - before they go on.

Or if the colony ship has hit one of those nuclear-bomb level pebbles, maybe the whole ship has to dump delta-vee trying to limp toward a rendezvous with one of those larger rocks in order to get some badly needed structural steel for repairs.