Sifrei Mitzvot sobre Deuteronómio 19:2

To establish six cities of refuge (for those who had committed accidental homicide), as it is said, “Thou shalt prepare thee a way and divide the borders of thy land” (Deut. 19:3).

The commandment to prepare six cities of refuge: To separate six cities of refuge from the cities of the Levites, that they should be designated for the one that slays a soul inadvertently to flee there; to fix and straighten the roads that correspond to the cities — and like the matter that they, may their memory be blessed, said (Makkot 10b) that they would write [signs] at the crossroads [saying], “refuge, refuge”; and clear the paths, so that there not be anything on them to delay the fugitive from his run. And about this is it stated (Deuteronomy 19:3), “You shall prepare the path for yourself and divide the border of your Land into three, etc.”

The root of this commandment is well-known and clear — that it be so that the inadvertent killer not be killed by the blood avengers.