הלכה על דברים 19:3

That is that He commanded us to prepare six cities of refuge, such that they be ready for the accidental manslaughterer, that a path to them be fixed, that it be straightened and that nothing be left that would impede the fugitive from running [to them]. And that is His, may He be exalted, saying, "You shall prepare the path" (Deuteronomy 19:3). And the regulations of this commandment have already been explained in Sanhedrin, Makkot, Shekalim and Sotah. And we have already mentioned (Sefer HaMitzvot, Positive Commandment 176) their saying [that the laws of these cities] are only practiced in the Land [of Israel] (See Parashat Shoftim; Mishneh Torah, Murderer and the Preservation of Life 8.)

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."