“You shall prepare the way for yourself and you shall divide the border of your land into three!” (Devarim 19:3)
It is a positive commandment to set aside six refuge cities in Eretz Yisrael from amongst the cities of the Levites. These cities will be reserved so that a person who accidentally killed someone may flee there. Included in this is an obligation to fix all of the roads leading to those cities as Chazal say that at each road junction they would write “Miklat Miklat” (refuge), so that the accidental killer will know where to flee to. They must also empty the roads so that there will be nothing stopping the refugee from running, and about this it says “You shall prepare the way for yourself and you shall divide the border of your land into three etc.”.
The root of this commandment is well-known, as it is to ensure that the one who kills accidentally should not be killed by the “redeemers of the blood” (close relatives of the dead person).
Amongst the laws of this commandment are that Chazal said that Moshe set aside three of these cities on the other side of the Yarden and Yehoshua set aside the other three in Eretz Yisrael, and that the three set aside by Moshe had no power to give refuge until Yehoshua had set aside the other three. If so, why did Moshe set them aside? He said, “A good deed that comes to my hand I will fulfil!” In the days of the king Moshiach we will add another three as it says “And you shall add another three cities on to these three cities!”. In truth, all of the cities of the Levites had the power to give refuge, as it says “and upon them shall you give forty-two cities; all of the cities that you will give to the Levites etc.” and we can extrapolate that all of the cities had this power. The difference between them is that those six cities of refuge gave refuge whether the killer knows about their powers of refuge or not, whereas the other cities of the Levites would only give refuge if he fled there with the knowledge that they would give him refuge. Another difference is that one who kills accidentally who goes to one of the six main cities of refuge does not need to pay rent, whereas in the other cities of the Levites, he must pay rent.
This commandment applies when Klal Yisrael are living in their land, and it is one of those commandments incumbent on the king and on the entire population.