Aruch Hashulchan SA, O’CH 345, Rav Menashe Klein in his Mishneh Halachas,
See Yabia Omer O’CH (9,33) in detail.
Repeating what is applicable for making an Eiruv:
Tosfas Shabbos (96,2) ד’ה הוצאה indicates it for carying out and there ד’ה הכנסה indicates it for carying in. Both cases is considered a מלאכה גרועה a weak melacha, as Tosfas 2,1 asks what is the differents carying from a private area to another private area where it is allowed to carry bibliically with karmeles (4 amos wide and less than 10 tefochim high) in between. See SA O’CH 345 There is an argument a public domain 16 wide 600,000 people walking through does that include cars? Reb Moshe holds they walk with their cars but others hold that a car is a a private domain for itself. See in great detail Yabia Omer O’CH (9,33) about making a airuv of tzuras hapesach. A public domain where houses are on each side might limt the movement Traffic lights also limit movement. Intersecting roads also are questionable.
The Aruch Hashulchan in O’CH 345 explains that in the olden time there was a main road where all other roads would meet. The houses were in an alley closed on one side and meeting this main road on the other side (movo). The main road had doors on each side but open at nigh 16 amos wide. This is derived from the mishkan were all the tents would converge to the Moshe Rabbenu’s tent where all 600,000 people gather and the wagons to the mishkan took up 16 amos.
Most agree that 600,000 is required. Rav Moshe in Igros Moshe O’CH (4,87) says that a city having 12 mil by 12 mil where this amount of people gather makes it a public domain considering it a platyeh godaloh. The Aruch Hashulchan says that each street is judged for itself.
The RMA O’CH 346,3 says that currently all of our streets are considered karmelis, only rabbinical. This is the the view of most poskim and therefore we can make an airuv with a tzuras hapesach without actual doors. Most poskim’s view is that cars don’t can included in the count. We have houses on each side of the road with street lights which limit the public movement. There are other roads circumventing the main road, so the main road is not as important as it used to be.