Q. Is one able to reserve a public parking spot? For example I am supposed to meet someone at a certain venue and I get there first and parking is tight. I see a parking spot open up and I go and sit down in that spot to “reserve” it for my friend, so no one else will take it, is that allowed?

A. Public parking spots users are subject to regulations and restrictions either from the owner or the city. By using the parking, clients implicitly agree to abide by the rules and Halacha dictates that they have to be obeyed.
Mishp’tei Hatorah (1: 85; 5-6) prohibits saving a place on a bus for someone who will board, on a later bus stop (see Bava Metzia 10a.) However when the trip is free, he mentions that Poskim disagree if it is permitted. He also adverts that in places where there is an established and accepted custom, the norm overrides.
Horav Shlomo Miller’s Shlit”a opinion is that if it is common and accepted behavior for users to save momentarily a spot for an evidently approaching friend’s car, it can be done if it does not create a Chilul Hashem.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a