It all depends on the where, when and why. If you're there as they are pulling the slabs, just stay very clear of the workers. Only reason anyone would object is if you are interferring with the workers or their schedule in my experience. Of course there's always differences between residential areas and business areas in some towns but I've never had any trouble while just walking on and detecting.
Some of the engineers may ask what you're doing, which group or component of the work you're with, etc.. I just tell them I'm looking for old coins and such exposed by the work. Only time I've ever had trouble is with one large construction outfit and the guy threatening me had no authority over the area I was detecting.
The parkways, when they get scraped are the best place to find old goodies but they will go fast if you are not there in or during the process very often. In the parkways you will be dealing with the landscapers and they are usually the most friendly, but they all do their jobs differently so some sites may be very good while others are not.
You have to keep up with them, they don't wait for you. But if they leave an area open for the weekend, it's best to go back to it even if you've already been over it and go over it again slowly and deliberately. Best part about this is meeting the people who live in the area and getting invites to do yards and asking the okay to do yards along the way.
Next best part about it, the digging is done for you on much of the site.