As often as possible, all the pedestrians and traffic will have been specifically placed. Sunday mornings are usually a good time to be able to close off a road. The general rule being you can close anything if you have enough money ;)
This ensures that when they change camera views, the same cars and people will be doing the same things at the same timings.
This, of course, is vital if the scene involves actors crossing the street, directly interacting with traffic, or a car chase etc, which will need to be shot from 17 angles across an entire day.
Of course, if they can't afford it, they have to work with what they can get.
Here note the two shots at the end, which is the most noticeable I can find on a quick look…
The same yellow taxi is pulling up to the lights each time, the white car & grey van stay the same throughout.
These two shots could not have been done simultaneously - one camera would be in the way of the other.
Though it's possible to do things like this with 'live' traffic, it tends to mean they need to do the scene in single long takes & try to edit from one of those single takes, so traffic is 'the same for just those two minutes'. This is cheaper, but hard to edit.
Earlier in the scene there is a set of buses going down the street. Now, this could have been done in one single take - the actors & camera getting exactly the shot the director needed in one go, whichever take they used they would have to use the entire take to keep the background match…
The reverse angles focussing on Sandra Bullock are shot in such a way that most of the time you can't see the traffic to tell if it matches Reynold's shots, which would push me more towards this part of the scene being done live.
There's also a FedEx van which is parked behind Reynolds, which is there the entire time. FedEx don't tend to hang around for hours, so this is either a placed vehicle, even if the others aren't, or they really did get all that in one take. Sometimes they'll just go with placed parking so the parked cars don't change through the day; it's hard to tell which in this case.
Sometimes they will go with 'live' traffic with one or two placed vehicles for significant parts, other times they will shoot the 'easy' bits with live traffic, then hold the road for sections they need to be able to repeat. I'd take a guess that's what they may have done for this scene. It has two distinct parts, which don't necessarily need to match each other. There's a significant set of shots in the middle where you really don't see enough traffic to be able to track it.
One part that does bother me is the first time you see her kneel, the parked cars aren't yet there…
This pulls me more towards the traffic being live. Maybe only the taxi is placed… but I still don't think they could get both those shots at the end at the same time. Maybe they were placed, but placed later in the day & they hoped no-one would ever examine it quite this closely. That does often happen, which of course makes deciding how any specific scene was done much harder to determine..
So, on to the people - the pedestrian who walks right in front of camera just after this just couldn't have been trusted to be there accidentally. He doesn't give her a second glance, just goes on with his 'day'. He will have done this 15 times that day already ;) This will apply to anyone near camera, right through the scene. You can see continuity in the guy with the brown bag as she first kneels, though the timing does step out by a couple of seconds; in effect, he walks past twice.
People at the very back of shot might be 'real', but I haven't spotted anyone being a 'movie tourist', also everybody's route is such that there is very nice spacing between them all. People don't all spread out this 'naturally' unless they've been told their exact placement & timing;) This is most noticeable right at the end, where they've got a nice wide of Sandra looking 'confuzzled' for a few seconds. Seven seconds* is a long time to hold the frame, so they need everything to look as 'natural' as possible for the duration. This will likely be the most 'organised' part of the scene.
'Movie tourists'… Ordinary members of the public in filming are terrible. They stop in the way of the shot, they see a camera so walk right the way past staring into it, they get their phones out & start taking pictures, they ask if they can be 'extras' without realising this entails being there the whole day doing the same thing over and over…
There might be 50 people doing various jobs this side of the lens. That's a spectacle the public cannot help but stare at. This does not work for any scene, at all. You can only really get away with this if you're filming guerilla-style with minimal crew & often hidden or obscured cameras.
*Re: holding the frame for a very long time… If you want to see this done to perfection, watch Sam Esmail's Homecoming which holds a shot at the end of each episode, for over a minute, with usually 'nothing much happening' yet many times must have been very carefully choreographed so as to not get dead spots. At some times they've just held the shot & used the 'best bit' but other times it pans from the last part of the action, then holds far longer than anyone would normally dare… [I've spotted other directors copying this recently… I'm looking at you Mr Clooney (The Midnight Sky)].
I thought, by comparison, this would be interesting. James Bond, Skyfall, just after the tube train crash, as Bond gets back up to street level in Whitehall.
This scene is all entirely staged. 650 Supporting Artists, all the vehicles, the road completely closed to the public for half a day, one Sunday morning in 2012. After each take, the whole lot had to go back to first positions & start over, exactly the same every time.
This is what a colossal budget will buy.