There are a lot of safety personnel in larger London construction sites as well. I've seen men basically acting as barriers while their mate does work above a footpath.

Also there are always people directing traffic in and out of a site, washing down every truck that leaves so there is no mud on the road, etc.

I would assume it is more a case of construction in a large city is more difficult logistically as there are stricter standards for noise, mess and pollution.

It was always my assumption that these type of jobs were on a rota. It would be pretty bad to make someone work the "human barrier/traffic light" job 52 weeks a year.

The people I mentioned above work at night, when the construction site is completely inactive. They're just living lights.