There aren't that many "top secret" construction projects. Maybe you'll be asked to build a toolshed for the President someday, or you'll install toilets in the pentagon, but... probably not. That being said, it is nevertheless of the utmost importance that, with rare exception, only those who are a direct part of a project should be allowed on a job site. Here's why:
- Your electrician's brother doesn't know your safety procedures. He doesn't know the on-site paramedic's number, he doesn't know how to operate a radio, he doesn't know where you keep your first-aid kit, he might not even know what an OSHA sticker means when he sees it on a container of flammable liquid. He may be a great guy, but that doesn't mean he has the proper training to manage the hazards inherent to a jobsite.
- Liability. If an outsider is hurt on your site, chances are that it's going to be a lot more complicated and a lot more expensive than if a worker, who knows the risks of the job, sustains an injury.
- Your reputation. It can be tough to get work in construction once word gets around that some teenager wandered onto your project and lost a toe while playing around with a belt sander.
There are exceptions, sure. Those exceptions do not include "We really wanted a pizza and didn't feel like meeting the delivery guy at the gate." Any visitor for whom you make an exception should be loaned a hard hat and briefed on basic safety, and you shouldn't be leaving unauthorized personnel to wander around and check out the sights on their own. Whether they're just dropping something off or taking a tour of the job site, make sure that they have a guide.
Construction sites are full of stairways that end in a thirty foot drop, doors leading out to patios that aren't quite finished, second floors with no railings, dangerous equipment, live wiring, you name it. Workers know their way around those hazards, visitors might not.