Does Security Have Authority To Stop Traffic?

I work Security for a Hospital in Ontario. We've recently been given a new policy that requires Security Guard's to physically stop traffic with stop signs when a helicopter is coming in or leaving. I'm questioning whether we have any authority to stop traffic? My employer argues that because it's private property the HTA does not apply. The road in question does run through the Hospital property but it's the main roadway through the Univerisity and is quite a busy road with buses, and other traffic. I would argue that it is considered a roadway/highway under the HTA even if it does run through private property IE Hospital and University. My question here is does Security have any authority to stop traffic? Personally I don't feel it's safe to. I think that because it's private property there are drivers who won't stop.

Thank you for any advice.

Yes, on private property, you would be allowed to stop traffic. I would obviously take every precaution when doing so, i.e. reflective vest, hand held stop sign, etc.