Cities: Skylines. How can I block a road to all but city services (police, fire)

Vanilla Cities:Skylines. I want to blockade all except city services from using a road. Any way to do that?

My city’s freight rail system is isolated from the international network, because our rails are broad gauge (actually so the network’s train jams can’t shut down my commerce).

So I have a "Freight Free Trade Zone" in my city (as is often done)… containing international freight container-ship and train terminals, and a domestic freight train terminal. That is to force all external, rail or ship delivered freight onto my internal freight rail network.

I’ve made several passes at this, and it works quite well.

(if trucks have direct access to one international freight terminal, it greatly increases town traffic and the backup at the freight terminal is literally over a mile long; I built out stack roads so I know.)

The problem is, these terminals require police, fire, medical and hearse services. They also need trash hauled away.

So… I’m looking for a way for those municipal vehicles to get road access only they will use and block all other vehicles. That way I don’t need to build a clinic, cop shop, fire station, etc. just for the trade zone.

I have tried having having one road, routed through a District with a "No heavy trucks" policy. All the trucks ignore it once they realize there’s no other roadway route.