A320 Captain reports inability to get vehicle traffic to give way after holding short of Taxiway T on Z at ORD.

Narrative

After we landed in ORD; our taxi instructions to go to the gate were; 2 left turns; C/U/E/Z hold short of T contact Ground there. We taxied uneventfully till we got on Taxiway Z and were at our hold short point prior to Taxiway T. There is a service road there that parallels Taxiway T. There were at least 50 vehicles traveling in both directions on this service road at that time. It was very busy. I've never seen that many vehicles traveling on that road. When I approached the service road there were 3 stopped vehicles headed west bound that were beyond the hold short line and the stop sign and one truck; beyond the hold short line and stop sign headed east bound on the other side. With the 4 vehicles over the hold short line it was unsafe for me to continue. I stopped the plane and waved those 4 vehicles on; trying to get then to cross so that I could safely continue. They wouldn't move so I flashed my lights and kept waving. The white van just sat there with me unable to pass. We contacted the Tower and they said they couldn't do anything about it that it was someone else's responsibility and department. Finally after about 3 or 4 minutes of me flashing my lights and waving; the second vehicle east bound passed the white mini van and started the ball rolling and crossed in front of us. The only problem was that now; everyone started crossing in front of us and wouldn't stop. I flashed my lights and contacted the Tower again with no possibility of help from them. I then inched forward to try and signal that I wanted to cross and again 3 vehicles on the westbound side and 2 on the eastbound side were well past the hold short line with their front ends right up to the taxiway. I tried to signal them to back up this time and they just sat there. I again complained to the Tower and still got a reply that it wasn't under their control and they couldn't do anything about it. At this point I have gotten as close as I thought safe and stopped while waving them to back up. They wouldn't back up. But they started a free for all; darting right in front of us as close as could be. I finally just moved extremely slowly and inched my way across while still complaining to the Controller. The second Ground Controller gave us instructions to taxi via T/W cross 4L and turn left on A to the gate. Needless to say I encountered the same aggressive driving after turning onto Taxiway W with the same service road. At this point I felt I had no choice but to cross that service road with 2 more vehicles well past their hold short lines. I did so very slowly using a lot of caution. I probably made the problem worse by trying to clear the vehicles out of my protected area. Perhaps; other aircraft continue with vehicles in this protected space and the drivers are used to being over the hold short line when the planes do pass. I have never seen this kind of traffic volume on that service road before. Maybe because of the time of day I am usually there or because there is just more volume then before. I will say that the vehicles have been quite aggressive in the past but this problem has gotten much worse. If you are going to let vehicles travel on service roads on airports that don't know or refuse to follow safe practices then maybe you need to have a traffic cop to keep it safe.