An Air Carrier Captain reported an aggressive braking action in order to avoid a charter bus which failed to stop on a DEN service road before crossing a taxiway.

Narrative

Charter bus runs stop sign. As we were approaching the ramp from the west; I saw a bus southbound on the service road. I thought it was going pretty fast and caught my attention. It soon became apparent that the bus was not going to stop. I applied firm braking and stopped before the service road. At that point; the Bus Driver saw us and stopped short. I saw someone in the right front of the bus waving his arms and falling toward the front window. I don't know if he was trying to warn the driver or fell because of the stop. Ramp Tower saw the incident and requested the bus number. By the time the bus stopped; it would have been at least under the wing tip had we continued over the service road. After we both had stopped; the bus continued south on the road. By the time we had continued onto the ramp; an airport operations van was chasing the bus. Since everybody on the plane was strapped in; no one fell or was hurt. I made a PA to let everybody know why we stopped short. After the flight; the Flight Attendants and passengers said they noticed the stop and appreciated the information. A couple of passengers commented that they saw the bus coming from our left side. On my way home; I called the Ramp Tower and they informed me the driver and escort were cited. I understand this was a charter bus and would be interested in what training these drivers get before being allowed to drive on the airport. Fortunately; this has not happened to me before (with a bus or service vehicle); and I surely hope it doesn't happen again; but it certainly is a serious reminder to keep track of everything that is going on all the way to the gate.