Date: 202110
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Aircraft: A321
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Ground ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Situational Awareness; Communication Breakdown
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Conflict Ground Conflict; Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Illness / Injury; Ground Event / Encounter Other / Unknown
Detector: Person Flight Crew
When detected: Taxi
Result: Flight Crew Took Evasive Action; General Flight Cancelled / Delayed; General Police / Security Involved; General Physical Injury / Incapacitation
Contributing factors: Environment - Non Weather Related; Human Factors
Primary problem: Human Factors

Narrative: Pushback and engine start was complete; and had received a taxi clearance from ZZZ ground control. I released the parking brake; turned on the TAXI light and started moving forward at approximately walking pace. There was some confusion about the taxi clearance so the First Officer (FO) and I worked to clarify our taxi clearance. During that time I stopped the aircraft briefly; and attempted to get clarification on a busy radio frequency. After some clarification I began to roll forward and begin my taxi toward the runway. As I approached the vehicle lane that crosses the top of the alleyway I saw a white vendor car passing right to left. I reacted and applied the brakes when the vehicle was approximately mid left wing. I stopped the aircraft to avoid a collision. I had to lean forward into the glareshield to see the driver and vehicle pass in front. I would estimate it at less than 20 feet. Shortly after I stopped the aircraft a fuel truck passed right to left. Upon stopping the aircraft I contacted the 1st Flight Attendant (FA) to check on their status. 2 FAs had been standing during the event. She was injured; shook up and another flight attendant who was standing in the forward galley was hurt with bumps and bruises to her torso. I coordinated a return to gate and requested paramedics to meet the aircraft to attend to the FAs. Ground control and company operations were both informed of the vehicle incursion at the time of the event. Multiple ZZZ [authorities] interviewed the flight deck and flight attendants for their report. Ultimately the entire crew was replaced and went to the hotel. A replacement crew arrived and the flight departed approximately 2 1/2 hours later.Dark operations; very active airport with ground vehicles; inattentive vehicle drivers. Our taxi clearance was confusing so my taxi may have appeared to be unpredictable. My taxi and beacon lights were on throughout the event and that should have given any road traffic an indication I may move at any time. Prior to departing gate area both pilots checked the immediate areas left and right and stated 'clear left; clear right' before aircraft moved. As we worked out the taxi clearance; I slowed and then stopped the aircraft. It is possible that prior to moving again I did not verify with the FO that the right side was clear. He may have been heads down looking at the taxi chart. I know that I cleared the left side and as much of the right side as possible. The vehicle actually slowed to a stop at the appropriate hold line while I was moving. The vehicle for unknown reasons then started moving into our path.[Suggest] Elimination of all road traffic from aircraft movement areas. Obviously that's not possible. Better vigilance on the flight deck clearing the area and recognizing vehicle crossings as a potential hotspot for collision. I should have not taxied the aircraft until the FO was completely engaged with clearing the area in front and to the sides of the aircraft prior to movement to provide more warning for ground traffic failing to yield.


Second narrative: We had both engines started; after start flows complete; after start checklist complete; flaps were set and after taxiing a few feet to the top of the alley we were holding short of the service road when we contacted metering for taxi sequencing abeam Gate X.  Metering told us to monitor ground.  Ground control gave us a confusing clearance to Runway XXL. Since both of us were confused we took time to check our charts and discuss a plan. After thoroughly discussing the route and our options we decided to contact ground again to get clarification.  Ground control was typically busy so we had to wait to get a word in so we were sitting there not moving for probably a couple of minutes.  At no time did we signal or wave any service road traffic to pass in front of us.  We both felt comfortable taxiing out of the alley as we were going to clarify our clearance before proceeding on [taxiway] 1.  As we started to slowly roll forward a vehicle moving right to left on the service road did not yield to us.  The beacon and taxi light were on.  The Captain was vigilant and quickly applied the brakes to stop the aircraft as it became evident the traffic was not going to stop (we weighed about 175;000 lbs).  I don't think we could have been moving at more than a few mph.  Our abrupt stop assured clearance from the first vehicle and a subsequent fuel truck that also passed in front of us.  The quick stop caused two flight attendants to lose their balance and stumble/fall during their demo.  After communicating with the FA's; ground; metering; ops and hearing of a possible injury we return to gate X.  FAs were evaluated by paramedics; they were all taken off the trip and went to the hotel.It would be speculation to suggest what might have caused the driver of the vehicle to not yield to us.  We were stopped as we clarified our clearance but the beacon and taxi light were on as we moved.  It was night and the ramp was typically busy.  Perhaps the driver did not expect us to taxi as we had been sitting at the top of the alley for some time.Continue education for all SIDA badged employees on the importance of giving way to aircraft.  Including that causing an aircraft to stop abruptly can cause injuries if flight attendants are up in the cabin.  Also; when an aircraft taxi light is on be aware the aircraft may taxi at any moment; even if it has been sitting for some time. Crews should assure the taxi light is on anytime they are going to move.  Vigilant monitoring of ramp and airport traffic by the flight crew is key to safety while taxiing; especially at night and in congested areas. Making a slow roll before adding much power to taxi in congested areas will also help add a barrier to potential ground conflicts.  A slow roll and taxi light on should give airport traffic a clear sign that the aircraft is beginning a taxi.


Synopsis: Air carrier flight crew reported they had to stop taxiing abruptly to avoid a collision with a vehicle that unexpectedly passed in front of the aircraft. The abrupt stop resulted in several flight attendants getting injured when they fell and requiring medical attention.