Date: 201505
Local time of day: 0601-1200
Location: DTW.Tower, MI
Flight conditions: VMC
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Pilot Flying; Captain
Anomaly: Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Taxiway
Detector: Person Air Traffic Control
When detected: Taxi
Result: Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: After normal pushback and checklist accomplishment; we were cleared by Ramp Control to taxi to Spot 5 and call Ground. As we started forward prior to starting a right turn to Spot 5; a catering truck drove down the truck road from left to right in our path. The Captain stopped the aircraft to avoid a conflict with the truck. A baggage tug with trailer then followed the truck down the same road right in front of the aircraft. I couldn't see the truck driver but the baggage tug driver was not looking toward us until he was directly in front of us. When he saw us it was too late to stop; he continued across our path. Neither was a Company vehicle. When the path was clear; the Captain continued the right turn and taxi. I was thinking of what to say to the Ramp Controller to report the trucks and I failed to notice the Captain was taxiing more West than southwest toward Spot 5; which was not clearly in sight yet. When I saw the spot off to our left front; I called Ground 'approaching Spot 5.' The Ground Controller asked if we had been cleared onto Taxiway K by the Ramp Controller. I then realized we had crossed the line dividing the ramp and Taxiway K. I told the Controller that we had not been cleared on to K by the Ramp. He said 'no big deal' and issued taxi instructions. The instructions were to taxi to 22L via K8 and Y. As we reached Y intersection; the Captain turned left; which confused me so I asked; 'Where are you going?' We were not taxiing fast but it was too late to turn back to the right. The Ground Controller called to tell us of the error and when I told him we had realized the mistake gave us new instructions to Runway 22L. The rest of the taxi was uneventful. There were no other aircraft in the vicinity. I believe the Captain had been distracted by the trucks and I did not monitor the taxi to Spot 5 as closely as I should have.We should not have let the trucks driving across our taxi path distract us from properly accomplishing our next task of taxiing to the runway. I also believe that I was more recently familiar with the DTW Airport than the Captain and didn't realize that he was confused about the exact taxi routing we should take. In the future; I need to verbalize my understanding of the taxi instructions to make sure we are on the same page. This is especially true at airports where taxiing is more complex.


Second narrative: Pushed off [the] Gate expecting a left turn to Runway 22L via Taxiway K9. Ramp [Control] advised; 'Right turn out to Spot 5; contact Ground at Spot 5.' As I began right turn; a white paneled van drove directly in front of me. I braked to a stop. After van passed; I again began taxi and a baggage tug drove directly in front of me. Again; I braked to an abrupt stop. After verifying no other vehicles were going to drive in front of me; I made a 180-degree turn; which took me to Taxiway K; (Spot 5 was on ramp Taxiway E); which was now behind my left wing. At Taxiway K; I made a left turn toward where I thought Spot 5 should be. My First Officer (FO) contacted Ground; who asked if ramp had cleared me to Taxiway K. This is when I realized my mistake. Ground advised to just go ahead and taxi K8 to Y. When I got to intersection of K8 and Y; I turned left. (Runway 22L required a right turn on Y.) Ground politely gave me directions back to Y in the direction of Runway 22L. I continued taxi to Runway 22L uneventfully. There was no conflict with any other aircraft or ground vehicle during entire event. There were no other aircraft moving on ramp or connecting taxiways. Once ground vehicles caused me to stop taxiing; I should have re-briefed the taxi to the FO while verifying position and taxi plans rather than discussing safety protocol of surface vehicles.


Synopsis: During taxi; while under the control of ramp control; crew of B737 became distracted by two airport vehicles crossing their taxi path and taxied onto an adjacent taxiway. After the crew checked in with Ground Control they were given an amended taxi clearance.