Air carrier Captain reported a critical ground conflict with a vehicle which crossed on the service road section in front of the gate at a high rate of speed. The Captain executed a sudden stop to avoid a collision.

Narrative

The flight landed after a period of snow; so the ramp area had deicing fluid; patchy ice and snow on the ground. While parking at Gate XX (ZZZ); the Captain turned off the taxi light to acknowledge the proper gate. Due to the position of the nose and main landing gear and length of a 757-300; taxi past the lead in line is necessary. While northbound on the XX line; the Captain began a turn towards the lead in line. With both wing walkers present and wands in the air; a vehicle crossed on the service road section in front of the gate at a high rate of speed (and not visible to the flight deck prior to the 90 degree turn). This required a sudden stop in the turn. The aircraft slid a couple of feet in the process. Once the vehicle was clear; the Marshal indicated it was safe to proceed. Extreme care was required to ensure sufficient power to complete the turn without unsafe use of thrust or extending now too far past the lead in line.