EMB-145 Captain reported while taxiing; a city vehicle moving very fast swerved near them; resulting in Captain aggressively stopping to avoid collision.

Narrative

On Date approximately XA25 CDT I was taxiing on [taxiway] 1 toward [Runway] XXL at ZZZ. I experienced an FMS 1 failure which caused the FMS to shut down completely. Upon restarting of the FMS and during its self-test the message RAM FAIL appeared on the FMS screen. I informed Tower of the issue and requested an area to park so my First Officer (FO) and I could assess. I was issued the clearance [taxiway] 2 left on 3 and hold short of 4. Upon reaching 2 I began a right turn as cleared. Without warning a vehicle came from behind us on our right side moving very fast and appeared to swerve right and continue down L taxiway. I was very concerned the vehicle may hit us so I aggressively stopped the aircraft. I did not feel or hear an impact which would leave me to believe the vehicle struck us. I immediately reported the incident to Tower control and was told they are contacting the city about the issue. I continued taxi as cleared and dealt with the FMS issue with Maintenance Control and returned to the gate for further maintenance inspection. The cause of this incident is a city vehicle moving incredibly fast from behind us in such a way we couldn't see him until we started our turn overtook us on the right appeared to swerve onto 3 taxiway and continue driving at a high rate of speed. I'm sure the city vehicle was on a ground frequency however I was on Tower as I was given clearance to switch from the previous ground Controller. I did not know the vehicle was behind me and moving fast until I began making the turn. My FO was looking to the right as I started my turn to clear me as was I and the vehicle was not in sight until he overtook us.